Power Supply problem

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

I've recently taken my DIY modular out of storage and installed two MFOS adjustable power supplies in it to replace the old 'Power One' style one I had in there.

I built both supplies, switched the modular on, and dialled in +/-15VDC on both PCBs no problem.

I have PSU 1 feeding Bus Board 1 with molex connectors and PSU 2 feeding Bus Board 2 with euro connectors. The bus boards' grounds are connected together.

When I plugged all the modules in -some on Bus Board 1 and some on Bus Board 2- I noticed the positive rail on Bus Board 1 dropped slowly until it hit about +5VDC.

At first I suspected thermal shut down, but all the regulators are heatsinked, and when I felt them, they were running cool. I didn't have this problem when nothing was plugged into them. I also noticed that I'm only measuring about 7VAC on the secondaries of the transformers.

I'm wondering if this is a current problem.

Each PSU PCB can provide a maximum of 1.5A.

Each PSU PCB is fed by a 80VA 115/ 230 > 2x15V transformer. The colour codes of the wires are as follows:

Primary
Blue - 0v
Grey - 115v
Violet - 0v
Brown - 115v

Secondary
Orange - 0v
Yellow - 15v
Black - 0v
Red - 15v

Maybe I have the wiring of the transformers mixed up. This is how I currently have them:

Primary
Grey + Violet tied together
Blue goes to my fuse
Brown goes to the neutral of my IEC socket

As I understand it, this wires my transformer up for 230VAC operation.

Secondary
Yellow + Black tied together and go to the CT pad on the PSU PCB
Orange goes to AC input pad on PSU PCB
Red goes to AC input pad on PSU PCB

Have I got this wired up correctly? Is this the reason I'm only getting 7VAC at the input of the PSU PCB? And because of this the regulator is struggling to make it up to +15VDC and shuts down?

Any help would be much appreciated!

Broadwave · Posted: Mon Mar 11, 2013 12:18 am Post subject:

@Letterbeacon

I have exactly the same setup, but it's working fine. I've only just woken up, but I'll take a look inside my synth and check everything once I've shaken myself.

But from what you've described, all the wiring seems ok.

EDIT - I've had a cup of tea and my mind's clear now Very Happy

I have the primary blue going to neutral and the brown to the fuse (but I don't think that will make much difference)

But... you've used a 15-0-15 transformer, and that may be the problem. I've got 18-0-18 which allows for the 3 volt drop that the regulators require to function properly.

Can anyone else chip in here?

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LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Hi Andy -thanks for the reply!

The reason I used 15v transformers is because they're rated for 2.67A. Doesn't that mean that if I'm pulling 2.67A the transformer output will be 15v? As I'll only be pulling 1.5A maximum, the transformer output will be higher.

Or have I got that totally wrong?

Are yours 80VA? How much AC do you measure at the input of the PSU boards?

Broadwave · Posted: Mon Mar 11, 2013 3:25 am Post subject:

Unfortunately, even if your transformer is capable of supplying 1.5A per rail, in reality you'll never be able to get that... The more current you draw from your regulator, the hotter it will get, eventually leading to thermal shut down.

As a guideline I only ever work to half the current required i.e for a 1.5A rail, I assume that I'm going to be able to safely draw around 700ma MAX before things get unreliable (voltage drops/overheating).

The regulators drop 2/3v for them to work efficiently. In other words, your 15-0-15 transformer effectively only provides 12/13v per rail (I'm simplifying the wording here, but I'm sure you get my drift). So trying to keep a steady +/-15v isn't going to work... which is why you really need an 18-0-18 transformer.

As an experiment, you could try adjusting your PSU output to +/-12v, then plug in a few modules to see if the voltage is stable - I suspect it will be.

Still no body else to chip in?

PHOBoS · Posted: Mon Mar 11, 2013 5:20 am Post subject:

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Thanks for the replies!

Yes, it starts to lower as I start adding modules. It's only one PSU PCB that does that though, strangely.

Looks like I'll be placing an order for two more transformers then!

The Bad Producer · Joined: Mar 08, 2009 Posts: 282 Location: The Manhole

I don't have anything to add to the good advice from AndyR1960 and PHOBoS, but did stumble across this thread last night, which is fantasticly interesting for PSU knowledge...

http://electro-music.com/forum/topic-51694.html

Full props to Osal for that!

Skrog Productions · Posted: Mon Mar 11, 2013 12:06 pm Post subject:

Hullo folks

In my experience the toroidal transformers pump out a little more than the marked output voltage , i used to run a 120Va toroidal 18 / 0 - 18 / 0 for my big blue coloured system, loading is 2amps per rail.
I was getting about 20V at the secondaries , my system is 95 % 12V modules and the resultant 8V drop over the 12V regs dissipated as heat , searing heat , i could have had a rotisserie module over the regs (mmmm roast chicken) hehehe , I now use a 15-0 / 15-0 Toroidal for my mostly 12V system and, have made 6 distro boards with regulators per distro with large cross sectional area cableing from bridges to each distro board everything runs sensibly now Very Happy

.

Your modular problem .... hmmm , try un-tying the 0V link that you say you've coupled between the two seperate distro boards , see if they behave ok ..... were they linked right at the capacitor 0V points on each power supply board ? or at the end of a chain of sockets for modules (distro boards)

Dave.

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Hi Dave,

That's interesting to know.

The grounds are tied together at the distro boards, a little way away from the filter caps. I'll try untying them and see what happens.

Skrog Productions · Posted: Mon Mar 11, 2013 12:20 pm Post subject:

i aslo had to tie my 0V connection at my capps to mains earth , i found a slight potential measured on my power supply between 0V & earth.
For me , this reduced a bad hum i was getting before.

Dave.

Skrog Productions · Posted: Mon Mar 11, 2013 12:34 pm Post subject:

i'd love to see a nice pvc shroud over the entire IEC socket rear & a wee cover over the 15A connector strip , safety first Smile

Dave.

diablojoy · Posted: Mon Mar 11, 2013 6:46 pm Post subject:

A long shot but Its possible you have the phase incorrect on your windings
if you have an old analogue meter its easy to check
you will see a very slight positive or negative kick of the meter on connecting the meter probes this gives you an indication of the winding direction
should be in series

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here

Broadwave · Posted: Tue Mar 12, 2013 3:41 am Post subject:

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Thanks very much for the info Osal. My 80VA 36VCT transformers arrived today so I'll have a go at hooking them up this evening.

capicoso · Joined: Nov 19, 2012 Posts: 128 Location: Argentina

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Sorry, I've only just seen your reply.

I bought a pair of 0-18v 0-18v transformers and put them in, but when I was installing them, I noticed the ground from one of the PSU PCBs had come loose from the screw terminal, which is I think was causing the problem.

Now I've got the new transformers in, everything works great!

Thanks again for your help.

Osal · Joined: Aug 16, 2011 Posts: 147 Location: Here