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Author Topic: 12v transformer no longer supplying full 12v - any ideas?  (Read 1177 times)

Ronski

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12v transformer no longer supplying full 12v - any ideas?
« on: March 05, 2021, 05:57:29 PM »

I have individual room stats which I fitted 15 years ago, these are power by a 12v transformer. A couple of weeks ago the room stats behaved weird, with just the back light glowing on most of them, a couple worked normally. It eventually cured itself after an hour or so.

Yesterday we had to have a new gas main run in, so the bolier was turned off overnight until early around lunchtime, this also turns the power off to the transformer and thus the room stats. After turning the boiler on, pretty much all the room stats just had an illuminated back light. So I took one off the wall and measured the voltage, it was just 6.6v. Checking at the transformer it was 8.8v.

For now I've jerry rigged in an old external hard drive power supply, which is 12v 2 amps and at least we have heating.

Any idea what could be wrong with the original transformer - I note the actual transformer has 9v printed on it?





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tiffy

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #1 on: March 05, 2021, 08:20:27 PM »

When you say transformer, it's more accurately a mains transformer with two 9 volts secondary windings, rectifier (2 diodes) and smoothing capacitors, can't make out if it's a regulated O/P., don't think so.
With the two 9 V windings connected as 9-0-9 and the 2 visable diodes as rectifiers you should get in theory full wave rectified DC of 1.4 times the individual windings AC voltage (off load) if my far distant theory serves me well, ie., 12.6 V. DC..

Regarding the PSU trouble shooting:
What is the DC voltage from the PSU off load?
What current is being drawn from the PSU, ie., is it dropping the voltage?
The 12 V, 2 Amp PSU you are now using may be a higher current rating that the unit in question and is capable of maintaining near to 12 Volts into what may be a faulty stat..

Try disconnecting one stat. at a time and see if the voltage from the original PSU is restored to 12 Volts.
If all that fails you may have a faulty PSU, very unlikely to be the transformer or the rectifier diodes, the most likely components to fail with age as always are the electrolytic capacitors (although they don't look leaky), these should be very easily replaced if necessary.
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Alex Atkin UK

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #2 on: March 05, 2021, 08:33:00 PM »

Are those capacitors on the right slightly domed, they look like it to me and would always be my first assumption of what has gone wrong.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #3 on: March 05, 2021, 09:04:54 PM »

Thank you both, I'm pretty sure when I measured it off load (one wire disconnected) it was about 9v, current draw was 0.5a with all the thermostats connected.

Unfortunately there is no current rating marked on the PSU.

I'll have another look tomorrow and see what those caps look like.

PS. The thermostats are all connected by Cat5e cable, so I suspect there is some voltage drop, hence the 6.6v at the one I measured, even with the temporary PSU there wasn't 12v, think it was just over ten, but 12v at the PSU end.
« Last Edit: March 05, 2021, 09:07:17 PM by Ronski »
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Alex Atkin UK

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #4 on: March 05, 2021, 09:25:54 PM »

Really trying to wrap my head around why they would be using two 9v windings to somehow obtain 12v.  There doesn't seem to be much going on with that PCB to do that.
« Last Edit: March 05, 2021, 09:47:43 PM by Alex Atkin UK »
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burakkucat

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #5 on: March 05, 2021, 10:02:52 PM »

Are those capacitors on the right slightly domed, they look like it to me and would always be my first assumption of what has gone wrong.

They certainly look slightly domed. I would consider replacing them and then re-checking the on-load output voltage of the PSU.
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tiffy

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #6 on: March 06, 2021, 12:43:38 AM »

Really trying to wrap my head around why they would be using two 9v windings to somehow obtain 12v.  There doesn't seem to be much going on with that PCB to do that.

A 9-0-9 V. permutation with 2 diodes gives full wave rectification, transformer center tap negative, joined cathode's of the 2 diodes positive.
A saving of 2 diodes (or use of a bridge rectifier) for full wave rectification?
Obviously a dual secondary mains transformer cost would far outweigh the cost of the diodes so not very logical really!
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #7 on: March 06, 2021, 02:14:32 PM »

Just ran some more tests. There are 13 thermostats in total, when first powered up some come on with the back light on others don't - there's a few different versions.

All following measurements are taken where the PSU connects to the thermostat wiring.

With the temporary power supply and all stats connected we have 12.21v and 0.59a current draw.

