capacitors

[Craig E. Luton]

I am replacing the capacitor is some coils I have that I will be needing this spring. I am using the replacement ones that are orange/yellow in color and are round in shape. I see on the Langs website that he has some other ones for sale, “High dV/dT”. Are they better then the ones I am using now? On some coils after replacing the capacitor they still test as “Bad Capacitor” I know that this subject has been discussed quite a bit in the past.
Thank you in advance, Craig

[Scott_Conger]

This has been heavily researched and ultimately custom caps were designed and manufactured for the hobby in order to have high quality long lived coils in the modern era. The hobby owes these folks a huge debt of gratitude.

You give no technical description of what you used for anyone to give a definitive answer to your question regarding suitability, but the fact that some of your coils still fail should give a clue. Using anything other than what is supplied by REPUTABLE and KNOWLEDGEABLE suppliers is simply wasting your labor and if they do work, will not likely last long. And shame on 2nd Tier Suppliers who sell junk as a viable replacement.

Support our suppliers and enjoy the fruits thereof. Those suppliers are not hard to find, and the capacitors are not that expensive.

Good Luck.

[Craig E. Luton]

Scott;

Thank you for answering. It would be a good idea if you went back and reread my post. If you had looked at Lang’s website you would have seen the capacitor I was talking about. It is orange in color. I bought them from Lang’s some time ago. I have been restoring cars since 1962 and am 85 now. Never in my working career have I bought parts form anyone other than a reputable wholesaler and Langs is a very reputable wholesaler and very helpful to talk to.
I also mentioned that this subject had been discussed some time ago. My question was and is “Is the “High dV/dT”” a better capacitor? I am asking for information from someone with experience and knowledge with/about the capacitor in question.
Craig

[Scott_Conger]

Craig

If you had in fact said initially that the round orange/yellow caps you bought were in fact from Lang's in the first place, then perhaps that might have been useful

Now that you've chastised me into looking at Lang's, I did, and I see that your "round" capacitor is in fact cylindrical, which was the first version of an acceptable capacitor in many years. The "high dv/dt" version has been around for several years now, too. I've pulled enough "round" useless radio capacitors out of coils to be wary of anyone mentioning a "round" capacitor. In reality, BOTH caps sold by Lang's are "high dv/dt" caps - designed as such...I don't know why Lang's specifically calls one that and not the other.

I have used both versions offered by Lang's and find no practical difference between the two other than the high dv/dt capacitor has a much friendlier shape and fits the coil with less effort than the bulkier earlier version, and does not require the glass insulator to be removed to make room like the orange capacitor frequently does. For that reason, and that reason alone, I prefer them. After many coil rebuilds using each capacitor I have yet to record a single failure, thus my remark that there is no practical difference in either regardless of their rating; they are both rated to exceed the working environment of our coils and each performs equally well.

If you are replacing caps and still recording a capacitor failure, it is very unlikely it is the fault of the capacitor.

Rarely discussed, but very useful for diagnostics and weeding out nearly unrebuidable coils, is the use of a Meggar during the rebuild process prior to installation of a new cap. Not likely an issue for what you're experiencing but then again I am not witnessing the capacitance test, either, to offer any further advice on that.

Good Luck

[Mike Silbert]

"high dv/dt" means that it can change voltage over the change in time faster than normal.
Or "High Change Voltage/Change Time"

When a capacitor can not absorb or discharge voltage as fast as the rest of the circuit it will heat up the capacitor.
Think of it as an undersized wire in a circuit.
The change to the high dv/dt capacitors was done to try to reduce heat buildup and try to extend capacitor life.

Coils run on the inductive kick and transformer principals plus even more.
The energy output (spark energy) is related to the length of time it takes for the field to collapse, among other things.
Go back to all the discussions about upper points drop, speed, and saturation in the past for more pieces of the puzzle.
The faster it works, the hotter the sparks it puts out.
The capacitor is only one of many things that makes a coil work well and it is part of the whole balanced system.

