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Dr. Barry L. Ornitz  on Oil Filled Capacitors 

Tom, Tarheel6@email.msn.com, asked about the possibility of PCB's in an old military power supply about 55 years old.  He noted that one of the capacitors had leaked.

I cannot say for sure about this particular power supply,
but polychlorinated biphenyls gained considerable
popularity in the late 1930's with large scale production
during the Second World War.  With its higher dielectric
constant than conventional mineral oil, it made for smaller capacitors for a given value of capacitance; thus it was popular in  military gear, particularly in aircraft
applications where its flame retardancy was a real plus. 
By the 1950's, it was difficult to find an oil capacitor
that did not contain PCB's.  So this places the likelihood
of the capacitor oil having PCB's very high.

Contrary to much of the misinformation passed around in
PCB'S OR NOT.   I am sorry to shout here, but you may read otherwise from other members of this list. I can say with certainty that they are wrong.  Speaking as the Old Tube Radios' "resident chemist" you should trust me on this.  [Actually I am a PhD chemical/electrical engineer.]  I have done considerable research on the subject of PCB's and as Jack would say, I know whereof I speak!   (*)

There are rather strict guidelines on the handling and
disposal of PCB containing materials.  A web search will
reveal considerable information here.  Look especially at
the various Environmental Protection Agency sites. 
Luckily, the typical capacitor found in a power supply for ham use is not large enough to fall under the action volume.  There are some really large oil-filled capacitors that show up on E-Bay occasionally, and at least one Texas ham advertising there was paid a visit by the EPA to insure he that had labeled and stored the units properly, and that his customers would realize the requirements too.

I would suggest you attempt to clean your supply yourself if you are willing to take the proper safety precautions.  First get a pair of rubber gloves and safety glasses or goggles.  The gloves should be neoprene or preferably the Buna-N nitrile solvent resistant gloves.  Latex and vinyl gloves are not suitable.  Go to a paint store and buy one or two empty, metal, one gallon paint cans, a gallon of mineral spirits paint thinner, a pint of acetone, and two inexpensive stiff brushes.

Working outdoors, place the power supply in a large polyethylene plastic tub.  If this is a military supply with sealed transformers and no electrolytic capacitors, you can begin work.  If the transformers have exposed paper insulation, and if you have electrolytic capacitors, you will have to remove them first.  Wear the gloves and safety glasses.

Remove the leaking capacitor and place it in one of the metal cans.  Using a screwdriver or putty knife, remove as much of the dried or gummy deposit as you can.  Place this in the can too.  Do not stir up any dust as breathing this could be dangerous.

Once you have removed all the dried or rubbery material
that you can, begin washing the chassis with half the paint thinner.  Use the stiff brush to scrub the chassis. 
Collect the liquid and reuse it until you feel you have
removed most of the residue from the chassis.  Pour the
contaminated thinner into the paint can too. 

Repeat the process with the second half gallon of paint thinner.  Clean your tools at this time too.  Pour off the paint thinner and let the chassis remain in the tub until most of the remaining thinner has dried.  Discard the brush into one of the paint cans too.

At this point, you need to wash the chassis in the tub with hot water and some detergent like Dawn.  Using the second brush, again scrub the chassis thoroughly.  This was the reason for removing the transformer; you don't want it soaked in water.  Any electrolytics would have been destroyed by the paint thinner earlier.  Rinse and repeat. The rinse water will be safe enough to pour out in your yard.  I would avoid taking it indoors to pour down the drain, however.

Rinse the chassis and tub very thoroughly with clean
water.  If you have hard water, mineral containing water,
or use a salt regenerated water softener, buy a gallon of
distilled water to finish the rinse.  Let the unit sit a
few minutes to drain and then pour the acetone over it. 
This will help remove the remaining water.  Note that the
mineral spirits and acetone may soften or remove some paint and MPF varnish.  This cannot be helped.

Let the unit dry several hours in the sun.  When you can no longer smell ANY residual solvent, it is safe to take back inside.  Remember that the acetone and paint thinner are quite flammable.  Any remaining acetone will evaporate quickly outdoors.  Before powering the supply up, make sure it has dried for at least a week or more and that any cloth covered insulation is not damp.

I would consider painting or varnishing the chassis with oil-based urethane varnish to seal and protect the chassis.  If you want to match the yellow MFP color, break open a High-Lighter pen and place the felt inside in a half-ounce of acetone to extract the dye.  This can be
added to the varnish.

The tub used to clean the chassis in should never be used for food, ice, or in contact with anything that might be consumed.  You might consider cutting it up and discarding it in another paint can if you do not wish to keep it for future cleaning.

