transformers - really dumb newbie question

[Mreff555]

Ok, so I know this is a really dumb question. Probably too dumb to post here, but work with me, I'm a bit frustrated.
I've worked with plenty of high voltage and plenty of low voltage circuits, but this is my first attempt with both.

I'm trying to build a regulated power supply, and I've never messed with a transformer before. It's a RS P/N 273-1512b

I followed the directions, on the High side I have it connected to a power cord with an inline 3V fuse. On the low side there are three wires, In the directions there are two arrangements for 12V and 24V

I was a bit intimidated by the transformer since I've never really played with one so I wanted to test it before hooking it up to my circuit.

When I give it power everything seems fine at first, the fuse doesn't blow. The 12V configuration only gives me 9V and the 24V configuration gives me 21V which both seem a little low. But the real problem is that the transformer appears to be overheating. after about 15 second it starts to smoke.

I don't understand. Everything appears to be hooked up right. Also, it doesn't pull enough power to blow the fuse. What am I doing wrong?

 

[srizbf]

in addition , if you can post a circuit of how you have connected or a photo of the setup ,
it will be easier to find the source of the problem.

 

[BradtheRad]

It's a Radio Shack 25.2 VAC, 2A, with center tap.

The 3 secondary wires can be described as a 12.6-0-12.6 arrangement.

Did you hook up a voltmeter alone (nothing else) to the transformer secondary? And got smoke, heat, under-spec voltage? This is a bad sign and the transformer may be defective.

Did you have the meter on a Volt setting, while connecting the leads to jacks that insert a low resistance (say the 10A setting)? You may or may not get a sensible reading.

Or are you making dual supplies from one transformer? With bridge rectifiers, regulator IC's, etc.? If so then it's all too easy to create inadvertent short circuits through the diodes. Such a schematic might look good on paper, but there are one or two 'gotchas' lurking.

Normally your output voltage should rise to 1.414 times the nominal ACV (minus diode drops). Therefore with no load you should get a reading of 34-35 VDC.

If you put a fuse in the primary (mains) side, it should have an amp rating based on the amount coming in at that side. Since the transformer is rated for 50W, you don't want much more than 416 mA at 120VAC going into the transformer. So the fuse should be 1/2A if your mains is 120VAC. If your mains is 240VAC then the fuse should be 1/4A. Slo-blo fuses are good in this case.

 

[pico]

I think the transformer has shorted-turn in the high side.

 

[Mreff555]

Hey guys,

Thanks for the great advice.

I did have the transformer attached to circuit of diodes which was open after the diodes. I'm pretty sure they were wired correctly.

So I before I try to exchange this transformer let me get one thing straight. I had never seen this 12.6-0-12.6 arrangement.

I want an input of 12V and an output of 12V, I instructions say 1 yellow + 1 black is =12.5V, 1 yellow + 1 yellow =25V.

Does this mean for the configuration I'm looking for, I wouldn't use the second yellow wire, or does it mean that I am supposed to short one of the yellow's with the black?

I know this is a pretty dumb question. I'm just a little paranoid about this part.

 

[srizbf]

it is a centre tap trfr.
yellow-black-yellow with two diodes for rectification.

may be you would have reversed the diode connection-a rare chance.

better attach a diagram or a photo of what you have connected.

 

[sheldonstv]

Heres a few circuits you may find useful......you have a 12-0-12v ac transformer.....you can either not use your center tap(black wire) and you should have 24v ac across the two other leads(yellow),between any single yellow and your center tap you will have 12v ac(use your meter to check).you can check the transformers ok with no load connected with your meter set to measure ac. has your transformer got a single primary winding or is it a tapped primary??

 

[Mreff555]

Thanks again. I think I had definitely hooked up the secondary side wrong.

I tried powering up the primary side with all of the wires open I wanted to measure the voltage bridging the two yellow wires as well as the center tapped voltage.
It doesn't hot anymore. at least not within the first minute or so, but it hums much louder than I would expect a 50W transformer to
Also the voltages are very low.



Am I correct in assuming I would still want to set my Multimeter to the VAC setting to check the secondary side?

 

[srizbf]

yes. set multimeter to ac when you are measuring sy of trfr.

set to dc when you measure after diode and across capacitor.

 

[Mreff555]

So that brings about another question. Many of the commercially available power supplies will have a common ground, +5VDC, +25VDC ports, or something like that. Is this because both circuits are used one using the center tap and one not using the center tap?