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APPENDIX V: AN ALTERNATIVE REPLACEMENT CONDENSER

Introduction

Unfortunately, the Panasonic condensers I tested so extensively are now out of production and no longer easily available. Although there never was a question that other proper high-quality substitutes were available, it takes time identify and test those alternatives. There were two obstacles to overcome before this could happen. First, I have enough Panasonic capacitors on the shelf to last me into the 22nd Century so doing this work had a hard time making it to the top of my to-do list. The other obstacle -- related to the first one --is it requires mind numbing work to page through catalogs and spec sheets to identify suitable candidates.

The specifications for capacitors are in different formats in different locations, not all of which are easy to find, so it takes sitting down for quite a while to systematically comb through catalogs and manufacturers' literature. Earlier this summer someone else did this for me and sent me a short list of capacitors that seemed to be appropriate given the specs I had sent him. Thanks to his work, I only had to go through a small number of web pages to arrive at a few prime candidates. Once I did that I ordered ten of each for testing.

Basically, an appropriate capacitor has to satisfy three criteria: have the proper electrical specifications, be of a size that fits into the cavity in the armature, and not dissolve in oil. I selected several that met the first criterion, possibly met the second, and would have to be tested to see if they met the third. Although one was back ordered and wasn't shipped until early September, the others arrived in early August. The construction of magnetos, and the size of appropriate capacitors, are such that every fraction of a mm counts and one can't be completely sure whether or not a capacitor will fit from the published dimensions alone. Anyway, two of the capacitors I ordered were just a teensy bit too fat to fit in a Lucas armature so had to be rejected.

Unfortunately, a few weeks after I started my tests the hot plate lost control and heated to nearly 200 oC sometime between the temperature checks I did every few days. This forced me to throw out those capacitors and start over using a different hot plate. However, I'm happy this happened a few weeks into this month-long test instead of many months into the years-long test I did of the Panasonics. On the right of the following photograph is one of these overcooked capacitors, next to one that spend 31 days at 102 oC in the center, and a new on the left.

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Vishay capacitors from left to right: new; after 1 month in 30W Castrol at 102 oC; after some unknown time up to a few days in 30W Castrol at 200 oC.

It is easy to see which capacitor in the above photography was accidentally subjected to 200 oC, but the only visual difference between the other two is the slight yellowing of the base material of the one that spent a month at 102 oC. This shows that oil does not attack the outer case of these capacitors.

Recommendation for a Replacement Capacitor

Since not everyone may want to read the details of the tests described below, I'll go straight to the conclusion. The capacitors I recommend as replacements for use in Lucas, BTH, and other rotating armature magnetos are a pair Vishay 0.082 uF "AC and Pulse Double Metallized Polypropylene Film Capacitors," manufacturer's number BFC238320823. These capacitors have pulsed current and voltage ratings of 1400 V/us and 630 VDC with a maximum operating temperature of 105 oC. These specifications comfortably exceed those needed to survive for years in the hostile electrical environment of a magneto. They are available from Digi-Key for $1.35 each ($2.70 for the pair required to be soldered in parallel) under part number BC1883-ND.

Relevant Properties of these Capacitors for use in Magnetos

At the left of the following photograph is a Lucas condenser from a K2F-type magneto, in the middle a pair of Panasonics I've already assembled into a package with bracket ready to install in a magneto, and on the right is a pair of Vishays.


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Left to right: Lucas condenser; a pair of Panasonic 0.083 capacitors soldered in parallel and ready to install; two Vishay 0.083 capacitors.

For a better idea of the size, the next photograph shows a pair of the Vishay capacitors in the cavity of a Lucas armature.


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Vishay capacitors in the end cap of a Lucas magneto.

Finally, as the arrows in the next photograph show, the Vishay capacitors are thin enough to fit below the lip in the end cap of the armature so they will not interfere with the coil of the armature when it is installed.


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Vishay capacitor in the end cap of a Lucas magneto.

