Have I identified this transformer correctly?

digitalhuxley said:

I'm trying to replace this old transformer.

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Why?
What do you think is wrong with it?

digitalhuxley said:

I feel like I can do 240V input to the replacement, and get 25V output, but do you think that is correct?

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Yes.
The replacement will do 24v, the original was rated at 25v, but at a quick guess 1 volt will not cause the end of the world.

One thing to check, are they of comparable sizes?
ie is the physical size of the proposed replacement similar to the existing transformer.

JimB

The output will be very close to 25 volts. The RS transformer has the primary voltage marked as 230 volts so with 240 volts on the primary the output from the 24 volt winding will be 24 x 240/230 = 25.04 volts. I agree with Max that the 1 volt difference is unlikely to make any difference. You do not tell us what it is supplying so this assumption is only a guess.

Les.

Hi! Thank you so much JimB and Les Jones. Really appreciate your help!

This is supplying a gas valve in an ancient 1970s boiler. A boiler technician visited, and with a multimeter said that the transformer was a failing part, but this wasn't the sort of repair he'd do. Another technician has said if we can source the transformer, he will fit it. Unfortunately the boiler, an Ideal Standard E Type CF 80/100 conventional flue gas boiler, is so old that not many people will work on it. It will be replaced, but it is inconvenient to do so right now, so the hope is with a replacement of the transformer, we can delay having to replace it.

The transformer itself is 6x5x2cm. The boiler itself takes 240v with a maximum 3A fuse.

It looks like the replacement I've picked is 8x6x6cm so it is possible the transformer could take quite a bit more total wattage than the one it is replacing.

I can see another replacement https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504662

This seems to be approx 6x5x5cm so is perhaps a closer candidate, but looks like it handles 20VA instead of 50VA. Naively multiplying 25x1 (voltage x amperes) to get power puts me at 25VA if I'm understanding correctly, so perhaps the smaller transformer wouldn't be rated correctly, even if both replacements are considerably bigger than the transformer they would replace.

What I do have is a wiring diagram that came with the boiler, here's the image:

digitalhuxley said:

This is supplying a gas valve in an ancient 1970s boiler. A boiler technician visited, and with a multimeter said that the transformer was a failing part, but this wasn't the sort of repair he'd do.

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It would be interesting to know the details of the 'Failing part" ?
Typically transformers burn out, shorted turn etc, those that go open is often due to a thermal fuse buried in the winding.
There is no outward sign on the present one.
Generally the load is not all that high for those applications.

OK, I think that your smaller replacement transformer would be an adequate replacement.

As for 20VA vs 25VA, yes your thinking is a little naive, just because a fuse is rated at 1amp, that does not mean that it passes 1 amp in normal day to day operation.

The transformer being condemned as faulty by the boiler man surprises me.
Transformers are usually very reliable, there is not much in them to go wrong.
Usually, but not always, failed transformers are burned, black, crispy and smelly.

But, for the expenditure of a few pounds, try it.
It cannot make anything worse.

JimB

MaxHeadRoom78 said:

It would be interesting to know the details of the 'Failing part" ?
Typically transformers burn out, shorted turn etc, those that go open is often due to a thermal fuse buried in the winding.
There is no outward sign on the present one.
Generally the load is not all that high for those applications.

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Hi! Unfortunately, I wasn't there, but I was told they checked inside the control box and concluded that the transformer was the problem - the failing part.

That is interesting what you say about the thermal fuse. I'm afraid I'm not where the boiler is at the moment so I can't do tests myself. I had a read up about transformers in general and it does look like after an amount of time like the 50 or so years this has been working, it is possible for them to cease working. But I do agree it is a bit odd, because given that the are wire wrapped around a core, what could go wrong? To me it could point to something further in the circuit that is faulty. But I'm a layman with a multimeter and what I can remember from school physics.

JimB said:

OK, I think that your smaller replacement transformer would be an adequate replacement.

As for 20VA vs 25VA, yes your thinking is a little naive, just because a fuse is rated at 1amp, that does not mean that it passes 1 amp in normal day to day operation.

The transformer being condemned as faulty by the boiler man surprises me.
Transformers are usually very reliable, there is not much in them to go wrong.
Usually, but not always, failed transformers are burned, black, crispy and smelly.

But, for the expenditure of a few pounds, try it.
It cannot make anything worse.

JimB

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Thanks JimB much appreciated. I agree, what could go wrong with wrapped wire around a core?

Perhaps there is a problem with the gas valve or something (that would - I assume - be more likely a point of failure due to it having a moving part). If the solenoid or whatever it was in the valve got stuck or something, I could imagine that creating a problem for the transformer. But then I'd also imagine the fuse could have blown. But as you can see, not an expert.

