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morpheus82 10th July 2006 08:44 PM

capacitors voltage rating
maybe a stupid question but...using a power supply scheme for a dual voltage(+/-35V) and 1 rectifier bridge the filter capacitors form a capacitor voltage divider; so the max voltage of the capacitors is 70V or 35V?because i remember that in a capacitive voltage divider it's mandatory to use the max imput voltage as max voltage for the I wrong?

richie00boy 10th July 2006 08:47 PM

In your example, you would need caps able to handle 35V or more.

Shredly 10th July 2006 09:05 PM

As long as the caps aren't connected rail-to-rail but rail-to-ground, and this is the standard method, 35V rated caps should be fine. DON'T connect one 35V cap directly from +35V to -35V.

Remember to check ripple current and the ripple rating of the caps, too.

AndrewT 10th July 2006 09:16 PM

what is the open circuit output voltage of the transformer when connected to a nominal mains supply voltage?

How high can the output voltage go when the mains supply voltage is at maximum?

In the UK this is +6%. i.e 254Vac for a nominal 240Vac supply

Now calculate the maximum peak voltage on each of the smoothing caps.

Select a voltage rating to suit the rail to ground voltage.

Some manufacturers trim this substantially to allow cheaper/smaller components to be used.

I have an amplfier that has 51Vdc on the PSU at 240Vac input and it uses 50Vdc caps. I think they should be shot for selling this potential death trap.

Shredly 11th July 2006 12:16 AM

I keep telling you about commercial design practices, Andrew. ;)

Good job telling the really correct way to judge what voltage capacitors to use; listen to him, raikkonen, he speaks the truth.

morpheus82 11th July 2006 06:46 AM

2 Attachment(s)
ok, guys, so if the secondary has 50vac,and I split it in 2 separate after the bridge, I think will be a good idea to use 50V capacitors instead of 35, right?the schematic is something like this...

AndrewT 11th July 2006 08:15 AM

your 2*25Vac will produce a nominal +-34Vdc.
The upper and lower voltage limits, assuming 6% regulation, will be 31Vdc to 38Vdc.
Your choice of 50V caps is just fine.

morpheus82 11th July 2006 08:43 AM

yes, considering also that italian mains will vary more than 6% i think it's a good choice, thanx a lot!!

head_spaz 12th July 2006 03:30 AM

Gonna need a 80v+ cap for C5.
It's gonna see the Peak to Peak voltage of both secondaries because the cap sees them wired in series.

morpheus82 12th July 2006 06:58 AM

wait wait...are you sure?that was my problem...i thought this too, cos it's a voltage divider...probably during a transient it will go upper 50 volts...damn...does anybody has a sure experience with this schematic??please!!!:bawling:

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