That is what it means and what it does.
Since a lot of my coils sit on top the heater (engine) I prefer to try to keep things as cool as possible and use the high dv/dt condensers.
It is up to the individual installer/user to determine whether the difference matters to them.

Mike

[Moxie26]

Good morning Craig.... With your experience, I'm sure you did check those capacitors before installing to find that they were good electrically . Putting a wire wheel brush to clean the external coil box contacts and top posts would give better true electrical values. The " Orange Drop " referenced capacitor, is the one that our major suppliers have sold , and used successfully by coil rebuilders without problem for years, even though recent production has been made in Mexico..... Keep up your good work..... P.S. analog meters IMHO show better results than digital in basic coil work.

[MKossor]

On some coils after replacing the capacitor they still test as “Bad Capacitor”

The key question is: How are you testing the capacitor that concludes it tests "Bad Capacitor"?

If the capacitor VALUE is 0.47uF +/- 20% and is of the correct type of capacitor (dV/dt > 600V/us) the capacitor is good for use in a Model T coil. Some capacitor test instruments (ECCT included) also test the dielectric insulating property of the capacitor in terms of leakage current which is typically very small. However, once the capacitor is in stalled in the Model T coil, there are other parallel paths for this leakage current to flow. For example, when the user's fingers contact both vibrator spring and cushion spring point elements when holding open the coil points to test the capacitor. Enough leakage current can actually flow through the user's fingers and register excessive on the capacitor test instrument.

Another alternate leakage path is primary winding wire insulation degradation over time that permits leakage from primary winding to the coil core and back through the vibrator spring when depressed against the coil core when the points are held open to test the capacitor. Yet another leakage path can occur if the top of the coil is painted black using a carbon based paint which is mildly or considerably electrically conductive.

A "Good" capacitor will typically have very low leakage tested by itself (less than 0.00000001A) or stated another way, very high leakage resistance (50,000,000 Ohms or higher). The alternate leakage paths can reduce that internal resistance down significantly that registers as "Poor" on the test instrument but not necessary affect Model T coil performance. The leakage resistance would have to be very bad for that to happen (well below 100,000 Ohms). So as long as the capacitor tests 0.47uF +/- 20% AND the parallel resistance is greater than 100,000 Ohms (100K Ohms). The capacitor will perform its function in the Model T coil as designed.


Technical basis for the need to only use high dV/dt rated capacitors for Model T (or any) ignition coil application
Power dissipation in an electrical circuit can be determined from Ohms low; P=I^2*R (I squared x R) measured in watts.
The current flow through a capacitor that is suppressing a continuous string of point contact voltage pulses can be calculated by the following formula: I(A)=C * dV/dt Where C is the value of the capacitor (0.47uF) and dV/dt is how fast those voltage pulses rise with respect to time. The faster the voltage pulses rise with respect to time, the higher the current flow through the capacitor. This is also known as capacitor ripple current. This is important because all capacitors have an internal effective series resistance (esr - not the parallel leakage resistance discussed previously). The esr is typically low but not zero, therefore, the capacitor current squared times this esr equals the power dissipated within the capacitor.

Pdiss(W) = I^2 * ESR

In summary, Ignition coil capacitor are subjected to very high ripple current due to the very fast rising voltage pulses that occur every time the coil points open. The high ripple current flowing through the capacitor internal resistance create a significant heat that can degrade the capacitor's internal insulation causing the capacitor to change value or fail outright unless the capacitor construction and dielectric material can withstand the power dissipation commonly specified by the maximum "dV/dt" rating (ie maximum voltage rise/time the capacitor can safely withstand from a repetitive series of such pulses.

[Craig E. Luton]

Thank you everyone. I do need to say how I know the capacitor reads as bad. To clarify how I am testing the capacitor, I am using a ECCT coil tester. The kind you set the coil in and press the test button and it tells you if your capacitor is either Bad, Good or Excellent. I test the coil then replace the capacitor and test it again. That is where I am getting the bad capacitor reading.

