Equating different value capacitors....

[jonnyGURU]

I used to know the answer to this when I built my own cross-overs when I worked as an installer.

What the math to figure out what the value of a capacitor is if it's run at a different voltage? If I have a cap that has a capacitance of 470uF @ 400V, is that the same as 940uF @ 200V or what?

I just can't remember how that works.

 

[HESDog]

I don't believe that capacitance changes with voltage. Will need to put caps in series or parallel to achieve the capacitance needed for a particular voltage level.

 

[dighn]

yeah, capacitance isn't supposed to change with voltage for a capacitor

 

[sharkeeper]

That's a max value. Exceed it and you will experience the smell of fresh honey roast cheerios.

 

[91TTZ]

Originally posted by: sharkeeper
That's a max value. Exceed it and you will experience the smell of fresh honey roast cheerios.

But they're not quite as nutritious.

 

[jonnyGURU]

Hmm.... Ok. You guys are probably right. I thought it was relative to the amount of capacitance the capacitor was capable of.

Here's what I'm thinking about:

A typical PC power supply has two 200V input capacitors in series. Each with a value of 1000uF. Some power supplies have a single 400V cap with a value of 500uF.

Where's the relationship? How can one 500uF cap do the job of two 1000uF caps? I'm confused.

 

[FrustratedUser]

Capacitance does not change with voltage.

 

[HESDog]

Originally posted by: jonnyGURU
A typical PC power supply has two 200V input capacitors in series. Each with a value of 1000uF. Some power supplies have a single 400V cap with a value of 500uF.

Where's the relationship? How can one 500uF cap do the job of two 1000uF caps? I'm confused.

In your example above, the two 200V @ 1000uF caps in series would be equivalent to one 200V / 500uF cap. One factor in your example would be cost. The 400V cap will cost more than an similar cap with lower voltage ratings. It's quite possible that the PS with the single cap is a more robust design / higher quality.

 

[dighn]

Originally posted by: HESDog

Originally posted by: jonnyGURU
A typical PC power supply has two 200V input capacitors in series. Each with a value of 1000uF. Some power supplies have a single 400V cap with a value of 500uF.

Where's the relationship? How can one 500uF cap do the job of two 1000uF caps? I'm confused.

In your example above, the two 200V @ 1000uF caps in series would be equivalent to one 200V / 500uF cap. One factor in your example would be cost. The 400V cap will cost more than an similar cap with lower voltage ratings. It's quite possible that the PS with the single cap is a more robust design / higher quality.

two 200v's in series should be ~400v?

 

[LordSnailz]

Originally posted by: dighn

Originally posted by: HESDog

Originally posted by: jonnyGURU
A typical PC power supply has two 200V input capacitors in series. Each with a value of 1000uF. Some power supplies have a single 400V cap with a value of 500uF.

Where's the relationship? How can one 500uF cap do the job of two 1000uF caps? I'm confused.

In your example above, the two 200V @ 1000uF caps in series would be equivalent to one 200V / 500uF cap. One factor in your example would be cost. The 400V cap will cost more than an similar cap with lower voltage ratings. It's quite possible that the PS with the single cap is a more robust design / higher quality.

two 200v's in series should be ~400v? or not?

capacitors are the not same as resistors, they are inversely porpotional, so it's (1/c1+1/c2)=1/cnew

 

[dighn]

i meant the voltage rating, not the capacitance. if the capacitances are equal i figure the voltage rating should be ~ double

 

[HESDog]

Originally posted by: LordSnailz

Originally posted by: dighn

Originally posted by: HESDog

Originally posted by: jonnyGURU
A typical PC power supply has two 200V input capacitors in series. Each with a value of 1000uF. Some power supplies have a single 400V cap with a value of 500uF.

Where's the relationship? How can one 500uF cap do the job of two 1000uF caps? I'm confused.

In your example above, the two 200V @ 1000uF caps in series would be equivalent to one 200V / 500uF cap. One factor in your example would be cost. The 400V cap will cost more than an similar cap with lower voltage ratings. It's quite possible that the PS with the single cap is a more robust design / higher quality.

two 200v's in series should be ~400v? or not?

capacitors are the not same as resistors, they are inversely porpotional, so it's (1/c1+1/c2)=1/cnew

Capacitors in Series: C(total) = C1* C2 / C1+C2
if the caps are identical: C(total) = 1/2 C

Capacitors in Parallel: C(total) = C1+C2+Cn

Only the capacitance changes when in series or parallel, not the supplied voltage. Every single cap will see the same voltage no matter what.

 

[dighn]

Originally posted by: HESDog

Only the capacitance changes when in series or parallel, not the supplied voltage. Every single cap will see the same voltage no matter what.

i don't see how the supplied voltage on each cap would remain the same in a series. if each cap see 200v, 2 in series would be 400v

 

[Danzilla]

two caps in series, while the capacitance isn't simplly C1+C2, 400V applied to the two caps would essentially be spilt 0-200 and 200-400, or 200V each.

 

[HESDog]

Originally posted by: Danzilla
two caps in series, while the capacitance isn't simplly C1+C2, 400V applied to the two caps would essentially be spilt 0-200 and 200-400, or 200V each.

seems that you have this confused with resistors. caps do not cause a voltage drop, they will "charge up" to the applied voltage. this is why you can have two 200V caps in series and still only (safely) apply 200V - each one will attempt to charge up to 200V. You apply 400V and you'll smoke em.

 

[jonnyGURU]

That's why I'm confused that they used a 400V cap. On the AC side, you'd still only hit it with no more than 230V (I guess the 200V caps are enough since that's all that anyone ever uses.) But if that's the case, and a 200V 500uF cap could've been used in place of the 400V 500uF cap and the two 1000uF caps in series equates to the one 500uF cap, then there's really no benefit to using one over the other.

That said... Since the caps in a power supply are actually used to filter common-mode interference, wouldn't more capacitors in series filter better than any one, or even two?

 

[HESDog]

I don't really know if there is any benefit from having one or two in the PS scenario you describe.

I'm guessing that it is related to cost or availablity factors. A common rule of thumb is to ues a cap that is twice the value of the applied voltage. This gives a nice safety margin on voltage spikes/surges. Use lower rated components and the bottom line is that you save money at the expense of quality and durability.

 

[davestar]

Originally posted by: jonnyGURU
That's why I'm confused that they used a 400V cap. On the AC side, you'd still only hit it with no more than 230V (I guess the 200V caps are enough since that's all that anyone ever uses.) But if that's the case, and a 200V 500uF cap could've been used in place of the 400V 500uF cap and the two 1000uF caps in series equates to the one 500uF cap, then there's really no benefit to using one over the other.

That said... Since the caps in a power supply are actually used to filter common-mode interference, wouldn't more capacitors in series filter better than any one, or even two?

series caps will attenuate a DC signal - not what you want for a DC power supply (assuming that's what you have). varying the cap value will change the corner frequency of your filter.