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Finding replacement for old pcb transformer

Thread starter #1
Hi guys, I'm trying to find the possible specification of this burnt out transformer on a gate controller board. Its 240V step down with output of 24VAC and 12VDC.
The back has 2 input pin and 4 output pins....please help!. Diagram attached shows the pin dimensions of the transformer. Thanks if anyone can help!
 

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Diver300

Well-Known Member
#2
You could see if something like this- https://uk.farnell.com/myrra/44440/transformer-22va-230v-2-x-12v/dp/1214619
will work, probably with a couple of new holes in the board,

Alternatively, mount a transformer separately and run wires. It's possible that you don't have to have the mains on the board if you have a separate transformer.

You could even have a completely separate DC power supply, which is how most modern equipment is made.
 
Thread starter #3
Thank you for your reply!
If I was to use the isolation transformer that you mentioned, would you wire it the same way as the
burnt out one? I'm assuming the existing bridge rectifier (RS204) is there to convert the rest to 12Vdc?
 

Diver300

Well-Known Member
#4
The existing transformer is an isolation transformer.

When you have a 12 Vdc supply from a transformer using full-wave rectification, that can be done in two ways. Have a look at

https://engineeringtutorial.com/comparison-of-half-wave-rectifiers-and-full-wave-rectifiers/

The point is that for 12 V you could use a transformer with two 6 V outputs in series, or it could be a single 12 V winding, or two 12 V outputs in parallel, or two 12 V outputs in centre-tap configuration. It would be a good idea to work out which is used in your board.

The 7805 IC in your picture is a 5 V regulator. I2 V is a generous voltage to run that from, but not unheard of. Circuits of that age often have several different supply voltages from one transformer, and some will also use the ac voltage for timing or other reasons.

You need to check how the circuit is connected.

A photo of the bottom of the board would help.
 

Diver300

Well-Known Member
#6
Thanks for that. What is the rating on the electrolytic capacitor (large component in the silver can, with blue sleeve with while band down it). Also what is the component that is behind the 7805? Can you see any voltage ratings on the relays.

Your transformer is wired as a full bridge. There is also connection to the mid point that is also used.
 
Thread starter #7
Thank you for going through this. From what I can see, the electrolytic capacitor says K9A on one side, and 35V, 2200uF, 85 degree C (M) on the other.
The component behind 7805 is 7812. Unfortunately, I cant see any voltage ratings on the relays.
 

Diver300

Well-Known Member
#8
I've just realised that only one winding of the transformer is connected to the rectifier, which does make sense for supplying the 7812.

As 24 Vac is used, you need a real transformer, not a separate power supply. However, you could mount the transformer separately and run wires.

The Myrra transformer is close to what you want. It is marginally larger, so you need to check clearances. The inner pins one the secondary are further apart, so you would need to drill two small holes in the board and wire to those once the transformer was fitted.
 

Diver300

Well-Known Member
#10
The only indication that I have about the current taken is the size of the transformer. The power rating of conventional transformers like the one in your first picture are basically a function of their size. The transformer looks as though the secondary has overheated. If that was a particular event that won't be repeated, that is less worrying, but I would not go down in size as that will be rated at a lower power. The encapsulated transformers are easier to find in PCB mount form, but may have a lower rating as they can't get rid of heat so well. If you are mounting elsewhere, you would be better with a chassis mount transformer. You could also look at torroidal transformers such as

https://au.element14.com/vigortronix/vtx-146-015-212/15va-toroidal-transformer-2x12v/dp/2817637
or
https://au.element14.com/vigortronix/vtx-146-030-212/30va-toroidal-transformer-2x12v/dp/2817644

I suggest using the larger one if you can fit it in.

Both of those have split primaries. You must join grey to violet, and insulate that connection from everything else, as it will be at mains voltage. Blue and brown connect to where the existing transformer input connections are.

You connect yellow and black to the two middle connectors on existing transformer output, and orange and red to the two outer connectors.
 

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