The metal paint cans containing the PCB contaminated waste should be sealed and labeled as such.  Many communities hold a yearly hazardous waste disposal day.  We have about three a year where I live.  The normal things they collect are old paint, motor oil, and garden insecticides, but they will take your waste too.  Be sure and have it labeled properly, and notify the person in charge of what the cans contain.

Some electric utilities will take contaminated transformer
oils for disposal, but they will rarely take materials such
as this.  It may be worth a few telephone calls to the
power company and local sanitation department to be sure.

This procedure is based on the assumption that PCB
materials are present.  This is the safe approach.  In
fact, I would approach ANY oil-filled capacitor made from
the 1930's through the 1970's as if it contained
polychlorinated biphenyls.  Between 1929 and 1977, Monsanto (the main manufacturer) produced some 1.4 BILLION pounds of PCB's, so there is a lot of it out there.  I have even seen documentation that some wrapped paper capacitors contained high molecular weight PCB's in the paper (solid at room temperature).  I doubt if these were common, however, so disposing of wrapped paper capacitors in a regular landfill is probably OK.

For anyone that is interested, I can provide a list of
trade names of commercial PCB oils.  The generic name for these oils is askarels, but every manufacturer used his own trade name.  Some you may have seen are Arochlor
(Monsanto), Chlorinol (Sprague), Diachlor (Sangamo),
Dykanol (Cornell Dubilier), Hyvol (Aerovox), Inerteen
(Westinghouse), and Pyranol (General Electric).

Please be safe, and please make sure your contaminated
waste is disposed of properly.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net

(*)  Most of the foreign trade names of PCB oils found on
the EPA website were provided by me.

(c) 1997, 2002 B. L. Ornitz



Dr. Barry L. Ornitz  on Electrolytic Capacitors 


> I know that aluminum electrolytics contain some ethylene
> glycol but the amount you will be exposed to is far less
> than what you can get when changing radiator coolant in
> your car.

Ordinary aluminum electrolytics are filled with a paste and are not really dry.  In fact the paste electrolyte is the cathode to the anodic film that actually forms the
capacitor.  The second aluminum foil just provides contact to the paste electrolyte.  Since the electrolyte must be kept moist to function properly, it often contains materials which absorb moisture from the surrounding humid air.  Ethylene and propylene glycol are often used along with glycerin and sorbitol (a sugar).  As long as the electrolytic capacitor is stored in a reasonably humid environment, typically 20% relative humidity or higher, the paste will remain moist.  In a hot and especially dry environment, the paste will dry out and the capacitor will quit working.  This is the failure mechanism for low temperatures too.  If the paste freezes, the electrical conductivity of the paste drops dramatically and the capacitance drops in proportion.  Fortunately many of the materials such as the glycols act as antifreeze so lower temperature operation is possible.  The conductive material in the paste can be any number of inorganic salts but boric acid and sodium borate are still popular.

None of the ingredients in aluminum electrolytic capacitors are terribly toxic or corrosive to the skin.  Proper washing with soap and water is all that is necessary if you contact them.  Of course, you would not want to eat them. 

There is a possible exception, however, with early "wet-slug" tantalum capacitors.  These are usually hermetically sealed units, and they contain sulfuric acid.  The modern solid tantalum capacitors use manganese dioxide instead of a paste, but even this is slightly moist.  About the only place most of us will encounter the sulfuric acid-filled tantalum capacitors is in the audio stage of the R-390 series of receivers.  Unless it is leaking, it rarely causes a problem.

Leaking electrolyte from an aluminum electrolytic capacitor will certainly cause a steel chassis to quickly rust.  Fortunately soap and water is all that is needed to clean it.

        73,  Barry     WA4VZQ     ornitz@tricon.net

(c) 1997, 2002 B. L. Ornitz



Basics of Electrolytic Capacitors

To begin with, there are three types of electrolytic capacitors - only  one of which is named appropriately. The original wet, or liquid- filled, electrolytic was the first type to be introduced. The later  "dry" and "solid" electrolytic capacitors are really still "wet". Non-polarized electrolytics will be discusses at a later time.

The process upon which all electrolytic capacitor manufacturing is based is the formation of an insulating anodic oxide film on a metal in the  presence of an electrolytic (ionic conduction) bath. Tantalum and  aluminum are the two metals of commercial importance here, but  electrolytic capacitors have been made with niobium, zirconium, tungsten, and titanium. According to Young ("Anodic Oxide Films", Academic Press,  1961), the first to patent an aluminum electrolytic capacitor was C. F.  Pollak in 1897. Tantalum and niobium capacitors were investigated by  Guntherschulze a few years later. The first commercial tantalum  capacitors came in 1925. 

