Problems with Radio Shack Transformers...

[FenderBender]

I'm doing a project for school, comparing switch mode and linear power supplies. It's due next Friday. Today is Saturday.

I decided it would be a good idea to try to get some work done on this, so I put together the high voltage stuff. Some of my most high quality wiring I ever did since it's going on camera. Practically intrinsically safe. Safety goes first especially when you are showing other people.

So I flip the switch and I'm feeding mains into my 12.6V CT "3A" Radio Shack transformer. I hook up my DMM. Voltage is present. Throw it up on the scope. Looks fine.

Not more than 5 minutes later of only being hooked up to a snubbed bridge rectifier and 2 filter caps...but no load I see a tiny blue puff of smoke. (It took me a while to figure out that it was the transformer, since I only saw it out of the corner of my eye).

But before I realized it was the transformer,  I'm trying to find what I did wrong. Bridge circuit is fine. No shorts, bridges. Fine. All dandy.

I remove the rectifier circuit and hook up my DMM. Like 0.03VAC. Supposed to be around 13-14VAC without a load. Just for sanity, I hook up the scope. Ziltch. Pretty much a flat line.

So I narrow it down the the transformer...and then when I think about it, I remember, blue smoke. Now it all makes sense! I wasn't soldering anything at the time I saw that.

Usually if something dies, I don't mind all that much, however...

1.) I used this thing for literally 5 minutes before it died. 5 minutes total life time. And it was never loaded shorted etc etc

2.) This is a project on a deadline and it dies!

3.) I wasted like $15 on this thing and now I have to spend atleast $20-25+ maybe $10 shipping more to buy a new one AND wait for shipping. Grrrr...I'm a student. I don't have an endless supply of money. Just ticks me off.

Funny part is: If you check the reviews on RadioShack.com, about half the reviews say exactly my case: "Dies within [insert time period <1hour here]", or "Blue Smoke and dead!"

Why would they insist on even selling something like this? With their name on it? Every time I try to support RadioShack, I swear I get f**ked over. Enough.

Well that's my rant!

[IanB]

Ah, that sucks. I trust you will be able to return it to the shop for a refund as defective.

If there happens to be Fry's near you they have a line of small transformers from Philmore that seem to have decent construction at similar or lower prices. It would save you the wait for shipping. I've tried two of those now and they have both behaved properly.

[TerminalJack505]

I'm like you--I like to support my local Radio Shacks when I can. 

The quality of some of their stuff is questionable sometimes, however.  Hopefully you can get a refund or replacement.

[Rerouter]

certain it wasnt something like a high frequency tranformer, if it was made for a higher frequency, it would saturate and heat up enough to smoke,

[Mechatrommer]

now... is the transformer still with you? can you check the resistance of primary and secondary winding using your DMM and report here. did you connect secondary to the mains? are you sure?

[amspire]

It is possible the transformer had a shorted turn, but it would have to be in the primary and if it was, I am amazed it lasted a whole 5 minutes.

A shorted turn in the secondary would not allow anything that looks like a normal voltage to appear on the windings, and I think the transformer would be buzzing like crazy.

Check to see if a winding has failed.

Otherwise even if it is a badly designed transformer, I cannot see it burning out in 5 minute. I think there is something about your load. Make sure you add a fuse between the transformer and the rectifiers next time.

You mentioned a snubbing circuit on the bridge rectifier. Not usually needed for a low power rectifier. Are you sure that the snubbing circuit was not incorrectly wired? Snubbing a rectifier is only needed the transformer output has a signifigant amount of leakage inductance, and I just don't think it is a problem for your 3A transformer. What parts make up the snubber?

It does not add up to me. I do not know what, but there is something about that rectifier circuit that is wrong, I think.

Richard.

[SeanB]

Why buy a transformer when you can get a pre tested one from almost any thrown out Ewaste? Those are very likely to be still working, the equipment they are in having died for some other reason.

[FenderBender]

It is possible the transformer had a shorted turn, but it would have to be in the primary and if it was, I am amazed it lasted a whole 5 minutes.

A shorted turn in the secondary would not allow anything that looks like a normal voltage to appear on the windings, and I think the transformer would be buzzing like crazy.

