Film capacitor as power-supply filter

If you ran your own sim, why did you not fix the error?

I ran your first one and decided that it was silly. Which it certainly was. -330 volts across an electrolytic cap!

OK, you buy a bunch of giant wirewound resistors, mounting brackets for same, diodes, fans, blast shields, warning signs, covers with interlock switches, safety goggles, rubber glove dispensers, fire extinguishers, and tranquilizers so you can sleep.

I'll put electrolytic caps in series and not worry.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

It didn't occur on my copy. Only on the posted version.

As I have already pointed out, the bottom cap had a typo.

Instead of 1,000uF, it was 100uF. Obviously something was wrong with the sym. It took a bit of sleuthing to find the problem.

You should have noticed this when you looked at it.

I have since developed a method that requires no bleeder resistors, no drain while power is applied, and will drain 800V off 2,000 uF of caps in 2 seconds when power is turned off.

1. You have no discharge path when power is turned off. Plenty of loads do not have constant power drain. This can leave dangerous voltages on the caps.
2. You can have reverse current on the lower cap when there is a constant load and power is turned off. Reverse current is bad for electrolytics.

I have already noted the obvious fact that it's sometimes reasonable to discharge the entire series string, as if it were a single cap.

Reverse current? Caps always have "reverse current" during discharge. The current is equal everywhere in a series circuit. All series caps have the same current.

Does ripple damage caps?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Then you need a dump diode.

When you have mismatched caps with the higher value on top, the top cap will try to push the voltage on the bottom cap negative. A dump diode will prevent the bottom cap from going negative.

This doesn't help when the top cap has a lower value than the bottom one.

As I pointed out, the capacitor tolerance is -50% to +100%, so your chances of getting matched caps is very low.

Of course it can if it too high, or causes reverse current through the cap at or near zero volts.

A bit more explanation. When the cap voltage goes negative, the current through the cap reverses.

This causes the formation of a thin aluminum oxide layer on the cathode.

This causes another capacitor to form on the cathode. This capacitor is in series with the capacitor on the anode, so the total capacitance is reduced.

So passing current through the cap when the voltage is near zero is a bad idea.

The dump diode will prevent this from happening. It bypasses the current from going through the cap.

Best Supporting Reference:

Page 10

Applying a reverse voltage will cause chemical reactions (formation of dielectric) to occur on the cathode foil, and, as is the case with overvoltage, the leakage current will rapidly increase with heat and gases generating and thus the internal pressure increases. The reactions are accelerated by the voltage, current density and ambient temperature. It may also accompany a reduction in capacitance and an increase in tan h as well as an increase in the leakage current. An example of capacitor reverse-voltage characteristics is shown in Fig. 27. A reverse voltage of as small as 1V can cause the capacitance to decrease. A reverse voltage of

2 to 3V can shorten lifetime due to a reduction in capacitance, increase in tan h, and/or increase in leakage current. A reverse voltage of even higher value can open the pressure relief vent or lead to destructive failures (Fig. 27).

NOTES:

There are many references on the web to reversed voltage on electrolytic capacitors. Many of them are just plain silly, such as the production of oxygen on the cathode. This is impossible due to the electrolysis equation:

2Al + 3H2O --> Al2O3 + 3H2 (Gas) + 3e

This shows that hydrogen is produced, not oxygen.

However, the web links almost universally indicate that reversing the polarity of an aluminum electrolytic cap will damage it.

Also, the original PDF would not allow copying the above paragraph. I simply passed it to "Remove restrictions in PDF files", at

Now I can copy information freely from the PDF.

The end of the series capacitor saga.

If the capacitor values can be held to within 25% of each other, then the bleed resistors needed to set the capacitor voltages to 50% of VCC can be reduced to 1.6W each.

The rational is the typical leakage spec is a maximum value. For example,

I = 0.01 * C * V (in microamps) = 0.01 * 1000 * 450 = 4.5mA

To allow worst-case conditions, the industry standard is to use 10 times this current for the bleed resistors. This results in high power dissipated in the bleed resistors.

However, manufacturers try to minimize the leakage current, and often produce capacitors with leakage currents one hundred times lower.

This means the bleed resistor values can be much higher and the power dissipated can be much lower.

This also allows a simple way to discharge the capacitor string within 2 seconds after power off. Simply use a relay to switch a dump resistor across the supply voltage when power is turned off. When power is on, the dump resistor is switched off and does not load the supply. As soon as power is turned off, the relay turns off and the normally closed contacts switch the dump resistor across the supply.

Cheap dump diodes (1N4007) are still needed to prevent reverse voltage on the caps, which can damage them.

Here is the ASC file that shows how this works.

Version 4 SHEET 1 1532 0 WIRE 752 -256 688 -256 WIRE 768 -256 752 -256 WIRE 864 -256 832 -256 WIRE 928 -256 864 -256 WIRE 1088 -256 928 -256 WIRE 1248 -256 1088 -256 WIRE 1472 -256 1248 -256 WIRE 688 -240 688 -256 WIRE 928 -240 928 -256 WIRE 1088 -240 1088 -256 WIRE 1248 -240 1248 -256 WIRE 1472 -176 1472 -256 WIRE 688 -144 688 -160 WIRE 928 -144 928 -176 WIRE 1008 -144 928 -144 WIRE 1088 -144 1088 -176 WIRE 1088 -144 1008 -144 WIRE 1248 -144 1248 -160 WIRE 1248 -144 1088 -144 WIRE 1248 -128 1248 -144 WIRE 928 -112 928 -144 WIRE 1088 -112 1088 -144 WIRE 1472 -48 1472 -96 WIRE 928 -32 928 -48 WIRE 1088 -32 1088 -48 WIRE 1248 -32 1248 -48 FLAG 688 -144 0 FLAG 928 -32 0 FLAG 1248 -32 0 FLAG 864 -256 D1C1 FLAG 1008 -144 C1C2 FLAG 752 -256 V1 FLAG 1088 -32 0 FLAG 1472 -48 0 SYMBOL voltage 688 -256 R0 WINDOW 123 0 0 Left 2 WINDOW 3 -61 161 Invisible 2 WINDOW 39 0 0 Left 2 SYMATTR Value PULSE(0 800 1 1 10m 50 100 1) SYMATTR InstName V1 SYMBOL cap 912 -240 R0 SYMATTR InstName C1 SYMATTR Value 1250uf SYMBOL cap 912 -112 R0 SYMATTR InstName C2 SYMATTR Value 1000uf SYMBOL res 1232 -144 R0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL diode 768 -240 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value 1N4007 SYMBOL diode 1072 -48 M180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D3 SYMATTR Value 1N4007 SYMBOL diode 1072 -176 M180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value 1N4007 SYMBOL res 1232 -256 R0 SYMATTR InstName R1 SYMATTR Value 100k SYMBOL res 1456 -192 R0 SYMATTR InstName Rd SYMATTR Value 1k TEXT 720 -352 Left 2 !.tran 0 100 0 1m TEXT 720 -384 Left 2 ;'Capacitor Discharge TEXT 1016 -352 Left 2 !.ic V(c1c2) = 0

--------------------

** Wish it was the end of PITAs like you. ** There is no general need for bleed resistors at all.

When the AC supply is cut off, the existing load bleeds them down very nicely.

Only in a few cases can the load disappear completely in normal use.

..... Phil