6 volt LT at 2 amps

[ppppenguin]

If I was being evil I'd suggest trying it If you're tempted to do this then use a lamp limiter and see what happens.

On transformers with 2 or more identical secondaries which you might connect in parallel, they should be marked. Often a dot at one end of each winding. So dot to dot, non-dot to non-dot. If they're not marked then connect one of each winding together and an AC voltmeter to the 2 remaining terminals. If the result is 0V, you've got it right and you can make the other connection. If it's double the winding voltage then you've effectively connected the windings in series and you were about to put a short circuit across them untill you realised.

If there's small difference then beware, the 2 windings aren't as similar as you though they were and you'll get a lot of unwanted current flowing in them. This might happen on a transformer with multiple 6.3V heater windings which were intended to powere different sets of valve heaters.

[pwdrive]

If you connected the secondaries in parallel the wrong way round you'd know about it soon enough

How's that?  Also if its bad then is there a way to check which goes were?

Two windings, both the same voltage output and impedance, if the windings are connected in parallel and in phase then when one end of one winding is going +ve during the AC cycle the corresponding end of the other winding will also be going +ve so current from one can't flow into the other, eg: connecting those identical voltage sources together means +ve to +ve and -ve to -ve, no current can flow from one winding into the other winding because for current to flow you need a potential difference, current can only flow from those windings if a load is connected across them, but if connected in parallel and out of phase, when one end of one winding is going +ve the end of the other winding will be going -ve so there's now a potential difference and current will flow and it will be large as there's not a lot to limit it.

EDIT: Post crossed.

Lawrence.

[ppppenguin]

Lawrence, the windings only need to be the same voltage, not the same impedance, to avoid harmful circulating current. If the winding impedances differ (such as if one is wound over the other and hence has more wire in it) then the currents won't accurately share when they are in parallel. This effect will be small in the sort of things most of us deal with.

[pwdrive]

Yes, thanks for that, I didn't say the impedances had to be equal although that could well have been implied from what I said, apologies for any confusion.

EDIT: Re written.

Lawrence.

[pwdrive]

Here's another way Dave can imagine it, two fully charged 12v car batteries connected in series to give 24 volts output, short the output and something will fry or explode, yet all that's been done in effect is to connected them in parallel but by connecting them in parallel the batteries have been connected in opposite polarity.

Lawrence.

[Crackle]

The wiring in Dave's breadboard will not lend itself to voltage stability.
Once it has been assembled together with the wiring from the main smoothing cap kept as short as possible and the rest of the components mounted close to the regulator I think it should work better.
Mike

[unrealdave]

Maybe post the voltages with the 5 ohm load connected.....The regulator output voltage, the regulator input voltage and the AC voltage input to the bridge rectifier.
Lawrence.

I will do as soon as I get a mo - I will also try a 13v secondary transformer.

Thanks for the other replies - I get it now and also will do as suggested to make sure dual secondary windings on unmarked tx's are put in parallel the correct way  instead of guessing .  It was an educated guess though as I looked up the manufacturer and checked the colours they used on other tx's, but if I had of guessed wrong would it have been REALLY bad or just heated up and a bit of smoke before cutting out the RCD?

Its sunny and Benita is insisting I clear up the garden and move everything I have stored under the chapel over the winter so cant play with anything at the mo   . 

[pwdrive]

Maybe post the voltages with the 5 ohm load connected.....The regulator output voltage, the regulator input voltage and the AC voltage input to the bridge rectifier.
Lawrence.

but if I had of guessed wrong would it have been REALLY bad or just heated up and a bit of smoke before cutting out the RCD?
 

It depends how much it's heated up, that would depend on any fuse rating or if no fuse how quick you are with the power on/off switch.

For an RCD to trip there needs to be a current flow between live and earth or neutral and earth.

Lawrence.

[ppppenguin]

As Lawrence said, an RCD won't trip on this kind of fault. It's a simple overload with all the current that goes up the live coming back down the neutral. If you're lucky a fuse will blow, otherwise you've got a few seconds until the dreaded brown smell and a little longer until the magic smoke emerges.