capacitors voltage rating

[morpheus82]

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 capacitors...am I wrong?

 

[richie00boy]

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

 

[Shredly]

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]

Hi,
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]

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]

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]

Hi,
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]

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

 

[head_spaz]

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]

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!!!

 

[richie00boy]

I have already told you the answer, as has Andrew in post 7. The C5 across the whole transformer does need to be rated higher... because it's across the whole thing.

I'm not sure what your confusion is now, please can you explain?

 

[morpheus82]

head_spqaz says i need 80v caps!!but i have +-35v and the caps are connected between voltage and ground, but he says that they see the whole differential voltage, so 70v, is it true?i think maybe during transient could be true...

 

[morpheus82]

ups...i'm an idiot!
c5!!ok,that's right!!sorry guys!!!i thought the other big filter caps!!!

 

[folkeb]

Sorry to barge in on the discussion, but I have a similar issue that has to be delt with! I have several (16) Mallory (org) 66000uF caps rated at 60v with a surge capability of 75v. I intend on using these in my Aleph 1.2 project. However, this amp should have approx. 60v on the rails. I am thinking of bringing the voltage down to just under 60v by the use of resitors. Is this safe, an how big a margin should I use? I would prefer to get as close as possible to 60v. What does surge actually mean? Peak value? For how long etc.

Thanks for any help!

Best regards,
folke

 

[richie00boy]

No you can't use resistors. They will be hot, waste power and make regulation even worse.

Aren't Aleph's class-a? With 60-0-60 volts and class-a that will be some monster heat machine. Are you sure on the rail voltage there?

 

[folkeb]

The aleph 1.2 are 200w class A amplifiers and they are monsters with a rail voltage of 60. The resistors are ment to be after the rectifier bridge and before the caps. Why can't I use resistors?

Tnx

 

[Christer]

I don't think regulation is much of a problem since it is a class A amp, but the resistors will be hot. Make sure they can handle the power required, with margin, and mount them so they get sufficient cooling.

 

[AndrewT]

Hi,
not sure I can help, but try to help me.
Is the A1.2 a single ended ClassA amplifier?
Is the standard PSU a rectifier followed by smoothing caps? i.e. an RC PSU.

or

Is it an RCRC supply with some resistance built in to reduce ripple being fed to the amplifier?

BTW. in both the above cases that first R might just be the resistance of the transformer secondary + cable.

 

[folkeb]

The following link show the principle of the PSU supply. My main question is whether or not it will be safe using capasitors close to their nominal rated limit, when their surge limit is somewhat higher. In my case - 60v VDC nom. tolerance and 75VDC surge. I was hoping to bring the voltage down to 58-59 volts with the use of power resistors. Hope the link will enable you to see what I 'm getting at. The resistors will be connected in series with the + and - out of the rectifier before the caps.

http://www.kk-pcb.com/power-12.html

The Aleph is Class A single ended.

 

[AndrewT]

Hi,
you have an RCLC supply with a very small R.

The problem is slightly worse than you described.

Have you measured the secondary voltage of the transformer when the mains input is exactly at the nominal specified voltage?

You need to correct for any variation here first.
Now add on the +6% mains voltage variation that is permitted in the UK.

You could end up with a considerable overvoltage when doing it this way.

BTW the Vsurge rating would take account of the +6% on the mains and other glitches that get fed into your house wiring.

Some spike attenuation can be added to the primary side of the amp or directly to the room wiring or even back at the distribution board. If your sound quality assessor approves of such things.

Part 2 to follow ( I just lost it when trying to preview)