CGS14 PSU

Dave Kendall

Hi all.

Am building PSUs right now, one of which is
this one . What effect has increasing the cap from 2200uF to 3300uF on the circuit if using a CT 18 - 0 - 18 AC transformer?

Also what is the danger (if any) in using 25V rated electros?
I've got a fair few of these, but not many 35V jobbies.

Cheers,
Dave

"Help me Electro Forum Kenobi - you're my only hope" Smile

andrewF · Posted: Wed Jun 18, 2008 1:28 am Post subject:

There is an equation flying around somewhere to help you work out the best size caps for a circuit like this. Maybe it's in Art of Electronics?
Anyway it won't make a HUGE difference using 2200 or 3300, though 3300 will help smooth out the higher voltage and make life easier for the regulators.

Best just to try it and see, if you can hear a 50/60HZ hum through your modules, or general noise, try bigger caps.

but the voltage factor. I recently had a mishap that blew out a positive regulator and resulted in about 60VAC going thru a dozen modules - lots of dead cmos! My caps were all 50V and 100V and survived.
Anyway, if you use 25V rated caps with an 18-0-18 transformer, they won't be happy if something goes wrong. They kind of burp and this greasy smoke comes out.

Best to build your power supply well and strong, it is the heart of the synth.

otherunicorn · Joined: May 11, 2008 Posts: 136 Location: Australia

With an 18-0-18 transformer, the recified voltage will be very close to or even a little over 25 volts. If the transformer puts out an over voltage (and they all do under minimal load) you will be wiping electrolyte from you equipment after they explode.
As for the 2200/3300 question, it really depends on your load. If your rated transformer current capacity x 0.7 is near what current you will be pulling from the supply, you are going to need the bigger caps, because you are at the limit of the transformer, resulting in a lower peak voltage, and thus less operational headroom for the rectifiers.

Dave Kendall · Posted: Sat Jun 21, 2008 3:43 am Post subject:

Hi guys.

Thanks for the concise replies. Smile

I have some 18-0-18 transformers rated at 75VA - but am getting nowhere trying to work out how much current that equates to in this circuit. The stuff I've read so far is a bit over my head I'm afraid, or I'm just not getting it.....

Is there a simple way to work this out for this circuit and the CGS66?

Thanks.

cheers,
Dave

otherunicorn · Joined: May 11, 2008 Posts: 136 Location: Australia

It can supply roughly 1.4 amps at the output of the supply. Of course this is more than the CGS14 or CGS66 can deliver, but both gould be rund from this one transforem with no problems.
For the sake of this discussion, 75VA / 36V = 2.08 amps. Rectification will convert that to roughly 50V across the outer rails with 1.4 amps available.
The voltages will vary with load due to reality getting in the way Smile

Dave Kendall · Posted: Sat Jun 21, 2008 5:20 am Post subject:

Thanks Ken.

I'll try and run the CGS14 and an EFM 3501 (similar to the CGS 66) from the same 75VA transformer - as they're what I've got right now)
I'll have to fing a way of cramming 3300 or 4700uf caps on the EFM board - perhaps chassis mount....... If I can't get that to work, I guess I'll be after a couple of CGS66s Wink

I guess the secondary AC outputs are simply paralleled to the normal ± and 0V inputs of the PSU boards....
Fuses before both sets of inputs would seem like a good idea too....

Many thanks for the help. Smile

cheers,
Dave

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Dave Kendall · Posted: Wed Jul 16, 2008 3:18 pm Post subject:

Hi.

Ken wrote