Alesis-style bipolar power supply?

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rascalseven

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I drew this based on the power supply in my modified Alesis 3630 compressor. 

Alesis-typebipolarsupply.png


[EDIT - I actually used 10uF electros instead of the 50uF marked at the reg outputs]

This is the power supply design after implementing the 3630 mod by he-who-no-longer-wishes-to-be-named-on-this-forum. 

I have a bunch of surplus power transformers with 14vac secondaries that I thought this might be a good design for (to make bipolar 15 and 18vdc from a single 14vac secondary).  The transformers that would be good for this design, so I thought I'd give it a whirl.  I have questions:

When I measure the voltage at pin 2 of the 7915 I get around -47vdc.  At pin 1 of the 7815 I measure +46vdc.  Wow.... that's a lot more voltage than I would normally expect at the front of these regulators.

Is this normal?  When I measure the output of the 7915 I'm getting almost -18vdc, not -15.  The + is fine at +15, however.  I've double, triple, and quadruple checked my wiring, and was just wondering if the regulators are gonna fail or what.  I'll stick another 7915 in to see if I got a mislabeled 7918 by any chance, but was really curious about the high voltage feeding these regulators, and specifically, if its a problem.

Can anyone shed light on this design?  I remember Keith saying that this is not an ideal psu design, but I can't find his post and don't remember his reasoning.  I just wanted to use my surplus transformers to make some bipolar psu's and thought this might be worth trying.

On another note, my transformers can be wired 28vac w/CT.  Do you think it'd be okay to use a more typical design with 15v regs?  That is do you think a 14vac transformer would provide enough rectified voltage to keep 15v regulators happy?

Thoughts??

Thanks,

JC

 
Each half of this is working as a voltage doubler.  Except for diode forward drops you would expect to charge the bulk caps up to the peak voltage, which for a real sinusoidal input will be square root of two times the AC rms voltage.  At low loading the transformer will be putting out a good deal more than with the rated load, depending on details of construction.

So the 40-something voltages are not indicative of malfunction or miswiring.  However, they are mighty high for 7815/7915 three-terminal regulators.

A full wave center-tapped bridge with the 14V windings in series would be pretty reasonable given what appears to be a fairly poor-regulation transformer, as long as the loading is fairly light relative to the transformer rating and as long as you are not anticipating very low-line conditions.  What is the current rating for the transformers?
 
Thanks!

The transformers are Stancor model SW-528.  Each 14vac secondary is @ 0.420A

The power supply will be for simple, stereo gain make-up amps using a 5534 for each channel, so there will not be much current demand.  I got these for $3 each at the local surplus store, so I'm looking for an appropriate +/-15vdc design to use them on.

JC
 
Hmm.  I did a quick sim and I see around 26V into the regulators, as to be expected with the voltage doubler(minus all of the losses and such).

On the other hand, most transformers need to see a specified load in order to meet output specs.  I suspect that this one needs more load than you are giving it and it's outputs are much higher because of that.  You might try adding some load before and/or after the regulators to see if you can get the tranny under control.

EDIT: You are testing this with a circuit hooked up right? You aren't looking at the voltages with open outputs right?
 
The power supply isn't hooked to anything, no.  I didn't want to hook it to my circuit with it producing +15/-18vdc.  So I began searching the supply for problems, and since I'm new to this design I was shocked by the high-voltages feeding the regulators.

Should I go ahead an connect it to a circuit to see what happens?  I honestly don't know what would happen with unequal voltages feeding the opamps, and just didn't want to damage anything if someone more knowledgeable could point out any problems with the psu first.

Thanks.

JC
 
The opamps won't care, that's like .6db of offset.  At full gain you'll clip one side a little sooner than the other.  I wouldn't hook it to the opamps just yet, use something like 5k from each rail to ground before the regulators and see what happens to the voltages.  Change the value of resistor up/down and see what happens.  Sim says that 1k might dissipate a little less than 1W but 5k is like 1/8th watt.

Other than that,  I can't see what else would be giving you such high voltages other than the lack of a load or mis-wiring.
 
If its a wallwart you're using here its always wayy more that its spec'd..
9V often puts down 16-18V :eek:
 
Yes, generally they can as long as your load isn't too high.  Pretty much the inverse of what I was saying about your current arrangement.


I've built a number of 48V supplies out of 22-0-22 transformers by over spec-ing the transformer and just trimmed the load to meet my voltages.
 
but of course.  I don't know why I was thinking that current was measured in RMS while we were measuring peak voltages.  ???
 
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