In a typical aluminum electrolytic capacitor, the positive electrode  (anode) is usually a thin metal foil. The negative electrode (cathode)  is the _conducting_solution_ in contact with the metal foil. In the  formation process, a current is passed from the foil to the solution.  Typically a hot aqueous borate solution is used with a constant voltage  between the foil and the electrolyte. As most of us have seen when "re- forming" a capacitor, the current is initially high but quickly drops  with the formation of a thin metal oxide coating on the foil. It is  this thin oxide coating that is the real dielectric in the capacitor. A  second aluminum foil electrode is used as a "counter-electrode". Its  sole purpose is to provide electrical contact to the actual wet  electrolyte that is the true negative terminal in the capacitor.  Hydrogen can sometimes be evolved at the counter-electrode (which is a  failure mode) and chemicals are often added to the electrolytic solution  to prevent this. Hydrolysis of the thin oxide film is another failure  mechanism and it may be minimized by the addition of other chemicals to  the electrolyte. Long periods of no or low voltage on an electrolytic  capacitor allow the hydrolysis reaction to slowly destroy the oxide film  and reduce the capacitor's voltage rating.

In the original "wet" electrolytic capacitor, the electrolyte remains as  a liquid solution. The so-called "dry" electrolytic is made with a  paper or other porous separator impregnated with the electrolyte between  the anodic oxide coated foil and the counter-electrode foil. The  electrolyte is generally boric acid, or ammonium or sodium borate with a small amount of water in ethylene glycol, glycerin, dimethyl formamide or gamma-butryolactone. [The DMF and GBL are used in low tempeature units.] Sometimes high molecular weight sugars like sorbitol are added along with many proprietary chemicals. The end result is a paste with good  electrical conductivity yet one which maintains its water content and  flexibility over the required temperature range. As long as the air to  which the paste may be exposed to has greater than a certain minimum  relative humidity, it will maintain its water content. This basically  means that storing Boatanchors in hot, exceptionally dry environments  will damage the electrolytics.

If one thinks about this construction, it is easy to see why electrolytic capacitors do not work well at radio frequencies. The actual anodic oxide film is a low-loss capacitor. However the external negative terminal is  connected to the counter-electrode. Ions must migrate through the  electrolyte to reach the oxide film. The electrolyte not only provides  internal series resistance, but at higher frequencies the ions may not  migrate fast enough to reach the oxide film before the polarity of the  external voltage reverses. At radio frequencies, an electrolytic capacitor behaves more like a resistor than a capacitor. At low temperatures, the  conductivity of the solution decreases as the ion mobility decreases. This reduces the effective capacitance. In addition to decreasing the tendency  for the capacitor to dry out, the ethylene glycol or glycerin also act as  antifreeze to allow the capacitor to function at decreased temperatures.

The so-called "solid" electrolytic capacitors are a more modern invention and they tend to be found mainly with tantalum construction. Manganese  dioxide (the black stuff normally found in regular carbon-zinc primary batteries) is used to replace the paste electrolyte. It is applied in  a high temperature process which tends to damage the anodic oxide. This  is normally partially repaired by repeat anodization in a dilute aqueous  solution. The result is a solid structure but it is still moist to some  extent to provide electrolytic conduction. These capacitors are very  limited in their voltage ratings, typically no more than 35 volts.

A truly dry construction has been achieved experimentally but I am not  sure if anyone ever commercialized the process. After the anodic oxide  film is grown, a counter-electrode may be evaporated or sputtered over  the oxide surface. At the time Young's book was published (1961),  stability was a problem. While such a construction should yield a non- polarized capacitor in theory, this was not achieved in practice.

73, Barry L. Ornitz WA4VZQ ornitz@tricon.net
(c) 1997, 2002 B. L. Ornitz


Non-Polarized Electrolytic Capacitors

There has been quite a bit of discussion lately on non-polarized 
electrolytic capacitors on Boatanchors. Some of the suggestions for 
replacing these capacitors with conventional polarized electrolytics 
have been unworkable at best. Some severe time constraints lately 
have kept me from posting, so I apologize to the list for being slow in
posting this. BTW, print this is a non-proportioned font for the ASCII
schematics to look half-way reasonable.

The best way to start this discussion is to explain how conventional 
electrolytic capacitors work. Once you understand this, the 
explanation of non-polarized electrolytics is much easier. The previous
article on electrolytic capacitors should be read first.

In conventional electrolytic capacitors, the anode foil (the plus 
side, the side upon which the oxide film grows) is normally "formed" 
before the capacitor is even wound. After the capacitor is assembled 
(anodic oxide covered foil, cathodic solution, and bare foil contact 
to the solution), the capacitor is generally "re-formed" to repair any 
minor damage in the manufacturing process. This is also why old 
electrolytics can often be re-formed to operate like new.

