filter capacitor in series
 

Hello
I know that this may be a dumb question but I have found conflicting
info about this on a few web pages.

If one takes say 5 capacitors each with a value of 500wv @ 150uf and
connects them in series the result wold be 2500v @ 30uf. Correct?

Now if one takes another string just like the one above, and connects
both strings across the output of the rectifier parallel to each other
does this still give the 2500v but @ 60uf? or does the voltage drop?

Thanks

Gordon

You are basically correct in your assumptions Gordon.

One thing to think about though is the balancing of voltages and currents
across the string of capacitors. This is done by putting a high value
resistor in parallel with each one of the capacitors.

The method can be seen in some of the plans for high boltage power supplies
in the ARRL handbooks. If the voltages are not equally divided among the
capacitors, one will get too much voltage and it will arc over internally.

This will effectively destroy that capacitor and then the rest of them will
go bad in rapid succesion.

You are basically correct in your assumptions Gordon.

One thing to think about though is the balancing of voltages and currents
across the string of capacitors. This is done by putting a high value
resistor in parallel with each one of the capacitors.

The method can be seen in some of the plans for high boltage power supplies
in the ARRL handbooks. If the voltages are not equally divided among the
capacitors, one will get too much voltage and it will arc over internally.

This will effectively destroy that capacitor and then the rest of them will
go bad in rapid succesion.

Gordin wrote:

Hello
I know that this may be a dumb question but I have found conflicting
info about this on a few web pages.

If one takes say 5 capacitors each with a value of 500wv @ 150uf and
connects them in series the result wold be 2500v @ 30uf. Correct?

Under ideal conditions (perfect capacitors with no leakage resistance,
etc.) the effective capacitance would be 30 uf and the voltage
capability would be 2500 volts. If the original capacitors were pre
charged to 500 volts, the series connected capacitors would produce
2500 volts. As a practical matter 5 capacitors connected in series
would require ballast resistors to equalize the leakage current, and
some voltage derating to allow for incomplete equalization.

Now if one takes another string just like the one above, and connects
both strings across the output of the rectifier parallel to each other
does this still give the 2500v but @ 60uf? or does the voltage drop?

Capacitors in parallel retain their voltage rating, and their
capacitances add.

Thanks

Gordon

--
John Popelish

Gordin wrote:

Hello
I know that this may be a dumb question but I have found conflicting
info about this on a few web pages.

If one takes say 5 capacitors each with a value of 500wv @ 150uf and
connects them in series the result wold be 2500v @ 30uf. Correct?

Under ideal conditions (perfect capacitors with no leakage resistance,
etc.) the effective capacitance would be 30 uf and the voltage
capability would be 2500 volts. If the original capacitors were pre
charged to 500 volts, the series connected capacitors would produce
2500 volts. As a practical matter 5 capacitors connected in series
would require ballast resistors to equalize the leakage current, and
some voltage derating to allow for incomplete equalization.

Now if one takes another string just like the one above, and connects
both strings across the output of the rectifier parallel to each other
does this still give the 2500v but @ 60uf? or does the voltage drop?

Capacitors in parallel retain their voltage rating, and their
capacitances add.

Thanks

Gordon

--
John Popelish

On Sun, 11 Apr 2004 12:31:00 +1200, Gordin wrote:

Hello
I know that this may be a dumb question but I have found conflicting
info about this on a few web pages.

If one takes say 5 capacitors each with a value of 500wv @ 150uf and
connects them in series the result wold be 2500v @ 30uf. Correct?

Now if one takes another string just like the one above, and connects
both strings across the output of the rectifier parallel to each other
does this still give the 2500v but @ 60uf? or does the voltage drop?

Thanks

Gordon

Gordon,

That is correct with a 'but' thrown in. If the capacitor values are
different voltage will be unevenly distributed. With the typical + or
- 20% tolerance using the string at 2500 volts is living on the edge.

It might work. If it doesn't you will find out in a few minutes. You
don't want to have your face in the circuit when it fails.

On Sun, 11 Apr 2004 12:31:00 +1200, Gordin wrote:

Hello
I know that this may be a dumb question but I have found conflicting
info about this on a few web pages.

If one takes say 5 capacitors each with a value of 500wv @ 150uf and
connects them in series the result wold be 2500v @ 30uf. Correct?

Now if one takes another string just like the one above, and connects
both strings across the output of the rectifier parallel to each other
does this still give the 2500v but @ 60uf? or does the voltage drop?

Thanks

Gordon

Gordon,

That is correct with a 'but' thrown in. If the capacitor values are
different voltage will be unevenly distributed. With the typical + or
- 20% tolerance using the string at 2500 volts is living on the edge.

It might work. If it doesn't you will find out in a few minutes. You
don't want to have your face in the circuit when it fails.

"Allan Butler" wrote in message
news:ex1ec.9943$rg5.37333@attbi_s52...

This will effectively destroy that capacitor and then the rest of them
will
go bad in rapid succesion.

And you will be among the select few who have experienced the Big Bang.

73,

"PM"

"Allan Butler" wrote in message
news:ex1ec.9943$rg5.37333@attbi_s52...

This will effectively destroy that capacitor and then the rest of them
will
go bad in rapid succesion.

And you will be among the select few who have experienced the Big Bang.

73,

"PM"

Gordin wrote:

Hello
I know that this may be a dumb question but I have found conflicting
info about this on a few web pages.

If one takes say 5 capacitors each with a value of 500wv @ 150uf and
connects them in series the result wold be 2500v @ 30uf. Correct?

Now if one takes another string just like the one above, and connects
both strings across the output of the rectifier parallel to each other
does this still give the 2500v but @ 60uf? or does the voltage drop?

Thanks


Others have correctly answered your question. I'll
add something: one should always derate the voltage
that a capacitor "sees" in a circuit from the cap's
rating. Using a 500 volt cap at 500 volts is asking
for trouble. (Or using 5 of them in series at 2500
volts.) If your circuit runs at 2500 volts, add at
least one cap to the string.

And do not forget the bleeder/equalizing resistors.
You'll need 2 resistors in series per cap. I'd use
2 51K 2 watt resistors in series across each cap, to
give a very good safety margin on the resistors' power
dissipation and voltage exposure. Others might recommend
a single 100K 2 watt resistor across each cap. But that
way, each resistor "sees" about 416 volts, and dissipates
a little over 1.7 watts. With two in series, each resistor
"sees" about half the voltage - and the power dissipation
capability is doubled.

Gordon