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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
trouble with CGS66 PSU (transformer problem?)
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fonik



Joined: Jun 07, 2006
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PostPosted: Fri Aug 01, 2008 1:28 am    Post subject: trouble with CGS66 PSU (transformer problem?) Reply with quote  Mark this post and the followings unread

hi all,

i am currently working on a PSU i built from the CGS66 schematics. i etched my own PCB, nevertheless everything should be fine.

however i experience some problems.

The output on the positive rail is quite fine (+15V). The negative is not, it's just about -6V.

The indicating LED for the negative rail need a loooong time to go down after switching of the supply, much longer than the positive one.

1st of all i verified the circuit and all values again. I used 2x 2200uF for each rail.

after the rectifiers i get 27.8V and -27.8V, which is to much for a 18-0-18 transformer, even with no load (according datasheet it should be around 20V).
- writing this i got an idea: what could be wrong with the wiring of the transformer? actually it is a dual transformer: 2x 115V primary and 2x 18V secondary. first of all i should measure the secondary AC.

however, the LM337 should be able to handle the +28V/-28V!?

all coments are appreciated. need a little help from my friends...

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Luka



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PostPosted: Fri Aug 01, 2008 2:48 am    Post subject: Reply with quote  Mark this post and the followings unread

is it a torroid transformer?
you might need to check your centretap

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fonik



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PostPosted: Fri Aug 01, 2008 2:59 am    Post subject: Reply with quote  Mark this post and the followings unread

it's a print transformer. nevertheless it has to get center tapped in the right manner.
thank you for the suggestion , luka. the secondary AC voltage will be the next thing i measure.

this is how i wired it now:
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however i measured +27V and -27V related to COM after the rectifiers (or at the input of the regulators). shouldn't the LM337 handle this? the LM317 obviously does.

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fonik



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PostPosted: Fri Aug 01, 2008 4:18 am    Post subject: Re: trouble with CGS66 PSU (transformer problem?) Reply with quote  Mark this post and the followings unread

fonik wrote:
after the rectifiers i get 27.8V and -27.8V, which is to much for a 18-0-18 transformer, even with no load (according datasheet it should be around 20V).

ouch. that's nonsense. Rolling Eyes
the datasheet talks about 20V AC. i measured the rectified DC...

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fonik



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PostPosted: Fri Aug 01, 2008 4:26 am    Post subject: Reply with quote  Mark this post and the followings unread

so the 27VDC i measure at each rail may a little bit high but reasonable (peak voltage - voltage drop over diodes)?

that would mean that the wiring of the transformer is correckt, and something might be wrong with the LM337 regulator. am i thinking right?

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Blue Hell
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PostPosted: Fri Aug 01, 2008 4:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Both the 317 and the 337 are specified for a max. of 40 V between in and out, so there is no problem there.
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Blue Hell
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PostPosted: Fri Aug 01, 2008 4:42 am    Post subject: Reply with quote  Mark this post and the followings unread

And the rectified voltages are what is to be expected indeed.

There is a minimum output current for the regulators, normally it is set up in such a way that that the voltage divider going to ground and back to the sense input uses just a bit more than that current. Most likely it is a wiring error ... you did see that the 317 and 337 have a different pin layout I assume ... (for the TO 220 housing at least).

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fonik



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PostPosted: Fri Aug 01, 2008 4:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
And the rectified voltages are what is to be expected indeed.

There is a minimum output current for the regulators, normally it is set up in such a way that that the voltage divider going to ground and back to the sense input uses just a bit more than that current. Most likely it is a wiring error ... you did see that the 317 and 337 have a different pin layout I assume ... (for the TO 220 housing at least).

hello jan,

yes, i quadruple checked the pinout of the 337 and the orientation on the my PCB, since a reversed regulator was my first suggestion. (actually i used expressPCB for both the schematic and the PCB layout and i used the softwares devices, so everything should be fine.)

will check it again. and again i will check the schematic i've redrawn from ken's. the redrawn schematic is the source for the PCB.

what if the regulator got too hot when getting mounted? wouln't it stop doing anything?

concerning the minimum current: i will try to power something from it. there is still the GMvoice waiting for a housing Very Happy

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zthee



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PostPosted: Fri Aug 01, 2008 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

This might be silly, but if you used this picture from the CGS site - http://www.cgs.synth.net/modules/pic/pcb_cgs66_psu.gif as reference while building. The you might have the 10uF capacitor on the negative rail backwards?

I put it in the wrong way and my negative side would never go over -9V.
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fonik



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PostPosted: Sat Aug 02, 2008 9:44 am    Post subject: Reply with quote  Mark this post and the followings unread

zthee wrote:
This might be silly, but if you used this picture from the CGS site - http://www.cgs.synth.net/modules/pic/pcb_cgs66_psu.gif as reference while building. The you might have the 10uF capacitor on the negative rail backwards?

good suggestion, zthee!
however, i built my PSU from schematic and made may very own layout (wanted to have larger heatsings and more space between caps and heatsinks, other connectors, specialy for my MIDI driven KLEE2 seq).

jan, you've been right. under load everything works fine! i powered a module from it and it's quite right now! thanks.

sometimes it's so easy...

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zthee



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PostPosted: Sat Aug 02, 2008 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

It's nice that you've solved it thought! Smile

And larger space between the heatsinks would've been nice!

I'm running my two CGS66 PSUs on 9,6°C/W heatsinks - and they get a little to hot for me being comfortable with it!
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Blue Hell
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PostPosted: Sat Aug 02, 2008 10:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah good it works, the feedback resistors are a bit on the large side then probably .. or it was designed to have a load of course ... maybe power indication LEDs?
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fonik



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PostPosted: Sat Aug 02, 2008 3:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Ah good it works, the feedback resistors are a bit on the large side then probably .. or it was designed to have a load of course ... maybe power indication LEDs?

i provided the PSU with two power indication LEDs (just because i indeed thought it would be their purpose to draw current from the regulators). seemed to work for the positive rail, not for the negative rail, though.
what is the feedback resistor? the one between adjustment and output of the regulator?
anyways, the PSU is for the KLEE2 and there will be enough load...

thanks again.

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PostPosted: Sat Aug 02, 2008 4:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

I see no problem with leaving it as it is, maybe try a low efficiency LED with a lower series resistor when you want to go fancy.
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fonik



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PostPosted: Sun Aug 03, 2008 10:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

case closed. thank you, jan.
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