In a non-polar electrolytic, both foils are pre-formed before the 
capacitor is wound. One foil is covered with the separator, which is 
saturated in the electrolyte solution and then the other foil is added 
on top. Both oxide layers face inward in contact with the electrolyte 
solution. The stack is then rolled up and packaged. _NO_ re-forming 
is possible with a non-polarized electrolytic (unless you want to use 
it as a DC capacitor). The voltage rating is entirely dependent on 
the initial forming of the foils and is reduced by any damage done 
while assembling the capacitor. The result is actually TWO 
conventional electrolytic capacitors in series.

---Foil/Anodic Film---Cathodic Electrolyte---Anodic Film/Foil---

+ +

If the anodic films are damaged by hydrolysis (a gradual process) or 
over-voltage (catastrophic), the non-polar electrolytic is destroyed.

So... Scott Robinson's suggestion of two conventional electrolytics 
back-to-back is, in reality, exactly what is inside a non-polar 
electrolytic. However, electrolytic capacitors are rarely high 
tolerance devices. Matching a pair is difficult and they really need 
to be the same capacitance.

When reverse voltage is placed across an electrolytic capacitor, the 
anodic film is chemically destroyed. Reverse voltages of less than a 
volt or so are usually tolerated well. So to protect the capacitors 
from reverse voltage when not exactly matched, it is a good idea to 
use back to back diodes as suggested by Herb Rosenthal, W5AN. 
[However, two back-to-back electrolytics really DOES make a non-
polarized electrolytic.] It really makes no difference if the two 
cathodes or two anodes are connected together as long as the diode 
polarity is correct.

| + | + |

Bob Roehrig, K9EUI, was worried about distortion with this suggestion. 
He is correct, but non-polarized electrolytics should NEVER be called 
on for low-distortion. 


In this circuit, the diodes will allow the capacitors to charge to 
about 71% of the peak voltage across the network, thereby keeping a 
proper bias on the capacitors. There will be some distortion involved 
with the diode's conduction but this occurs mainly during the first 
cycle of the AC applied. [In the above schematic, during the positive 
swing of the input (left), the right capacitor will charge to the peak 
positive voltage. During the negative swing, half the charge will be 
passed to the left capacitor.]

Ed Tanton, N4XY, suggested an alternative way of connecting the 
diodes. This is my understanding of what he suggested:
| + |

I am surprised that no one suggested a bridge arrangement as an 
alternative to this. This one needs only ONE capacitor and since 
modern silicon diodes are cheaper than capacitors today...

| | + |
| = |
| | |

Actually neither one of these circuits work as intended!

Think about it a little. In both cases, the capacitor(s) charge but 
then they essentially no longer pass current. Likewise, they have no 
way to discharge. It is fine to store energy in a capacitor, but you 
have to have some way of getting that energy back out for the 
capacitor to be useful. [In respect to Ed, though, if there were such 
a thing as a polarized inductor, his method of connection would be 
correct. This might be approximated by an inductor having a 
permanently magnetized core.]

So if you really need a non-polarized electrolytic, buy one or buy a 
motor starting capacitor. Just remember that non-polarized 
electrolytics really are NOT designed for AC use. They are for DC 
circuits whose polarity might change OCCASIONALLY. In intermittent 
use, like the motor starting capacitors, using them with AC might be 
OK. Just expect them to eventually fail.

For critical applications use real non-electrolytic capacitors. They 
are big and bulky but they work. For audio, avoid the non-polarized 
electrolytics wherever you can. Adding diodes can protect the 
capacitors a little and they really add little to the distortion that 
is already present in electrolytic capacitors.

73, Barry L. Ornitz WA4VZQ ornitz@tricon.net [Copyright 2002 B. L. Ornitz]

For almost all audio and radio applications, film resistors work fine, but 
with one big caveat: their voltage rating. Actually, this is a problem 
with all modern resistors. Resistors of the 90's are typically rated 
for no more than about 350 volts for 1/4 through 1 watt sizes. The 
old carbon composition resistors could handle much higher voltages 
but you really were taking a risk. With age, their voltage rating 
typically drops and their resistance increases. Film resistors tend 
to be much more stable with aging, especially the metal film resistors.

73, Dr. Barry L. Ornitz WA4VZQ ornitz@usa.net

Resistors 101 - the freshman course?

Carbon composition resistors are made by compressing a mixture of 
graphite and clay into a cylindrical shape, somehow making connections 
to the ends (usually with end caps), attaching wires, and potting the 
mess in phenolic resin. The resistance is determined by several 
things. Foremost is the chemical composition of the original mixture. 
Add a lot of graphite and the resistivity is low.

Add a lot of clay or other filler material and the resistivity is high. 
Note I said resistivity and not resistance - this is important.