[Moxie26]

Quoting from Mike Kossor's post above .... "However, once the capacitor is in stalled in the Model T coil, there are other parallel paths for this leakage current to flow. For example, when the user's fingers contact both vibrator spring and cushion spring point elements when holding open the coil points to test the capacitor. Enough leakage current can actually flow through the user's fingers and register excessive on the capacitor test instrument. ". ........ When you checked capacitor with the ECCT, did you place/insert a paper insulator between the points for testing , .... or held points open with fingers ?.. I've done the same thing myself in the past and I stand corrected.

[Craig E. Luton]

Robert;
I held the lower point down with my fingers. I didn’t touch any part of the coil, only the point holder. I will try again using a piece of cardboard.

[Craig E. Luton]

I tried using a piece of paper between the points and still got a bad reading.
Craig

[ABoer]

Craig , when you use news paper you get a wrong reading .
You need a pice of plastic for isolation .
Toon

[Craig E. Luton]

Anthonie

I used the back of a writing tablet.

[Moxie26]

Craig... Have you repotted your coils yet? Just open up the sliding trap door and check your capacitor connections....wires were cleaned, twisted and soldered ?, are any of those bare wires touching other bare wires ? ... Are wires securely soldered to the bottom of the terminal posts?.... Pictures could help since we're not there physically .

[Craig E. Luton]

Yes, and I checked continuity and everything is good, with no wires touching. New doesn’t mean good. I am wondering if some of the capacitors could be defective or if I am doing something wrong.

[Moxie26]

If you want to consider removing that capacitor, and replacing with another new one ?.....

[speedytinc]

Rig up a capacitor externally between the upper & lower points.
When you test fire the coil, do you get a strong spark? Is there minimal arcing @ the points?

[MKossor]

I am using a ECCT coil tester.

Since you are using the ECCT Capacitor Test function, If the the test result is "Poor" illustrated below:

ECCT Capacitor Result with Leakage.jpg (45.02 KiB) Viewed 247 times

Referring to the ECCT Instruction Manual for guidance: Section 4.3 on Page 8 in bold red text: Capacitors that test Poor but are in the range of 0.47uF to 0.56uF do NOT need to be replaced. The reason is explained in detail previously.

An added benefit of using the ECCT Advanced Features Software is a clearly displays the parameter testing poor is Leakage Not capacitor value. In hindsight, the leakage test should not have been this sensitive because such a high value (> 100,000) has no significant impact on ignition coil operation. Always judge the need for capacitor replacement on Capacitor VALUE, NOT LEAKAGE! Capacitors that test LOW in value (0uF to 0.22uF) warrant replacement. 0.33uF is marginal but still within the typical 20% tolerance range.

Capacitor Leakage Test Failure.jpg (16.66 KiB) Viewed 247 times

[DHort]

Craig

I cut a quarter sized circle of plastic out of a milk carton for my insulator. Works great and cheap.

[ABoer]

Craig ;

One time I had a problem with a capacitor .
-The Capacitor out side the box was OK , but inside NOT .
-In the end we foud it .
-It was the BLACK paint on top of the coil box .
-After I [took of] the black paint all was OK

Greeting from Holland
Toon

[perry kete]

DHort,

I cut a quarter sized plastic disk out of my milk carton but now I have milk leaking all over the refrigerator!

[big2bird]

DHort,

I cut a quarter sized plastic disk out of my milk carton but now I have milk leaking all over the refrigerator!

Lay the fridge on its back. Duh.

[TXGOAT2]

Some types of paint can have a low electrical resistance. Most any metallic paint, including gold or silver colors, and some black or dark colored paint can cause issues. For the coil box top piece, cleaning off all old paint, soot, etc and re-sealing the wood with something like Krylon Ignition Sealer ought to give good results. A new top piece would be best, if available
Paint could also cause issues around the high voltage contact button. Years ago, I had a new KW coil. It had what looked like a very thin coat of black paint or stain. It did not hide the grain of the wood. It may have had a vegetable dye for pigment.

[speedytinc]

I learned from Mr. K's instruction on leakage test failures.
We have been holding the lower point down.
Having the point bar touching the core is no good & there is the possibility of fingers touching the upper & lower point bars.
So an insulator between the points is the way to go.