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Idiot's guide to transformers ?


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Hi everyone, I need a 12v 5A supply to run an incubator. I've made up Ron J's thermostat with appropriate resistors, and while I could run that with his transformerless power supplly sircuit, I really want the heat source and fans to run at 12V too, and that needs more amps.

I thought I had a suitable transformer, rated 72VA 12V, (I found it in Dad's shed - Dad was a Marconi's research engineer ;) ), but when I tested it (with a meter), it shows 12V to start, but dies rapidly. I can't see anything wrong with the coils so presume that the one and only component (rectifier? it's Russian) is the problem. Would it be possible to fix/replace this? I am on a tight budget, so can't afford to buy a complete new power supply.

I couldalso use some general information on transformers, and especially on rectifiers, and voltage regulation, or w/e it's called, too, as I have another tranformer, stripped of everything except the input and output wires.  :-[

Thanks for reading this. :) Hope you can help. :) :)

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Have you tested resistance on the coils? Just to ensure that theyre not shorting or anything?

If that all checks out, try replacing the rectifier unless the rectifier and transformer is as one package then that may be a small problem. With your other transformer, try reading each winding to get which one is the secondary and primary by using the resistance option. The primary coil with be the one with the most resistance.

Bridge rectifiers really aren't majorly expensive and all you need is a correctly rated one (Peak of 12v = 12 x 1.414 = 16.9v but bridge rectifiers are normally quite high voltage components for voltage spikes etc. Aslong as the voltage is within the range, you would really just need to see if the current rating is suitable. a 6A 200v bridge rectifier off Rapid would cost: ~

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Hi pyrohaz,
Thanks for replying. :)
I can see quite easily on both transformers which coil is which.
It looks like the output coil on the 72VA one may be shorted, after all, as it only measures minusculely more resistance than the meter's internal resistance. :( The winding is really thick wire though, about 2mm, so I wouldn't expect much resistance, but I don't know diddly.  :-\
The other one has kind of multiple windings for the secondary coil, with half a dozen wires coming off, each marked with a different voltage, from 0(!) to 30. They are much thinner wire, which may explain why they do show slightly more resistance, (increasing with each additional coil).
I think I'll try replacing the rectifier, now that I know what it is, hopefully the 'bloke in the shop' will be able to find me the right one, thanks. :) Is it normal for it to be on the + side of the input? You say they are usually for high voltage, so I presume it is ok?

Most of what you've said about rectifiers is unfortunately over my head, but at least you've put me on the right track. :) 

Thanks too, for the regulator chip number. I should be able to use that to protect the thermostat circuit, and maybe the fans? (PCU case fans 2off).

I may be getting ahead of myself here, but would it be possible to leave the bulbs (heat source) on 12V AC and just rectify the input to the circuit (and fans?) ?  (The heat source is connected by a relay, operated by the circuit.

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Hi, good luck with getting the transformer working, unfortunately if the outputs are sorted, it might be unfixable :/. If the transformer in itself is quite large, I can imagine the secondary coil is that large for the current capability of the transformer.

Hopefully the 'Guy in the shop' will be able to give you a hand with your rectifier though it should just be a silicon rectifier with enough current capability :) here is an example of one: http://www.rapidonline.com/Electronic-Components/Discrete-Semiconductors/Bridge-Rectifier-Diodes/6A-Bridge-rectifiers/29672

The brand or anything shouldnt matter too much aslong as it rectifies.

For your idea with the bulbs, running them on 12v AC would be fine but for the fans, yes you will need DC. If you don't run your bulbs off the same DC supply, your filter capacitors will not need to be as large. :)

The 7812 will only be able to provice 1A current if heatsinked. If you require more, you might want to look towards a switching regulator to save efficiency and reduce heat :).

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You probably won't be able to buy the Russian rectifier diodes any more. They're almost certainly obsolete and inferior to modern diodes. I'd recommend replacing the old diodes with modern ones, even if they work.

If replacing the diodes doesn't work, try replacing all of the electrolytic capacitors.

I doubt there's anything wrong with the transformer itself, it's normal for the secondary resistance to be too low to show on a multimeter. To test the transformer, connect the power to the primary and measure the AC voltage on the secondary.

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Have removed the defunct whatever, and now the voltage appears to go up and down. Figured that's something to do with it being AC, and hooked up a lamp, to check. Lamp stays nice and bright, no ups and downs, so when the shops open I'll nip out and see if I can get a suitable rectifier, and a 7812. Many thanks. :)

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If your meter is on the AC setting there shouldn't be too much fluctuation although there will be some because the mains voltage isn't perfectly stable.

If the transformer is 12V, the LM7812 is unsuitable because there isn't enough headroom.

The LM7812 requires at least 15V to regulate properly, the diodes will drop 2V at full load and there will be some ripple across the filter capacitor. In other words 12√2 - 2 - 3 = 12V which means that any ripple will transfer to the output. Then there's the fact that the mains could be on the low side which will make matters worse.

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Have finally managed to find LM7812, and mine are definitely not that. :)

The mystery transformer component was a thermal protection switch, and I'm hoping the by being careful and not overloading it, the transformer will be fine without it.

In fact most things are sorted, the main sticking point is not knowing what rating of bridge rectifier I need. I have two in my box of bits, but apart from the 'forward voltage' I have no idea what rating they are.

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