The second thing that determines the resistivity is how much pressure 
under which the material is compressed. A lot of pressure and the 
resistivity goes down.

Now resistivity is measured in ohm-cm. You take a 1 cm cube of the 
material and measure the resistance from opposite sides. Of course 
you have to have perfectly conducting electrodes on both measuring 
surfaces. The funny thing is that if you had a 1 meter cube of the 
same material and did the same resistance measurement, you would 
measure the same resistance. The key fact to learn here is that 
resistivity is only a function of the material, but resistance is also 
a function of the geometry of the material.

So now you have to assume a geometry for your resistor. This is the third
thing and the final one that determines the overall resistance. I know you
hate equations but try to follow this one:

Resistance(ohms) = Resistivity(ohm-cm) * Length(cm) /
Cross Sectional Area(cm^2)

Thus, resistance is measured in ohms. To manufacture a resistor of a 
given value, you start with a chosen chemical composition, compress it 
to a chosen density in a cylindrical form, also of chosen cross 
sectional area. The only thing you have left is the length of the 
composition, and this what is usually adjusted slightly to trim the 
resistor to its desired value. Alternately, you could grind away a 
little of the center of the resistor to adjust it too. [In a pinch, I 
have used a triangular file on carbon composition resistors to do just 
this. Always seal the cut with epoxy afterward to keep the moisture 

This overall process is complex and not under the best control. Thus 
carbon composition resistors were originally manufactured in 20% 
tolerances. As things got better, 10% tolerances became possible and 
finally 5% tolerances. But this was really about the limit because too 
many "little" things could change about the manufacturing process and 
upset production.

Being essentially a straight rod of resistive material, carbon 
composition resistors had minimal inductance. A 1 Megohm resistor had 
approximately the same geometry as a 1 Kohm resistor, the major change 
being in the composition of the graphite mix used. Thus, inductance 
changed minimally with resistor value. Resistance did change, 
however, with temperature, humidity, and time.

The film resistor was created to produce a more uniform resistor, and 
one that could be manufactured to close tolerances easily. To do 
this, the design of the resistor was changed entirely.

Instead of using bulk resistivity, the surface resistivity of a thin 
film of material is used to develop the resistance. A ceramic rod is 
somehow coated with this film of resistive material. [The actual 
process can vary: sputtering or chemical vapor deposition may be used, 
or resistive inks may be coated on the surface and then fired, etc.] 
But the same equation above still holds.

If you want to make a higher value of resistance, make the film thinner
(reducing its cross sectional area), make it longer, or make it out of 
a material with a different resistivity. In practice, all of three of 
these techniques are used. Low value resistors use thick films of low 
resistivity coatings while high value resistors use thinner films of 
higher resistivity coatings.

In manufacturing these resistors, several things can be adjusted. The 
material coated on the ceramic form is usually not changed for every 
value within a given range. Instead, its thickness is usually 
changed. However controlling this to a very narrow tolerance is not 
easy. It is possible to coat a very large number of ceramic rods 
uniformly in one batch, but the next batch may be somewhat different.

This is where the spiral cutting or laser trimming is involved. If you
ablate part of the coating, you increase the resistance. However, 
this may mean having to remove large portions of the resistive 
coating. An alternate method is to cut a spiral into the material. 
What this does is make the effective length of the resistive path 
longer. The result is an increase in resistance.

Now this trimming can be done on a batch of resistors based on 
statistics of the batch, or - if you are willing to pay for it - on 
each individual resistor with computer controlled feedback. The 
latter is used to produce high precision resistors. After the 
trimming, a conformal coating is applied to seal and protect the 
surface. Today this is more often an epoxy or urethane polymer than 
the phenolic resins of old.

So now we have a ceramic rod coated with a resistive material that may 
be cut in a spiral shape. Without the spiral, the inductance of such 
a resistor would be virtually the same as that of the composition 
resistor. There would be some slight differences at very high radio 
frequencies (UHF and microwave) because of skin effect relationships, 
but you would never see any difference in audio or low RF frequencies. 
[The skin depth at any frequency becomes greater as the resistivity of 
the material increases.] But what happens with the spiral?

This creates inductance, of course. How much? Well this again 
depends on the geometry. We can use the ARRL formulas published in 
all their handbooks, or we can use Nagoaka's equations (published in 
the Radiotron handbooks), or we can even go the "bible" of inductance, 
"Inductance Calculations" by Grover. In all cases, however, we can 
see that the inductance is proportional to the diameter and the square 
of the number of turns, and inversely proportional to the overall 
length of the coil.