As an aside, remember that it was someone other than me who took on the task of narrowing down the list of all possible types of capacitors that are on the market. All he had to work with were the specifications I gave him based on the measurements I made on operating magnetos. It is no coincidence that the Vishays he found have the same metalized polypropylene construction as the Panasonics. Because surviving high pulsed currents for extended periods requires thick metal electrodes, which in turn makes the capacitors fat, it is also no coincidence they occupy approximately the same volume as the Lucas and BTH condensers they replace. Also, like the Panasonics, the 0.16 uF version of the capacitor from the same family is too thick to fit into the available space so a pair of the thinner 0.082 uF capacitors has to be soldered in parallel.

Why These Magneto Capacitors Could Not be Any Smaller

To illustrate an important point made in the previous paragraph, the next three photographs show the internal construction of a Vishay capacitor.


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Cross section of a Vishay capacitor. The shiny flat electrodes are at the left and right sides, but the individual layers of the capacitor appear black in this micrograph.


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Higher magnification view of the lower left corner of the above photograph showing the metal layers connecting to the flat electrode.


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Micrograph at approximately 300x magnification showing the individual layers of the Vishay capacitor. The layers are 7 um thick, but the waviness was caused by the fairly crude cross sectioning process I used (a slitting saw on my mill).

The flat electrodes at the far left and right connect directly to the external leads, and have cross-sections of 0.0011 square inches. This is equivalent to the cross section of #19 AWG, wire which is approximately the same as the #20 wire used for the magneto's primary. This is no coincidence, since these capacitor electrodes have to survive the same high current pulses as does the primary coil. From there the current spreads to the metal layers that make up the capacitor itself (along with the polyethylene layers that separate them).

There are only two ways these magneto capacitors could be made smaller: reduce the thickness of the metals, which would reduce their ability to handle the high current pulses without burning out; or, reduce the thickness of the dielectrics, which would reduce their ability to handle the high voltage pulses without being destroyed by arcing between the layers. Actually, these particular ones could be made a tiny bit thinner without affecting their electrical properties. Vishay uses the same outer shell for a family of 630 VDC capacitors of values from 0.082 uF (i.e. this one) up to 0.11 uF. Looking again at the top micrograph it can be seen that there is a white plastic spacer between the actual capacitor layers and the outer shell. The thickness of this spacer is incrementally reduced as more layers are added to give higher capacitance values within this family until there is no more room left. At 0.12 uF the outer shell is replaced by one that is 1.5 mm thicker and the process starts over. If Vishay used a different shell for every capacitance value, this particular one could be made ~1 mm thinner.

Other Possibilities

There certainly are other capacitors with appropriate sizes and specifications. However, my goal here was to identify just one appropriate replacement capacitor that is in current production, not all possibilities. A potential replacement capacitor from a different manufacturer was out of stock when I placed my order in August and wasn't shipped to me until a few weeks ago so I won't finish testing it for a few more weeks. However, although its electrical specifications are even more robust than those of the Vishays, it is slightly thicker so would not fit in Lucas and BTH armatures. I ordered some of them anyway because if they pass my tests they will be fine for magnetos that happen to have deeper cavities. Also, sometime in the future (years? months? weeks?) these particular Vishays almost certainly will go out of production, so someone will have to go through this testing process again when that happens.

Tests on the Replacement Capacitors

For those interested in more details, the tests I ran on these capacitors were an abbreviated version of the ones I described on the Panasonics in Appendix II:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=467734#Post467734

Although the tests were not as extensive as the ones I conducted on the Panasonics, taken in combination with the manufacturer's specifications they are enough for me to recommend them. If I did not already have a lifetime stock of the Panasonics of my own, and even if it were not possible to conduct any additional tests on these Vishay capacitors, I would use them myself.

As I wrote in Appendix II, one typical accelerated lifetime test of electrical components is based on the observation that most chemical reactions approximately double in rate for every 10 oC increase in temperature. This "doubling rule" makes it possible to derive a lifetime estimate without having to conduct an experiment that runs for decades. Assuming it applies to the chemical processes at work breaking down the dielectric material of the Vishay capacitors, if they still functioned after a month in 30W Castrol at 102 oC this "doubling rule" would predict they would function for at least 20 years on a motorcycle parked in a storage shed at 22 oC (72 oF).

