Capacitors ripple current question

[Helmholtz]

To measure ripple current, insert a 1 Ohm resistor between the minus contact of the cap and ground and measure AC RMS voltage across the resistor.

Of course the resistor could be connected to the positive terminal of the cap as well. In this case all existing wires connected to the plus terminal have to be lifted and connected to the top of the resistor to force all cap currents through the series resistor.

[nickb]

How can be determined/estimated in a very simple way the ripple current performance for a filter capacitor, please ? I have to replace screen grids filter caps in series battery into a Sound City PA 100 and I just have two small FT rated at 0.2 A ripple current.This is ridiculous small relative to originals (big cans rated at half an Amp).Can I use it safe, please? Thanks

Here's quick and dirty method. There is a resistor that feeds the screen filter caps. Measure the RMS voltage 'Vin' on the OPT center tap that is on one side of this. We'll assume that this is a sine wave, it makes little difference to the result. The ac current through the filter resistor R and thus the filter caps C is

Iripple = Vin/ sqrt[R^2 + (1/2/3.14/f/C)^2]

For the Sound City R=100, C=16uF, F=100Hz and I'll guess Vin= 10Vrms

the ripple current is 5/(100^2 + (1/2/3.14/16uf)^2) = 10/141 = 70mA rms.

[J M Fahey]

I love simple solutions to complex problems.

Edit: this goes both to Helmholtz and Nick B

[trobbins]

If you were wanting to make sure the screen cap ripple rating was going to survive what the tubes could just survive, you could increase the screen power dissipation from 425Vx30mA = 13W, up to a magically uniform 32W level (8W per tube), and use PSUD2 to indicate the capacitor rms ripple at 100/120Hz.

[catalin gramada]

Thank You gents.
JMF, I will put a circuit as soon I can do the measurements. Till then a simple sketch of power supply with grounding points to chassis as looks in mine. Plate voltage 480-490v around.

[catalin gramada]

I ordered FT 2x32uF rated at 0.4A max ripple. There are not so dramatic size differences in between. Originals are just around half an inch longer. I had to peel it off to keep it grounded in circuit as in original.



Now understand what "military style" means - the wires are not hooked but knotted, really pain to remove it.

[nickb]

I ordered FT 2x32uF rated at 0.4mA max ripple. There are not so dramatic differences in between. Originals are just around half an inch longer. I had to peel it off to keep it grounded in circuit.

[ATTACH=CONFIG]51017[/ATTACH]

I hope you meant to write "0.4A max ripple"

[catalin gramada]

And here the promised schematic. Not mine,but founded one here by kind of Steve L.Thanks Steve.

Sound City PA 100 Schematic (2).pdf

Except bigger 500-490v voltage in mine everything is identical. It is quiet. Definitely a choke and little more filtering can make it dead.

[Dave H]

Except bigger 500-490v voltage in mine everything is identical. It is quiet. Definitely a choke and little more filtering can make it dead.

The ripple current of the screen node cap will be much lower after a series choke. A 0.2A rated cap will be fine.