Now this should start making sense. In a fixed wattage of film 
resistor, the overall geometry is fixed. This means that in the same 
wattage, the inductance should go up as the square of the number of 
turns. Yet for a fixed coating thickness of constant resistivity, the 
resistance should go up only as the number of turns (approximately, 
since the cut is not infinitely thin).

But there are more details. It is really impractical to make a very 
fine spiral in these resistors. You only have so much space and the 
width of the cut must be reasonable. The question is really how many 
turns are needed in the spiral. If you made the resistive coating 
thickness perfect, you would not even need to trim the value by 
cutting a spiral. But as I said before, this is not practical. So in 
reality, a spiral is usually cut to adjust the value.

In low value film resistors, the spiral may be quite minimal. If you 
can take the conformal coating off the film resistor (not so easy), 
you can count the turns. Most that I have seen have less than 5 turns 
for values below 1 Kohms or so. Some trimming techniques do not even
produce a full spiral.

For very high value resistors, say 100 Kohms or more, it becomes 
impractical to make the coating too thin. Thus more turns will be 
needed - but remember there are manufacturing limits. I am sure it 
depends by manufacturer, but my guess is that about 20 turns would be 
an upper limit. To get a proper feel, I looked at a 1 Gohm (1000 
Megohm) resistor. It is 6 inches long and 1/4 inch in diameter. At 
12 turns per inch, it has a total of 72 turns. The turns are spaced 
such that their width is about the same as their spacing. This is done 
to maximize the voltage rating. From geometrical scaling, 20 turns 
would seem to be a reasonable upper limit in small film resistors too.

Taking 20 turns as the upper limit and the dimensions of typical 1/4 
or 1/2 watt film resistor, this places the upper limit of the 
inductance in the low microhenry range. Is this enough to do anything 
at audio frequencies? Hardly! But if I needed a 50 ohm termination 
resistance at 150 MHz, I would probably not use a film resistor. But 
I might parallel a number of higher values to lower both the 
equivalent resistance and inductance. Chip resistors, like those used 
in surface mount circuitry, have lower values of inductance than do 
the film resistors.

Chip resistors are manufactured similarly to film resistors except the 
original substrate is flat to allow surface mounting. Instead of a 
spiral to adjust their value, chip resistors may be cut in a zigzag 
pattern to increase their resistance. Because of this, chip resistors 
have very low values of inductance.

So after all of the above explanation, can I give anyone an *EXACT* 
value of inductance for a 1 Megohm resistor. No. But give me the 
resistor to remove the coating (since they will be different from one 
batch to another and even more so from one manufacturer to another), 
allow me to count the turns and measure the geometry, and I can come 
pretty darn close. But why is this even necessary. Do you want a 
reasonable upper limit on the inductance to judge the effects on audio 
through low RF frequencies? If so, use a value of about 1 microHenry. 
Then calculate the inductive reactance at the highest frequency you 
are interested in. At a megaHertz, this is about six ohms of 
inductive reactance. Now do the vector sum of this six ohms with the 
resistance value and see how much difference it makes. With a 100 ohm 
resistor at 1 MHz, the effective impedance is SQRT(100^2 + 6^2) or 
about 100.18 ohms - not very much difference. As the resistance 
increases, the effect of a little inductance gets even smaller.

In fact, the biggest reactance seen with resistors, be they carbon 
composition or film resistors, is parallel capacitance. The end caps 
of a resistor are typically a few millimeters apart. This produces 
some capacitive effects alone. The conformal coating on film resistors
increases this somewhat. The thicker encapsulation on carbon 
composition resistors produces even higher capacitance. Without going 
into details on how this capacitance can be estimated, allow me to 
just say the typical 1/4 to 1/2 watt resistor typically has about 2 to 
5 picoFarads of shunt capacitance across the resistance. At a given 
frequency, you can calculate the effect this extra capacitance has on 
the effective impedance of the resistor. Basically, it become worse 
as you increase the frequency, but it is rarely a problem until you 
have very high resistance at VHF and higher frequencies.

Barry L. Ornitz (c) 1997

Since this was written, I have received questions about the noise 
performance of various resistors. Carbon composition resistors are
generally considered the worst since their resistivity depends a lot on
the contact resistance of individual granules of graphite. This contact
resistance changes greatly with temperature, mechanical vibration, and 
especially humidity. As I mentioned earlier, carbon composition resistors
tend to increase in value as they age (sometimes going as high as several
times their original value). This is mainly due to the loss of proper 
contact between the granules. 

Carbon film resistors have much better noise characteristics than do the
carbon composition units. They should maintain this property as they age,
but I expect that their noise will increase with time. Since carbon film 
resistors are the low-end replacement for carbon composition units, this
is to be expected. Carbon film resistors do suffer thermal overload much 
quicker than carbon composition units because of their lower thermal mass.

