Bought a bit of audio gear from the US at a decent price even allowing for carriage. Guessed it might be 110v - but it is actually marked 120v 60Hz.
Looking inside, it has an analogue power supply - I was rather expecting an SMPS.
The transformer primary has two winding wired in parallel which seem to be coil 1 black start brown end, coil 2 red start orange end.
Red and black are paralleled as are brown and orange. Red/Black goes to neutral, brown/orange to the off/on switch.
So it looks like I may be lucky and simply altering them to series will do it for 240v. Link brown and red and 240 goes between black to orange? Does it matter if one winding has the connections reversed when they are in series?
Draws a picture to make sure, yes that is correct. B-)
Yes the fields generated will cancel and it won't work. It may also get rather hot and go bang.
Check the size of the smoothing capacitors, IIRC they don't need to be quite as big at 60 Hz as 50 Hz. Not critical unless this thing is going to be driven hard, which may cause the supply to have rather more ripple than intended.
If they do cancel out, the transformer current will depend on the primary winding resistance, as there will be hardly any inductance. This means that the transformer will let out it's magic smoke.
If you series the split primary windings in counter-phase, you'll have created a 'non inductive' wirewound resistor of perhaps circa ten ohms for something like a 200VA transformer (it all rather depends on the VA rating of the transformer).
That should blow a 13A plugtop cartridge fuse in about a second or so. Obviously, a 5A fuse would be a better bet if you're going to 'test and hope' sans any form of continuity testing to identify the correct pairs to associate with each winding.
The thing is, assuming you make use of the fact that you need to connect either the red or black to the orange with the remaining unused wires connected to neutral and on/off switch, you can either end up shorting one of the split windings in isolation and connect the other 120v winding across the 230v supply or end up with them wired in series phase aiding as per requirement.
Now, if you get it wrong, the same 200VA transformer will present a mere 5 ohms with the shorted out winding effectively making it a non-inductive resistor with just a small amount of leakage reactance in series. If you get it right first time by dumb luck, the fuse doesn't blow and the amp will work just fine.
Provided you're happy to sacrifice a 5A plugtop fuse and you use your noggin, it's quite possible (if not recommended) to arive at the correct solution in no more than 2 goes of the "Suck It And See Method"(tm).
As it happens, in this case, there's no risk of damage to the rectifier/smooting capacitor pack other than failing to short out the out of circuit winding when applying 230v across the single 120v winding.
BTW, closer scrutiny suggest you may have correctly identified which colours belong to which winding. As I re-read it, you need to either connect brown to red or else orange to black using the remaining connections to go to the neutral and switched live terminals.
It's a fairly standard looking transformer with the wires exiting from the top of the bobbin and on every other ones of this type I've got they go Winding 1 start finish Winding 2 start finish in a line. So for 240v you just link the centre two and apply the 240v to the outer two.
Quite - which is why I was asking. I wondered if the wires were standard colours in the US. I do have a US transformer here with twin primaries, and that has a red and a black but purple and yellow.
However, some exhaustive Googling found a pic of the insides of a device from the same maker 240v connected.
It goes Red and Brown linked. Black to neutral, Orange to Line. Only other difference (apart from obviously fuse value) is the 240v device has a DP mains switch, the 110v a single pole one. I won't loose any sleep over that. ;-)
I'll risk that. No big deal to change if it proves a problem. Thanks.
Both voltages are common in the USA. So it will be precisely for this pupose. (So there probably exists information somewhere (manufacturer/internet? Label inside somewhere/users manual) on how to change the voltage connections.
You commonly see a similar idea on three phase motors too.
That could spell trouble keep a *very* careful eye on it for signs of overheating. US made kit for their domestic market tends to cut it extremely fine on the saturation of the transformer and the shift in frequency from 60Hz down to 50Hz will allow a higher current to flow. One of my first jobs was to determine why a particular US made colour monitor died prematurely and sometimes even caught fire in the UK.
Yes. Get it wrong and the two windings magnetic fields will cancel each other out resulting in an almost non inductive load. Check with an LCR meter to establish which way round produces a 2L load as opposed to an almost pure resistive one with a tiny inductance.
And really keep an eye on it for overheating. The change from 60 to 50Hz can saturate the transformer cores on some US kit! Their electronic designers often can't imagine anywhere outside the USA!
Japanese stuff is always OK anywhere as they have half 50Hz UK and half USA 60Hz frequency depending on which part of the country you live in!
True enough but the tendency of US electronic designers to use cheap and nasty transformers that are only marginally safe from saturating at 60Hz is a potentially serious problem for overheating at UK 50Hz.
And no there is no wire colour standard. I once bought several transformers with the same pec but from different places and none had the same colour ewires, and one just had tags with no visible wires at all.
Sounds like a P ioneer piece of kit, but I'm going back a lot of years now.. Leans on old walking stick. Brian
What is interesting is the legend on the rear panel also shows 100v 60 Hz and 230v 50 Hz as choices - presumably with a different transformer(s)?
But this one was marked 120v 60 Hz and I can't see why they'd make the transformer for that with two windings in parallel if couldn't be altered to 240v.
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