Power supply transformer is trying to sing...

[David]

Help (maybe!)

I'm in the process of building a power supply.  The first transformer I used failed utterly.  Turns out there's no output from the secondary.  Hooked temporary wires to xfmr #2.  Nice, steady AC output.  Hooked it up to my rectify/regulate board.  Started humming!  Took it off.  Hum went away.  Should a transformer hum when it's under load or did I screw something up in my typical, inimitable fashion?

Man, I hate this.  Every time I try something new...   :x  :x  :x  :?

[Aharon]

It shouldn't hum unless the laminates are loose or you're shorting the transformer(your first one failed right?).
What schem are you using?.
There's a couple at guitargadgets.com ready to use with PCBs I think.
Aharon

[Ansil]

check diodes to make sure they are not prematurely dying and shorting out is a good start.  what voltage is it putting out before and after rectification. is it close to the intended voltage.

[David]

I don't know about the laminates, but I wonder if it could be possible that the metal casing around the coils is vibrating.  Those pieces seem pretty loose and wiggle easily.  As far as shorts, I suppose that's possible.  HE(*, when I do something, ANYTHING's possible!!  Anyway, I guess I'll (re)build it on breadboard stage-by-stage to figure out what I did wrong.  I do know that there were no shorts in the diode bridge.  As far as anything else...  well, that's why we test, right?

The circuit I'm using is 1/4 of Scott Swartz's "Four Output Power Supply".  I made my own bridge from 4 1N4007s.  My transformer is a 24VDC 1A secondary. I used the outside wires because I was concerned that there wouldn't be enough voltage under load to feed both the diode bridge and the 9V 1A regulator.

[niftydog]

Those pieces seem pretty loose and wiggle easily

that's no good. You're talking about the thin metal plates that form the body (and core) of the tranny?

They should be tight. Chance you might be missing some of the laminations...

[David]

No.  The transformer itself has metal covers around the coils. These are what are loose.  I think the laminations, as you called them, are OK.  Good question, though.   This is getting me closer to the root cause.

[David]

But...

I rebuilt the power supply on breadboard, starting with just the transformer and diode bridge.  No noise, about 14-15 volts DC (just like R.G. predicted!)  I added the regulator back into my test circuit.  This is SUPPOSED, I say SUPPOSED to be the NTE equivalent for the 7809 1A positive regulator.  OK, fine.  That being the case, why the heck is it only putting out 7.27 volts?  Could I have screwed up something else (what else is new?), or does NTE mean (as I've always suspected) Not Totally Equivalent?

Seriously, though, does a 9V regulator really only put out 7.27 volts?  Would I be better off to just bite the bullet and order a 7809, or could I press an adjustable regulator (what is that - LM317?) into service and get an amp out of it?

[niftydog]

why the heck is it only putting out 7.27 volts?

This is with no load on the supply?

It's broken... or wired incorrectly.

You can use an LM317, requires slightly more work, but it isn't too hard.

bite the bullet and order a 7809

order? you nowhere near an electroincs shop? They're fairly common devices.

[David]

why the heck is it only putting out 7.27 volts?

This is with no load on the supply?

It's broken... or wired incorrectly.

You can use an LM317, requires slightly more work, but it isn't too hard.

bite the bullet and order a 7809

order? you nowhere near an electroincs shop? They're fairly common devices.

Load:  my DVM
I know the diode bridge is wired correctly because the output is polarized  exactly like Scott's schematic.  It's ALWAYS possible I wired the regulator in wrong.
I am indeed near an electronics shop.  Unfortunately, they're basically a NTE store.  They didn't stock and wouldn't order the 7809.  Yes, yes, I know, order from Steve.  Don't seem to have much of a choice this time.

[niftydog]

Unfortunately, they're basically a NTE store. They didn't stock and wouldn't order the 7809.

wow.  :shock:

the mind boggles... must be some great kick backs involved there.

[David]

wow.  :shock:

the mind boggles... must be some great kick backs involved there.

Great isn't the word I had in mind...  Mail order, here I come! :cry:  :cry:

[David]

...  I am an idiot!

I went back to the lousy NTE shills that supplied my regulator to gripe about not carrying 7809 chips.  In the process, I indicated that I had done my testing without the filter capacitors in the circuit.  The shill behind the counter then said that the regulator couldn't hit its full output voltage without the capacitors.  Well, this sounded like junk science to me, but I thought I'd give it a try.  D'OH!  Dang if he wasn't right!  I can't believe it -- rock-steady 8.97 volts!

I have just reinstituted Unbreakable Rule #2:  ALWAYS breadboard a circuit BEFORE you build it.

What is Unbreakable Rule #1, you ask?  Try to find an article related to what I want to do on GEO, read it carefully 10 times, memorize it, and then use the information to keep me from doing something stupid!