The original power supply has a no load voltage of 12.56v.

Connect all the stats and it drops to 8.52v and 0.43a.

I got one of my spare thermostats and connected just the one to the original PSU. Voltage dropped to 10.97v and a current draw of 0.09a with the back light on, and 0.04a when it switched off.

The two large capacitors are domed, they are certainly not flat.

Tiffy, what do you mean by "regulated O/P"?

Thanks.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #8 on: March 06, 2021, 03:18:07 PM »

I've now removed the PSU so have taken some more pictures









Is the general consensus to remove the capacitors and source some replacements?

Caps are 16mm diameter and about 32mm tall, with a leg pitch of 7.5mm. They have (M)1485 and 4700F 16v, - 40 to +85c on them.

Found this and various others on RS https://uk.rs-online.com/web/p/aluminium-capacitors/7621679/ I'm a bit confused by the lifetime, a load life of 10,000 hours, which is not much over a year?

PS. Postimage links seem not to be working at the moment, hopefully the pictures will come back up later.
« Last Edit: March 06, 2021, 03:24:31 PM by Ronski »
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Alex Atkin UK

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #9 on: March 06, 2021, 03:38:37 PM »

As I understand it the lifetime is quoted at the maximum operating temperature.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #10 on: March 06, 2021, 03:40:28 PM »

I think you could well be right, I was just looking on another website and it said lifetime at maximum temperature.
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tiffy

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #11 on: March 06, 2021, 03:51:08 PM »

13 thermostats, you must live in a mansion?

The important parameters regarding the electrolytic capacitors is the value and working voltage (16 v's or greater), physical mounting should not be an issue, virtually any radial mounting, good quality devices should suffice.
If you do remove the capacitors, check on an ohm meter, with correct polarity resistance should increase to virtually infinity as the capacitor charges, low resistance reading will indicate a leaky unit which should be replaced.

A regulated PSU incorporates active components which will maintain a constant voltage regardless of load within the parameters of the regulator and current capability of the source.
An unregulated PSU's voltage output will decrease with load even when operating within it's design parameters.

From the voltage/current readings you have supplied, your PSU would appear to be unregulated.
As the temporary PSU would appear to be capable of maintaining 12 volts with total current of 0.59A's and all stat's connected it would appear that the original unit is faulty or one or more of the stat's has started to draw more current than it used to.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #12 on: March 06, 2021, 04:08:57 PM »

Thanks Tiffy, perhaps I'll get my soldering station out tomorrow and remove the two caps.

Mansion - I wish  ;) It was a four bedroom detached with one tiny bathroom when we bought it about 19 years ago, we bought it because we could easily extend it, it's now a five bedroom (master with en-suite), with an additional downstairs room, small utility room, small cloakroom/server room and much bigger kitchen. OK its a mansion compared to some, but it took me well over a decade to finish all the building work, and there's still some finishing off left to do.

It's surprising how quickly the stats add up, two rooms have two although we've never used the heated floor in either room.

  • Bed 1
  • Bed 2
  • Bed 3
  • Bed 4
  • Bed 5
  • Bathroom
  • En-suite
  • En-suite floor
  • Cinema Room
  • Hallway
  • Lounge/diner
  • Kitchen
  • Kitchen floor


 
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tubaman

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #13 on: March 06, 2021, 04:15:08 PM »

If the top of those caps is not flat then they are dead and they need to be changed. As said earlier the hours life in the spec is based on running at the max temp (85C in this case) so should not be of concern in this application where I doubt they get even warm. If they've been running for 15 years then they've done just fine!
Make sure to note the polarity if it's not marked on the PCB - if you put them in the wrong way around the results can be a bit explosive!
 :)
« Last Edit: March 06, 2021, 04:17:24 PM by tubaman »
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #14 on: March 06, 2021, 04:29:58 PM »

Thank tubaman, I've got pictures and it very clear to see which track is negative as it goes to the negative terminal for the stats connection.

It's far from ideal where the PSU is, it's in a recess behind a fridge in the small utility room with heating pipes running below it, and next to the fridge is a floor standing boiler, it gets quite toasty in there - we tend to put the washing on an airer to dry overnight.

I'll get a couple of the caps I linked to above and replace them - we have an RS account at work, so postage is free.

PS. Images now show in my earlier post.
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