wall wart PSU replacement question/problem

[Rob Flinn]

Hi

10 years ago I made a box that was intended to replace 3 identical wall warts that supllied 3 identical Alesis graphics in a PA system. The box had 1 large overated transformer that fed via individual fuses each graphic, & also lit 3 Led`s that were in the circuit so you could tell if a fuse had blown. The wall warts are AC so no rectification was required.

Problem:

If more than 1 graphic is plugged in the fuses blow in the SPU box apart from on 1 of the graphics. This is without consitency on which fuse blew.
Would the phasing of the wiring going to each graphic cause this sort of problem? If one box was wired 180degrees out phase for example? Or is it bad practise to supply more than one rectifier from a single transformer?

 

[Rob Flinn]

Hi Butta, thanks for the reply.

The fuses were after the secondary one for each graphic. They don`t need to be slow blows because the transformer is not torroidal.

also, the connections are crucial-- make sure that the units are all wired in parallel. for two units, just connect all of the grounds, then one AC hot to one unit and the other hot to the other unit.

I`m not quite sure I understand this, let me try & verify what your trying to say.
The graphics run off AC 15v. So I have a mains transformer that has some thing like a 15v secondary. So if we say that the secondary has a live and neutral (just to give them a polarity). Is what you`re saying that the polarity must be observed for each rectifier, in each graphic. Surely a bridge rectifier is the same circuit whatever polarity you hook up the AC ???

 

[Rob Flinn]

rf

Thanks for that. I`ll have to open up one of the Alesis to see if the rectifier is identifyable.

 

[dayvel]

I'm pretty sure Alesis gear uses a rectifier circuit like this-

to provide bi-polar voltages without a center-tapped trannie. So you need to keep track of the ground sides of your supplies.

 

[Jonathan Hayward]

If the 3630 is anything to go by ( it was using the 9VAC input to get plus and minus 15V - was the first thing in circuit a capacitor? I can't remember) there will be some strange, low cost and rather tenuous arrangement for the PSU. I would definitely look under the hood of each one.
As you're using a decent TX, why not just turn your box into a pro standard regulated supply, use the existing leads for plus and minus, and bolt on some extra grounding (which it probably needs anyway).

 

[Jonathan Hayward]

just seen dayvel's post, that's probably it.

 

[Rob Flinn]

Thanks for all this information guys. When I get a chance to check the thing over the first will be to make sure the polarity is observed for each unit. I suspect this is the problem.