Taming a power supply

[sr1200]

I inherited a power supply with a transformer 115v primaries and 15v secondaries, going into a couple of 2200uf caps and a rectifier.  Voltage regulators are 7918a and 7818a. Im getting 18v (positive and negative) which is about 3 volts higher than I'd like.  Would i be able to place a trim pot before the voltage regulators to bring that voltage down, or is there a "correct" way to go about it?

 

[joaquins]

7915 and 7815 are what you want... if you want 15V.

The other way is to use LM317 and LM337 so you could trim it as you want...

7X18 are made to get ±18V, with a trim you could get higher, to get lower in a ± supply could be made but not recomended, go for LM3X7 and you will get better results or just 7X15 and nothing else to change.

JS

 

[jwhmca]

It would be nice to see the supply / schematic , but you should be able to do a direct replacement with 7815 , 7915

 

[sr1200]

I have no schematic or anything on this, i inherited it from one of the board members when I bought a kit they couldn't get to work... maybe it didn't work b/c the max voltage rating for the IC's are 15v..  chips were getting crazy hot. might have to replace em.  But i will def put in the 15's thanks for the heads up on that, didn't put 2 and 2 together on that.

 

[jwhmca]

Which "chips" were getting hot.... I can think of WAY more "chips" that would be totally fine on 18v instead of 15v than those which wouldn't...

Maybe there was something else wrong... or wired backwards

 

[sr1200]

5532N's (for the Neumann W492 eq) circuit calls for +/- 15v

 

[jwhmca]

Yes, bu they can take up to a 22 volt supply per their spec sheet.

 

[sr1200]

then something else in the circuit doesn't like more than 15v.  When i was putting together the 2 boards the guy didn't finish i tested with a +-15v test supply and everything was fine.  Pop the 18 and all hell breaks loose. lol

 

[mjrippe]

Then I would check that PSU before even putting in the 7815/7915.  It could be putting out AC as well as DC or have some other issue that is causing hell to break loose!

 

[sr1200]

How can i go about testing for AC?  (scope?)

 

[jwhmca]

Meter on AC

 

[sr1200]

so i would check between the + and - of the 2 VR's im guessing?

 

[mjrippe]

Set your multimeter to AC Volts, put the neutral/common/black probe on a good ground point, put the positive/red probe on V+ or V- and see if any AC voltage is present.

 

[sr1200]

OK, yeah that was confusing, usually i check AC across 2 point of an AC line (like if there are 2 secondaries off a power transformer)