Check to see if a winding has failed.

Otherwise even if it is a badly designed transformer, I cannot see it burning out in 5 minute. I think there is something about your load. Make sure you add a fuse between the transformer and the rectifiers next time.

You mentioned a snubbing circuit on the bridge rectifier. Not usually needed for a low power rectifier. Are you sure that the snubbing circuit was not incorrectly wired? Snubbing a rectifier is only needed the transformer output has a signifigant amount of leakage inductance, and I just don't think it is a problem for your 3A transformer. What parts make up the snubber?

It does not add up to me. I do not know what, but there is something about that rectifier circuit that is wrong, I think.

Richard.

The snubbers were 100pF caps across each diode. The diodes were Schottkys rated at 60V PIV and 10AIf. Maybe that's the problem?

And when i say 5 minutes, it was actually probably less. I just can't remember exactly.

[amspire]

The snubbers were 100pF caps across each diode. The diodes were Schottkys rated at 60V PIV and 10AIf. Maybe that's the problem?

And when i say 5 minutes, it was actually probably less. I just can't remember exactly.

100pF will not cause a problem. When you get the new transformer, start by connecting the rectifier circuit to the secondary via a resistor - like 100 ohms. It should fully charge the capacitor. If there is a problem in the rectifier circuit, you will blow a resistor and not a transformer.

Richard

[grenert]

What kind of readings did you get before it died for:
Secondary ACV?
Rectified and filtered DCV?

What were the capacitance and voltage rating of your filter caps? 

I agree with the amspire's earlier suggestion to check the windings.

[sonicj]

take that chit back! the advantage of shopping at radshack is that you can walk in and choke someone if you need to.
-sj

[IanB]

If we give FenderBender the benefit of the doubt and assume he did everything right, then my guess would be on the primary or secondary winding termination. Each end of the winding is connected to a flying tail and then secured inside the bobbin. If there is anything poor about that electrical connection inside the bobbin it could go high resistance or open circuit.

[FenderBender]

Amspire, et al:

I think you are right.

It must be my rectifier or something....Though I'm 99.99% confident that it should work. After all, it's just a bridge configuration with 2 caps on it...odd.

I wasn't home all day, decided to play around before I go out to watch the big game. Checked the primary and secondary. Both seem in good shape. About 0.4? on the secondary and 14? on the primary. No overload or very high resistances. And the blue smoke perhaps was just my imagination??? I don't know.

I do have a 13.5VAC 2A transformer that I scrounged from a old speak system if this does really fail me.

Would a problem be caused by Schottkys instead of normal rectifiers?

Decided to change the thread title to something less harsh since it MAY be user fault.

 

[IanB]

Shouldn't be a problem with the schottkys if you are wiring things up correctly.

I've been doing the same experiments a lot lately: connecting transformers through rectifiers to filter caps and adding a load. I've done it both with standard diodes and schottky diodes. Nothing funny has happened, though I have been very careful about my wiring and making sure I have no mis-connections or accidental shorts.

Maybe you should retrace your steps and build up the circuit step by step. First just power up the transformer and measure the secondary voltage with the AC range on your meter. Then add a suitable dummy load like a bulb and see how much the AC voltage decreases. Then insert the rectifier, put a small load on the DC output and measure the DC voltage without filter caps. Then add filter caps, ensuring the correct polarity, and measure again.

[FenderBender]

Right right. Of course I have to root for my Giants now, but I'll totally get to it. Thank you for your help.

My only other suspicion is my soldering. I'm doing this on perfboard Instead of running wires, for some of it, I just made bridges between the pads, but I've done that so many other times...

Grr..Maybe I Was just having a bad day.

[Mechatrommer]

Checked the primary and secondary. Both seem in good shape. About 0.4? on the secondary and 14? on the primary.

something wrong with ohm reading on primary, i dont know your transformer but from my experience it should be in range of hundreds, not 14 ohm, maybe the primary insulation already tossed, and this is a sign of bad transformer if its just few hours of purchase.

[FenderBender]

Alright let's go check it again...

However, I did some more tests today, and all looked fine.