AlanRatcliffe
Hmmm... true. I'm afraid we are beyond my knowledge and will have to wait for someone else who knows more to chip in.
Gearhead
No takers? I ain't no electronics wizard but it is my hunch that your power supply draws blind current which is heating up the tranny. As in, the phase difference between the voltage over the tranny and the current through it is going towards 90 degrees. Again, I dunno. Could you post the schematic you are using?
makepeace
Thanks GH ?
Its basically this:
Except, 2200uf caps instead of 10000uf, and only 1 regulator circuit.
The transformer is toroidal, this:
http://docs-europe.electrocomponents.com/webdocs/0da0/0900766b80da0cc2.pdf
1x 240v primary, 2x 18V secondaries. i've only wired up 1 secondary to the board, the 2nd one has its leads separated and unused. could that be why perhaps?
My regulator is a 7809 rated at 1A, here:
http://docs-europe.electrocomponents.com/webdocs/0609/0900766b806090e4.pdf
Gearhead
Looks like it was drawn in MultiSim... Should be possible to simulate all faults until you get the one you have. Anyway.
First thing to do is to check (again) if you really stuck to this schematic.
Second step is to simplify the circuit until some of it does what you want. Say, take out the 7809 and the 7805 and see if you get 12V while not heating up the whole thing.
On a side note: if you take the rectifier out of circuit and compare the voltage on the open two secondaries, and they are reasonably close, you could put them in parallel and decrease the current through the one.
makepeace
summin funny is goin on her'.
rebuilt the thing, after getting bored of rechecking. as i said, the only regulator i'm using is the 7809. replaced the 2200uf's with a 4700uf, so now i have:
Vin -> tranny -> rectifier -> 100uF -> 4700uF -> 9V regulator -> 100uF -> Vout
i also wired up a similar circuit, but with an 18V regulator to the 2nd secondary, just to route out that problem.
i do get 9V out of my 9V reg, and there is no hum, so it is doing its job correctly.
left it on for a while, came back to find that the insulation around the wires has melted to the plastic wrapped around the coil. not ideal. tbh, i think its this transformer's fault. i'm going to write to RS and hear what they say. afaik, this is a very simple circuit that should do its job no prob. the problem seems to be the transformer design.
will definitely try paralleling the two secondaries, it says you can in the datasheet. otherwise, i'll get over the toroidal and just use a cheapy 12v tranny i have here while i badger RS.
thanks ?
Gearhead
Despite the spec saying you can parallel the primaries, I would still measure them, especially since you may have a short.
makepeace
tested both secondaries, one puts out 19.1VAC the other 19.3VAC.
paralleled them, same thing. still gets moerse hot.
makepeace
Eventually solved my problems with this. For interest sake, the toroidal burned out a while back - seems like it was a manufacturing defect and RS offered to replace. For those interested, my solution is to take your common wall-wart - those unregulated linear jobs that have switchable voltage and polarity - desolder and remove the two slide switches, desolder and remove all secondaries but the 12V winding, switch the 1000uF cap for a 4700uF, solder in a 7809 across the rectified output, where the polarity switch used to be and add an RC filter across the output: small series resistor (I used 220 ohms, but the smaller the better, of the order of 10 ohms should be the best) with a 100uF shunt to ground. The series resistor is the trick here to get rid of the high frequency noise of the regulator.
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