Replacing the transformer at least would show us where to next investigate if it still didn't seem to work.

Thanks again for your help.

I agree with Max that the transformer does not look to have overheated. I would want to confirm that ther was no output on the secondary before ordering a new transformer. It is very unlikely that the secondary has gone open circuit. It is normally the primary that fails. I suggest that you first check the output voltage yourself.
The 20 VA transformer you mentioned would probably work bur I don't like connecting windings in parallel in case they don't have EXACTLY the same number of turns. There is this transformer that has two 12 volt secondaries which could be connected in series to 24 volts.
You would have to mount them externally in some kind of enclosure as they would not fit in the space available.

Les.

Les Jones said:

I agree with Max that the transformer does not look to have overheated. I would want to confirm that ther was no output on the secondary before ordering a new transformer. It is very unlikely that the secondary has gone open circuit. It is normally the primary that fails. I suggest that you first check the output voltage yourself.
The 20 VA transformer you mentioned would probably work bur I don't like connecting windings in parallel in case they don't have EXACTLY the same number of turns. There is this transformer that has two 12 volt secondaries which could be connected in series to 24 volts.
You would have to mount them externally in some kind of enclosure as they would not fit in the space available.

Les.

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Thanks Les! Unfortunately I can't check it without getting a flight. But I can certainly provide advice to the tech who will come look at it next on what to check.

What you said about the windings in parallel, and the secondaries and the other transformer is very interesting and made me realise I didn't fully understand. I drew out how I would understand I'd need to wire up the two transformers - wonder if you'd be able to correct any mistakes/advise if I've got things wrong?

It hadn't occurred to me that I should use both of the secondaries with 504-662 (although your concern about the number of windings has been noted, new to me - thank you!)

Another thing to check is that the fuse clips are tight enough to properly connect to the fuse - that style of open fuseholder can easily have the contact arms bent out when removing a fuse, and a bad connection there would give a similar failure to a transformer fault??

All they need is the fuse removing then the ends squeezed slightly by hand so the fuse it a tight fit when clipped back in.

(The top part looks like it has been sprained, though it may just be the lighting).

digitalhuxley said:

What you said about the windings in parallel, and the secondaries and the other transformer is very interesting and made me realise I didn't fully understand. I drew out how I would understand I'd need to wire up the two transformers - wonder if you'd be able to correct any mistakes/advise if I've got things wrong?

It hadn't occurred to me that I should use both of the secondaries with 504-662 (although your concern about the number of windings has been noted, new to me - thank you!)

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Typically transformer windings that are intended to be connected in either series or parallel, are wound bifilar, i.e both together at the same time, this make them more likely to be equal to each other when connecting in parallel .

rjenkinsgb said:

Another thing to check is that the fuse clips are tight enough to properly connect to the fuse - that style of open fuseholder can easily have the contact arms bent out when removing a fuse, and a bad connection there would give a similar failure to a transformer fault??

All they need is the fuse removing then the ends squeezed slightly by hand so the fuse it a tight fit when clipped back in.

(The top part looks like it has been sprained, though it may just be the lighting).

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That is a good call, that fuse holder doesn't look so great. Would be great if it was just a dodgy clip or wire.

MaxHeadRoom78 said:

Typically transformer windings that are intended to be connected in either series or parallel, are wound bifilar, i.e both together at the same time, this make them more likely to be equal to each other when connecting in parallel .

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Thanks! That is reassuring.

Really appreciate the help from everyone. Many many thanks.

I would definitely get another boiler guy if he/she can't change a simple transformer! Did they try selling you a new boiler?

Shagreobe said:

I would definitely get another boiler guy if he/she can't change a simple transformer! Did they try selling you a new boiler?

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Not at all, it's not within the scope of a boiler repair guy - repairing electronics to component level is well above their skill set, and the transformer almost certainly won't be available as an individual spare part.

In the UK gas appliance repairers are STRICTLY regulated (and I'd presume it's similar in any civilised country?), and attempting to bodge an incorrect transformer in one would probably use them their Gas Safe Certification.

The impression I got from the first guy that he would be good as long as he could buy replacement parts from a parts list, like so if he could buy a new control box for this model of boiler he would have done it. I don't know how many of this boiler still exist in the country but I imagine the parts were discontinued a couple of decades ago. Luckily due to its simplicity, it has been pretty reliable.

As Nigel alluded to, I think this is why we have had to find someone who operates on the fringes a little, who will just look at the boiler electrics from first principles. Luckily the era in which it was made makes the circuitry relatively simpler.

My worry is probably it is the gas valve that has actually gone, and it will be more complex (or not possible) to find a suitable replacement. But we'll see!