The lowest noise film resistors are the metal and metal oxide film types.
These tend to remain stable over long periods of time, and most can handle
short-term overloads quite well.

When repairing older equipment, it is usually a very good idea to inspect all 
resistors for signs of damage. Cracked phenolic cases, or charred spots on the 
case are a dead giveaway that the resistor needs to be replaced. However,
I have seen many carbon composition resistors drastically increase in value with
no external visible evidence of damage. It is worth measuring the resistance 
of all resistors when restoring very old equipment.

As I mentioned earlier, the maximum voltage rating of most modern resistors is 
around 350 volts. In actuality, this was the case years ago but most equipment 
designers knew the phenolic cased resistors could usually withstand much more 
voltage than this. A few paid the price, however, like Heathkit who replaced 
many CRT’s in their color TV kits when 19 cent resistors failed by having over 
1000 volts across them. Today’s resistors are generally smaller than those of 
yesteryear, so if you need a higher voltage rating, use additional resistors in 

Barry WA4VZQ ornitz@usa.net (c) 1998

To answer the question about how much inductance can be found in 
a spiral cut metal film resistor, I calculated the inductance for 
a ribbon coil which would be the same dimensions as the film on a 
1/4 watt film resistor. These calculations were based on:

5 complete turns for the winding
Length = 0.15 inches
Diameter = 0.07 inches
Winding Pitch = 0.03 inches
Strip Width = 0.025 inches
Coating Thickness = 0.001 inches

To calculate the inductance, I used equations for "Helices of 
Rectangular Strip" from:

Grover, F. W.: "Inductance Calculations", Instrument Society 
of America, 1946, ISBN 0-87664-557-0, pp. 164-166.

The equations present a correction term to the equivalent 
cylindrical current sheet inductance. The correction for edge 
insulation is based on geometric mean differences for rectangles. 
In addition to solving some monstrous equations, the solution 
required several one dimensional and one two dimensional cubic 
spline interpolation of tabular data.

I obtained the following results:

Sheet Inductance = 36.36 nH
Correction = -13.40 nH
Inductance = 22.96 nH

We can compare this result to that given by traditional formulas 
for solenoid coils:

ARRL Equation = 16.87 nH (ARRL Handbook)
Nagaoka's Equation = 16.98 nH (Radiotron Designers 

The increase in inductance is due to the width of the strips 
produced by the spiral trimming of the resistor.

To model this resistor, we place this inductance in series with 
the resistance. We also need to add the shunting capacitance of 
the ceramic body and the conformal insulation. This is typically 
about 2 to 3 picofarads.

We can also include the inductance of the resistor leads using an 
equation by Rosa.

Rosa, E. B.:"Bureau of Standards Paper", 169, 1912.

For 1/4 inch leads on each end, this adds (for non-magnetic leads 
of 0.6 mm in diameter) an added inductance of 2.92 nH on each 

The equivalent circuit for this 1/4 watt resistor (forgiving the 
crude ASCII representation) I came up with is:

3 nH | | 3 nH
2.5 pF

If you calculate the impedance of this equivalent circuit, you 
will find that the capacitive reactance dominates the response 
for resistor values over approximately 100 ohms. For resistances 
below 100 ohms, the inductive effect dominates. 

To get an idea of how good this film resistor behaves over a 
frequency range, the following table gives the approximate 
frequency for which the resistance increases or decreases by 10 
percent over its DC value.

Resistance, Ohms Frequency for 10% 
Change in Resistance
1 3 MHz increase
10 30 MHz increase
100 300 MHz increase
1000 30 MHz decrease
10K 3 MHz decrease
100K 300 KHz decrease
1Meg 30 KHz decrease

My conclusion is that for resistor values over 100 ohms, film 
resistors can replace carbon composition units with no worries.
The inductance from the spiral design is just not significant 
unless you go to very low resistor values.

[Copyright 2002 B. L. Ornitz]  All Rights Reserved

haz-ma1.jpg (100205 bytes)

(check to enlarge this Resistor Graph)


Transformer Ratings
Copyright 1999 B. L. Ornitz


A number of methods of "guestimating" transformer ratings have 
been published. Most methods do quite well for large 
transformers, but fail to give reasonable numbers for small 
transformers. This is because small transformers have a much 
greater core loss than do large transformers. Small transformers 
are better able to dissipate heat than are large transformers, 
so they are often designed with higher flux densities (and 
higher core losses) - in an effort to save weight and cost. 
Also, in small transformers, wire gauge cannot be used to 
estimate the ratings as values from 200 to 1500 circular mils 
per amp ratings are in use. 

