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Author Topic: 12v transformer no longer supplying full 12v - any ideas?  (Read 1246 times)

tubaman

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #15 on: March 06, 2021, 05:13:23 PM »

I've just looked at the caps you linked to and initially thought they were very expensive but see that they are 125C rated. That's total overkill for your application but won't do any harm of course. Standard 85C or 105C rated caps are much cheaper and would do the job perfectly well.
 :)
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #16 on: March 06, 2021, 05:38:09 PM »

There are also these Rubycon ones at 98p each +vat 105c

https://uk.rs-online.com/web/p/aluminium-capacitors/7497161/

or these Panasonic one for 1.41 each + vat

https://uk.rs-online.com/web/p/aluminium-capacitors/7083709/

Actually the Panasonic ones are on back order, so I'm guessing Rubycon ones will do?

No point over paying if they are up to the job.
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tubaman

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #17 on: March 07, 2021, 07:56:57 AM »

The Rubycon's should be just fine for your use.
 :)
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #18 on: March 07, 2021, 08:36:19 AM »

Thank you, I'll get a couple ordered.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #19 on: March 09, 2021, 08:42:33 PM »

Well that didn't go quite to plan.

Un-soldered the old capacitors, soldered on the new ones, all well and good so far.

Connected up a mains lead, set my multi meter to volts, connected it to the 12v terminals.

Turned on the mains just to catch a glow from the 12v circuit fuse as it blew  :no:

Looked at my multimeter and realised I still had the lead plugged in for measuring amps  :wall:

So now I've had to order some T800mA 20 x 5mm fuses  :blush:
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Alex Atkin UK

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #20 on: March 09, 2021, 09:06:27 PM »

Could be worse, at least it had fuses and you didn't do what I did and put the meter across the mains in that mode.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #21 on: March 09, 2021, 09:08:23 PM »

That is true, and at least I know what they fused the supply at, so a max of around 800mA.
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tickmike

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #22 on: March 09, 2021, 10:02:38 PM »

Yes , first job the capacitors.

Also just have a smell of the transformer (with it unplugged of coarse  ;) ) You will know if it's been getting hot a it will smell burnt.

If the box is not in an area that it can get water splashed on think about drilling some vent holes in the top and bottom of the box for some ventilation.

The two small round items near the capacitors and fuse 2 are full wave bridge rectifiers.

I think using CAT5 for the supply cable is giving you a lot of volt drop.

Not sure what thermostats you have but some have a resistor near the by-metal strip to heat it up an make it flip over quicker so when that's in circuit it would be pulling more power from the PSU.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #23 on: March 09, 2021, 10:23:13 PM »

I've not noticed any burnt smell, burnt electronics is a pretty nasty smell.

The thermostats are early Heatmiser units, with LCD display from circa 2006 and are all electronic with either built in or external temperature sensors - so no by-metalic strip's. Heatmiser specified the CAT5 cable to be used, and I bought the cable so I know it's the right stuff, but given the length and number of stats there will definitely be some volt drop, with the temporary PSU I think it was around 2v drop which the stats cope with.
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #24 on: March 20, 2021, 11:17:13 AM »

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.

I got time today to fit the new fuse, to my surprise the little LED that indicates the fuse is OK lit up, so those capacitors had held the power since the 9th March.

No load voltage is 12.52v, so very close to before, but with one stat connected it is now 12.12v, a lot better than before.

Now to go and pull the fridge out and refit it.

I seem to be collecting electronics repair jobs, our clock has stopped working - just corroded battery contacts, but my DS415+ Synology NAS has succumbed to the Intel Atom C2000 flaw, but I just need to solder a 100ohm resistor to the motherboard and hopefully that will fix it.

Oh, and another job I have is to replace many of the buttons on the thermostats as they sometimes don't work, I've had the buttons for years, just never got around to it.
« Last Edit: March 20, 2021, 11:20:43 AM by Ronski »
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Ronski

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Re: 12v transformer no longer supplying full 12v - any ideas?
« Reply #25 on: March 20, 2021, 11:49:53 AM »

Success, power supply refitted and it's now 10.17v under full load  ;D

Thank you to all those who offered advice.
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