Further, although I've never measured a temperature as high as 52 oC in an operating magneto, but if we take that as a worst-case upper limit and assume that during the next 20 years the motorcycle also covers 30,000 miles at an average speed of 30 mph, that would be 1000 hours (42 days) of operation at that temperature. That time on the road is equivalent to an additional 31 hours at 102 oC. Specifically, the test itself had two Vishay capacitors spend 745 hours immersed in 30W Castrol at 102 oC, which is equivalent to 30,000 miles at a very high operating temperature plus 20.8 years storage at 22 oC (72 oF).

One of the specifications that caused me to select these Vishay capacitors is their 630 VDC rating is significantly higher than the voltage they will experience in operation, and thus they will be understressed in a magneto. Although I could have assembled an external voltage divider to allow me to test their resistance at 630 V, I just used the nearest built-in setting of 500 V. Although lower than their rating, this is still significantly higher than they will experience in operation. Also, the Milspec requirement calls for testing magneto condensers at only 400 V, specifying s a resistance greater than 2 MOhm at that voltage.

Before starting the test my General Radio Megohm Bridge found 8 TOhm at 500 V one minute after applying the voltage (the polypropylene in the capacitors slowly polarizes so their resistance continues to slowly rise). This exceeds the Milspec resistance requirement by a factor of at least 4 million. During the month-long test it was convenient to use a less specialized megohmmeter limited to 20 MOhm to make quick checks. However, at the end of the test, after removing the capacitors from the hot oil and letting them cool to room temperature the resistance at 500 V was the same ~8 TOhm after 1 minute as it had been at the start. I found no degradation of the electrical properties whatever caused by the test, with the yellow discoloration of the white plastic base shown in a photograph above the only change I could determine.

Having passed the resistance test, I next used a Hewlett-Packard 200 Watt high frequency pulse generator to subject the capacitors to current pulses of 1000 V/usec, which is more than 10x higher than the pulses I had measured using a magneto tester. I did this test at the equivalent of >1 million rpm so that I was able to simulate "30,000 miles" of operation of a twin magneto in only a few minutes. Although I did this test at room temperature, Vishay does not derate the AC or DC voltage of these capacitors for temperatures below 90 oC so this test, in combination with their published specifications, indicates they would have been fine if I had tested them at an operating temperature of 40 or 50 oC.

After having been subjected to a month in 102 oC oil, followed by "30,000 miles" of 1000 V/us current pulses, I made one final measurement of the resistance. The resistance at 500 V was the same ~8 TOhm after 1 minute that the capacitors had when they were new. Since this exceeds the Milspec requirement for the resistance of a magneto condenser by a factor of at least 4 million, my conclusion is these Vishay capacitors, like the Panasonics, should be good for at least 30,000 miles of operation plus 20 years in storage .

Summary

Although these tests were not as extensive as the ones I conducted on the Panasonics, the Panasonics are by far the most extensively tested magneto condensers that I am aware of, which means these Visays are now the second-most extensively tested condensers available for use in a magneto. Taken in combination with the manufacturer's specifications, and the fact they have the same internal polypropylene structure as the Panasonics, I would have no hesitation using them myself if I did not already have a very large stock of the Panasonics.

If you want to install a new capacitor in your rotating armature magneto and not have to deal with it again for a very long time indeed, based on my tests I recommend either the Vishay BFC238320823 or the Panasonic ECQ-P4823JU. These metalized polypropylene capacitors have capacitance values of 0.082 uF so have to be soldered in parallel to give the 0.16 uF required total. The Vishay capacitors are currently available from Digi-Key for $1.35 each under their part number BC1883-ND. Note, however, that there are other polypropylene capacitors on the market with much lower V/us ratings because they use thinner metal electrodes so it is essential to check the specifications if you plan to use any other than these two specific ones that I have tested.

It is worth emphasizing that 30,000 miles plus 20 years is only a lower limit because, other than discoloration of the white plastic base, these Vishay capacitors showed no sign whatever of degradation when I ended the test after a month. There is no reason to expect they would not continue to last quite a bit longer.