I suggest elsewhere on this web site that the capacitor soakage effect may be partly due to dielectric compression and or loose winding and neither is a linear effect ~ If the plates of a capacitor can be pulled together by their opposite charges then the capacitance should change at twice the rate of an applied a.c. current provided the plates can respond fast enough or they hit a resonant overtone (or undertone ?) frequency Considering the way dielectric compression may work I have conducted many tests looking for the production of 2nd harmonics by paper in oil capacitors which could explain why they sound "musical" and "warm" although producing distortion ~ At present these tests have NOT proved conclusive so here are some more d.c. tests related to capacitor soakage or I would like to say dielectric compression Many HiFi audio components are now cryogenically frozen and claim a "better sound" which I think is complete **** especially for thermionic valves ~ But what if a capacitor with a compressible dielectric is frozen? ~ Would the dielectric compression "effect" while still "frozen solid" as ~ measured by a rise in open terminal voltage after discharge ~ be reduced ? |
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In the picture are 2 pairs of identical capacitors that both exhibit "soakage" as defined by the fact that when charged for a long time and then short circuited they produce a voltage across their terminals when the short circuit is removed ~ This voltage rises slowly with time provided the measuring equipment load is very high or connected intermittently | |||||
The capacitors in front covered in polystyrene insulation are 8µF 1000V paper and NITROGOL which is a thick insulating oil ~ In the background are a pair of 16µF 500V TCC electrolytics ~ Both caps in each pair were from the same production batch unused and about 40 years old
Nitrogol and electrolytic capacitors both have porous paper as a "spacer" between the plates ~ I cut open several samples of each type and found the rolled up cores of aluminium foil and paper were easily deformed ~ especially the electrolytics ~ But after freezing to about 18˚C both types become very hard to deform ~ The Nitrogol was like a very hard wax while the electrolytic appeared to freeze to ice All capacitor types were tested as pairs using the same technique ~ At room temperature they were charged in parallel to their working voltage using a Fluke 343A supply for at least 10 minutes ~ Then the Fluke voltage was quickly reduced to 0V which shorts the caps and when discharged to 0V the +Ve leads were removed at the same time leaving the Ve leads connected to the fluke along with a volt meter Ve lead The volt meter used had 10MΩ input impedance and its +Ve lead was only connected to each capacitor for a second to measure the terminal voltage so there was very little loading between readings ~ Although I have equipment that can measure and record readings against time ~ I did not want this to become an absolute figures experiment plus the voltages of 1000V and 500V did not allow for its safe use I measured the terminal voltages of the Paper/Nitrogol pair at room temperature after discharge taking readings at random intervals between 30 seconds and 2 minutes ~ Each pair of 1 second long readings were taken as close together as possible ~ The rise in voltage after discharging was the same for each capacitor ~ The voltage gradually went up to about 10% of the initial charge voltage and then although still rising was becoming reduced by connection of the meter This test procedure deliberately avoids absolute timing and temperature measurements as these only tend to confuse the issue plus the results were well clear of small errors so such accuracy was not required ~ Both pairs of capacitors always measured within a few % when at the same temperature as checked over several days at different ambient temperatures When however one capacitor was frozen to 18˚C for several hours and the other heated to +60˚C for several hours { 40˚C above and below ambient } the following typical results were obtained |
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8µF 1000V Nitrogol Capacitors Initial charge 1000V for >10mins
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H and C Capacitors swapped Initial charge 1000V for >10mins
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+60˚C
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18˚C
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Time
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+60˚C
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18˚C
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5.5V
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2.0V
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↓
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7.8V
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1.6V
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12V
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3.0V
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↓
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14V
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2.4V
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22V
|
4.6V
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↓
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20V
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2.9V
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24V
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5.0V
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↓
|
24V
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3.4V
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27V
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5.4V
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↓
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27V
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3.6V
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32V
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6.0V
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↓
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31V
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4.0V
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38V
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6.5V
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↓
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33V
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4.1V
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42V
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6.7V
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↓
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41V
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4.6V
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44V
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6.9V
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↓
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44V
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4.8V
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48V
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7.0V
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↓
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54V
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5.4V
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69V
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8.2V
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↓
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68V
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6.0V
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As seen in the pictures the capacitors were thermally insulated to slow their return to ambient temperature and they were marked H and C to indicate their initial Hot and Cold conditions ~ Several tests were done with the capacitors' swapped ~ Each time the pair had finally returned to ambient the test was repeated ~ With both capacitors at the same temperature the rate of voltage rise was always the same irrespective of the initial temperature or the common charge and discharge time |
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The same tests under similar conditions were made to a pair of 16µF 500V electrolytics only this time they were placed in card tubes to slow the temperature change plus the metal cans at 18˚C tend to stick to your fingers and at +60˚C is easier to handle in a card tube
Again the pair were from the same production batch and charged to their full working voltage for the tests at room temperature but for the hot and cold tests the voltage was reduced to 450V because previous new old stock electrolytics had broken down when heated A typical set of results are: |
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1970s 16µF 500V electrolytics
Initial charge 450V >10mins |
H and C Capacitors swapped
Initial charge 450V >10mins |
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+60˚C
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18˚C
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Time
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+60˚C
|
18˚C
|
2.4V
|
2.1V
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↓
|
3.6V
|
2.5V
|
4.0V
|
2.7V
|
↓
|
5.5V
|
3.3V
|
5.7V
|
1.6V
|
↓
|
7.2V
|
2.9V
|
6.0V
|
1.1V
|
↓
|
7.6V
|
2.8V
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6.1V
|
0.5V
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↓
|
7.8V
|
1.5V
|
6.0V
|
0.24V
|
↓
|
7.8V
|
0.9V
|
5.9V
|
0.22V
|
↓
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7.7V
|
0.68V
|
|
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↓
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7.4V
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0.3V
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" They heard the call and they wrote it on the wall ~ For you and me we understood "