Help with transformers please

[Woodburner]

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 could also 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.

 

[Sceadwian]

Use an ATX power supply from a old computer, or just buy one, they cost as little as 20 bucks, and will provide more amperage on the 12 volt line than you need, it's fully regulated short circuit and over current protected and all that's required to turn them on is short the green wire on the motherboard connector to ground, via a low voltage/current switch. A whole lot less messy than dealing with a custom design, and much cheaper.

 

[Reloadron]

I would strongly suggest Sceadwian's solution. You would get a well regulated 12 VDC supply plus additional supplies like 3.3 and the ever popular 5 volts, all well regulated and filtered.

As to what you have, transformers themselves don't generally fail but things like the rectifiers do as well as the capacitors, especially in the case of old units. The capacitors tend to dry out and fail. I am not saying that you should not attempt to troubleshoot and repair what you have, simply that it likely would not be practical or more economical.

Ron

 

[Woodburner]

Tried that it needs to be pugged into a m/b to work. :S If you know a work around, I would be glad to read it.

ETA: Opps missed the workaround. will try that thanks.

 

[Diver300]

I'm going to go the other way and suggest that you buy a new rectifier, although a computer PSU will probably work fine.

The first thing to do is find the 12 V AC output of the transformer, and connect a car headlight bulb to it. That should light and you can leave that running for an hour or so to properly test the transformer. If that works, look for a suitable rectifier.

If you are running heaters and a fan, have a separate rectifier for each. Then you can put a smoothing capacitor on the feed to the fan, while leaving the heater with no smoothing. That way you can use a much smaller capacitor.

 

[Woodburner]

The PSU is kaput so back to square one anyway actually so thanks very much.
Would a 12V 20 or 50W bulb be ok? And should I leave the existing rectifier, or remove it? It's on the live side of the input to the tranformer, but from what you say, I could leave the bulbs (heat source) on AC and just rectify the supply to the themostat and fans.

Not sure what you mean by a smoothing capacitor, don't I need a errm voltage regulator?

 

[Reloadron]

The PSU is kaput so back to square one anyway actually so thanks very much.
Would a 12V 20 or 50W bulb be ok? And should I leave the existing rectifier, or remove it? It's on the live side of the input to the tranformer, but from what you say, I could leave the bulbs (heat source) on AC and just rectify the supply to the themostat and fans.

Not sure what you mean by a smoothing capacitor, don't I need a errm voltage regulator?

Something isn't right here. You should be running AC from your mains to the transformer primary (input) then rectification takes place on the transformer secondary (output). Thus converting the transformer AC output to DC.

Generally speaking transformers don't fail. Unless severely overloaded or the insulation breaks down they last forever. There isn't much to a basic transformer to go wrong or fail.

Could you post a picture or rough drawing of what you have?

Ron

 

[Sceadwian]

What PSU is kaput? Did you kill an ATX power supply? If you did you're doing some dramatically wrong, they're virtually indestructible. They're thermally protected DI/DT and short circuit protected and automatically shut down if they lose regulation or are overloaded without damage to the supply. Cycling the power will reset them and allow proper operation again, if the ATX supply is going into shut down every time you turn it on you're doing something very wrong.

 

[Woodburner]

No, it was already kaput. Presumably was the cause of a dead computer, and just got left out of the skip. Don't know what's wrong with it, and don't really care, tinkering with them is well out of my comfort zone.
Can we get back to my original question please?

Oh and I have disconnected the rectifier, or whatever it was, so can't show you but I can assure you it was definitely between the live wire from the mains plug, and the primary coil.
It works fine now, but is now AC output, obviously. 50W bulb works too.
Just need some more info on rectifiers, and regulators please.

The guy in the shop who really knows his stuff wasn't there today. So I just got an over spec rectifiing diode, and hope that that and the 'regulator MC78T12CT' s that I found in a bag of bits, will do what I want them to!

 

[Reloadron]

OK, back on track:

Hi everyone, I need a 12v 5A supply to run an incubator

A 7812 will not work as a 7812 would need over 14 volts in to regulate and a 7812 has a maximum current out of about 1 amp.

On your transformer secondary if you want DC you need a full wave bridge rectifier to convert your AC from the transformer secondary to DC. Now if you are just (at that point) wanting to run a heater or whatever, you don't need the voltage regulated.

Besides the heater(s) what else do you want to do?

Ron

 

[Woodburner]

As well as two 20W lamps, it needs to run a 12V thermostat circuit and two 12V fans. Preferably one lamp and one fan via the thermostat's relay.

Someone else, somewhere else, mentioned '7812' but I got the impression they were referring to an IC, not this, even though it has those numbers on it. I looked up the regulators on RS and they're rated 12V 3A, so am pretty sure it's not the same. This is exactly what I have : ON Semiconductor | Semiconductors | Power Management | Linear Regulator | |MC78T12CT

I've managed to find out what the original component was. It was a UP63 by UCHIYA. Which raises more questions than it answers! I presume the '3' indicates a now defunct variation that incorporated a rectifier. :S

I've got a 6A silicon rectifier, with a reverse voltage rating of 600V, to fix the transformer, and will use regulators on the circuit board to protect the circuit.

 

[Sceadwian]

So you've decided not to use an ATX power supply?

 

[Reloadron]

OK, I still agree with Sceadwian in that you can buy an ATX computer PSU for about $20. However, if you want to work with what you have you might want to make it like the attached. Whatever was before your primary I haven't a clue but matters not. You should be able to run your mains (being careful) to the transformer primary. Next on the secondary you need a full wave bridge rectifier if you want/need DC. Lamps or Heating elements it matters not AC or DC. I have no clue what your thermostat is? If it is electronic the you likely want the regulated 12 VDC. The attached should give you an idea. The rectifier is actually a full wave bridge (in reality 4 rectifiers in one package). Anyway, you may want something like the attached.

Ron

 

[Woodburner]

Yeah, they cost a lot more outside the USA, and then there's postage or a premium price to get one from a shop.
I've found another one, but it's up in the attic, still in a computer case, and may also be kaput, even if I can get it down. (It went up so must be able to come down! I just need to remove some of what went up there after it did! lol)
Tbh, the transformer, if I can get it sorted, is much neater.

I've managed to find some basic info on transformers, and with that and deciphering the writing on the component, it becomes obvious that it wasn't any sort of rectifier. It was some sort of protection for the transformer itself, originally cutting the supply if the primary got too warm. I can't find a replacement, though, so, yes, I will just have to be careful not to draw too much current.

The diagram is very helpful thanks, especially for the use of capacitors.

I'm still struggling to figure out what rating diodes/rectifier I need. I have two bridge rectifiers, but no idea how to tell what rating they are apart from the 'forward voltage' , but I don't know how that relates to current etc. :S


I'll have to go back to the shop because, surprise, surprise, none of the dozens of capacitors I have is anything like 2500uF 25V or even 2500uF 16V. I seem to have lost all the ceramic capacitors too. Hopefully there will be a more knowledgeably chap on the counter this time!

 

[Reloadron]

If the bridge has any part number on it Google may be your friend. For example I have a few bridges lying here labeled KBPC3506 and a Google gets me a data sheet telling me 600 volt 35 amp. Don't know your location but if you are in the US I could send you some parts that would help. I don't want anything for them. Additionally, the drawing is a rough that should help get you going.

Ron