For large transformers in commercial service, a 40 C temperature 
rise is the norm (40 C or 72 F temperature rise over room 
temperature). It is best to keep transformer temperatures low; 
90 C or 194 F should be an absolute maximum unless you know the 
transformer has insulating materials that can withstand higher 
temperatures. [But if you know this for sure, why are you 
bothering to estimate the ratings?] Experimental evidence has 
shown that transformers generally deteriorate twice as fast for 
every 8 C (12 F) rise in temperature. So keep the transformer 
as cool as possible (a reason why I always use solid state 
rectifiers instead of tubes, reducing the transformer load by 
eliminating the rectifier filament power). A good rule of thumb 
is to operate the transformer for several minutes at the 
expected load. If you can hold your hand on it without it being 
too hot to touch (about 60 C or 140 F), the transformer is 
adequate for that load. Watch out for high voltage, however. 

For amateur use, a typical transformer can be used to supply 
higher levels of power than in commercial service because of the 
intermittent current demand. In powering single sideband 
transmitters, even higher power levels can be obtained; while 
the peak power may be high, the average power may be quite low. 
With the above information in mind, the following equations 
should make some sense. While we are talking power here, we 
should actually use volt-ampere units rather than watts since 
the transformer generates heat with both real and reactive 

Commercial Service (also use for receivers): 

Average Volt-Amperes = 1.4 * Transformer Volume (in cubic 

Average Volt-Amperes = 28 * Transformer Weight (pounds) -

Thus a 10 pound transformer would be rated for 180 volt-amperes. 
Such a transformer would be about 5 inches on a side. A potted 
military transformer might be somewhat larger and heavier than 
this for the same rating. The potting adds weight but only 
causes a slight improvement in cooling. 

CW Service: 

Average Volt-Amperes = 60 * Transformer Weight (pounds) -

SSB Service: 

Average Volt-Amperes = 70 * Transformer Weight (pounds) -

So the same transformer as above would be rated for 350 V-A for 
CW and 450 V-A for SSB service, while only 180 V-A for 
continuous duty. It is easy to see why SSB allowed weight 
reduction in military gear. 

For small transformers, as noted earlier, higher core losses can 
be tolerated. If you can measure the effective core area, a 
better estimate can be obtained. With typical E I laminated 
cores, measure the cross sectional area of the center of the 
"E". Then use the following equation. This is good for small 
transformers up to about 500 volt-amperes. Note the area is 
raised to the 1.5 power. 

Average Volt-Amperes = 40 * Area (sq.in.) ^ 1.5 

Thus a transformer having laminations 1.75 inches thick with the 
width of the E being 1.5 inches would have an effective area of 
2.625 square inches. This would give an estimate of 170 watts 
for this transformer.

The ultimate test of a transformer’s ratings, as mentioned 
earlier, is determined by its temperature rise. If, after a 
while of operation, the transformer is not too hot to touch, you 
are operating it at a safe level. If you need just a little 
more power from the transformer, provide for better cooling. 
This may be nothing more than providing better ventilation 
around the transformer, or it may mean a small fan is needed. 
Very old transformers, with insulating materials not rated for 
today’s standards, need to be run cooler than modern 
transformers. Reducing the load on old transformers can also 
prolong their life, so consider using solid-state rectification.
If you decide to operate at a higher transformer temperature,
do so with caution and knowledge that you are reducing its life.

It should be noted that the replacement of solid-state 
rectifiers for vacuum diodes will result in a higher voltage 
output from the power supply. Thus a resistor may be needed in 
series with the diode to maintain the same final voltage. This 
resistor will generate heat, of course. But it will generate no 
more heat than the original tube did. In fact, it will generate 
less heat as the filament power is not being used. Placement of 
this resistor may be important for adequate cooling so remember 
this when substituting silicon diodes for tube rectifiers. With 
typical rectifier tubes, the transformer will see from 10 to 15 
watts less heat it has to dissipate when going to silicon 


For chokes, some other considerations are in order. Chokes 
typically have an air gap to reduce core saturation. Thus they 
should be operated at current levels where their inductance is 
still adequate for the filtering they perform. Higher currents 
lead to saturation and a decrease in inductance. Wire resistance 
is an issue too since with excessive resistance, the voltage drop 
may be high. Even if the voltage drop is tolerable, and the 
inductance is still adequate, the choke must also be able to 
dissipate the heat produced by core losses and its resistance. 
As in transformers, limiting the temperature rise is a good way 
of preventing damage to chokes. The same as with transformers, 
a 40 C (72 F) temperature rise is the normal maximum. The old 
rule of thumb follows with chokes too. If you can safely hold 
your hand on the unit after several minutes of operation, the 
choke is not operating at too high a temperature. Again, watch 
out for high voltage.

Barry L. Ornitz, PhD WA4VZQ [Copyright 2002 B. L. Ornitz]



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