Need to find a 120v to 12VAC 250MA transformer

Thread Starter

gregsenne

Joined Jan 27, 2021
5
I need to replace a transformer that is soldered to a circuit board and I've tried everywhere, I thought it would be easier to find a 250ma 12vac transformer.
On my current transformer, the primary is open.

It says on it INPUT 115V 60Hz
Output 12.4V 250mA
E219749
Also on the top of the transformer stamped into the plastic it says RS 35X15M1

Thanks for any help
 

Attachments

Thread Starter

gregsenne

Joined Jan 27, 2021
5
Does it have a thermal fuse inside.
I just undo the core and replace the fuse
I pulled back the tape around the winding, because it was burnt, and found this square thing. Is this the thermal fuse?
I ohmed it out and got nothing.
But I also ohmed it between the other terminals (like bypassing the fuse) and was still open. If I ohm it between that far left peg and either side of that black square thing (assuming it's the fuse), shouldn't I get something?

transformer1.jpgtransformer2.jpg
 

Thread Starter

gregsenne

Joined Jan 27, 2021
5
If it helps, the transformer went bad because it was shorted out on the secondary side. Secondary side still Ohm's out, but the primary does not.

I looked at those links you guys sent me, and that is very helpful! I see transformers that look very similar. But I'm still confused on exactly what to order. The only info I have to go off of is 115V -> 12.4V 250mA.
The website is asking for VA, and also parallel and series.
Like this:
1611844491677.png
 

bertus

Joined Apr 5, 2008
21,364
Hello,

Your transformer is just over 4 VA (12.4 X 0.25).
The 16XE12 will be to weak (12.6 C.T.).
The 16XF12 will work fine (12.6 C.T.).

Bertus
 

sagor

Joined Mar 10, 2019
439
I pulled back the tape around the winding, because it was burnt, and found this square thing. Is this the thermal fuse?
I ohmed it out and got nothing.
But I also ohmed it between the other terminals (like bypassing the fuse) and was still open. If I ohm it between that far left peg and either side of that black square thing (assuming it's the fuse), shouldn't I get something?

View attachment 228942View attachment 228943
Yes, that is a thermal fuse. I had one go out on a controller board this year. There were no signs of heat, tape was still soft and flexible. Sticky stuff was still sticky on the tape. My fuse was rated at 2A, 130C. 130C is very high a temperature rating. But, there were no signs of overheating at all. Seems these fail randomly at times, possible due to some stress. Mine lasted 15 years before "popping".
They are available, if you wish to replace, at places like Mouser or Digikey.
However, in my case, I bridged that device (removed and jumpered) and installed a smaller 1A fuse externally as a safety measure. My transformer is only around 24VA, and at 240V source, it would normally draw only about 100mA. Controller board has been running ok now for a few months with no issues.
 

Thread Starter

gregsenne

Joined Jan 27, 2021
5
So if it is just a fuse, shouldn't I get continuity between the left post and ONE of the two sides of the fuse? I've probed between each side of the fuse and EVERY terminal on the transformer and can't get continuity.
FWIW I also don't get continuity when checking the fuse
 

sagor

Joined Mar 10, 2019
439
Hm, it may be possible that something is blown on the primary side, though one would think the internal fuse would have protected it. Only exception I can think of would be a nearby lightning hit, where there was a large surge through the transformer, blowing the fuse but also arcing through the primary and blowing something.
Some of the wires on the primary are very fine. Make sure none is broken where it is soldered to a lug/leg. When removing the blown thermal fuse, it is easy to break that wire.
 

Thread Starter

gregsenne

Joined Jan 27, 2021
5
If one of the wires in the winding is broken, would that cause it? Looks like there are a few wires right around there the fuse was that might have become broken. Not the one that goes to the lug, just one on the outside of the coil

The transformer was fried because the 12v stuff got wet and shorted
 
Top