29-12-2011, 08:32 PM
I think that worrying about the fuse rating is moot here - unless you've found a fuse that lasts forever at 2A and blows straight away at 2.001A. Such a fuse would never survive the inrush current anyway.
The best way to protect that transformer is to add a thermal breaker. These open after a short delay as something inside needs to warm up. But, the element warms up faster than the transformer windings, and that's the key point. For an example, see here: http://www.markhennessy.co.uk/isolating/
It's not obvious from the pictures, but the IEC mains inlet is fused. I can't remember the rating, but it could be 5A or 6.3A - I could take a look if anyone needs to know. The transformers are 500VA each, and the thermal breakers are nominally 2A. These transformers/thermal breakers had done some 20 years in a radio studio before I rescued them, so I guess the arrangement works
Al, you're dead right to say what you've said about phase, and you've covered a lot very comprehensively. But in my humble opinion, I felt that it was worth expanding slightly on how to ensure the phase is definitely correct before the two secondaries are connected together in parallel - it's also worth stating what the outcome of getting this wrong might be (not wanting to scare you Rob!).
Rob, if you have access to a 'scope, this might well be very easy to you. But, as you raised the question, I'll assume that it's worth spelling this out - but apologise in advance if it's stating the obvious. Also, it's worth stating to anyone else who might be in a similar position.
If you connect them in parallel, as per Al's third drawing, but get the phase wrong, there will be a "fight". With luck, the primary fuse will fail. But it could be much worse...
One approach to determine phase without using an oscilloscope is to wire the secondary windings in series. If you get 240V that's great - add the dots as shown is Al's first diagram, then proceed to diagram 3. But, if you get 0V, you know that you've wired them out of phase - but luckily, with a series circuit, the windings will dodge each other rather than fight it out
And, even when you're sure you've got everything right, can I suggest that this really is a good use for a lamp limiter. If the lamp remains on at full brightness, you've probably got a phase error on the secondary side.
Hope this helps,
Mark
The best way to protect that transformer is to add a thermal breaker. These open after a short delay as something inside needs to warm up. But, the element warms up faster than the transformer windings, and that's the key point. For an example, see here: http://www.markhennessy.co.uk/isolating/
It's not obvious from the pictures, but the IEC mains inlet is fused. I can't remember the rating, but it could be 5A or 6.3A - I could take a look if anyone needs to know. The transformers are 500VA each, and the thermal breakers are nominally 2A. These transformers/thermal breakers had done some 20 years in a radio studio before I rescued them, so I guess the arrangement works
Al, you're dead right to say what you've said about phase, and you've covered a lot very comprehensively. But in my humble opinion, I felt that it was worth expanding slightly on how to ensure the phase is definitely correct before the two secondaries are connected together in parallel - it's also worth stating what the outcome of getting this wrong might be (not wanting to scare you Rob!).
Rob, if you have access to a 'scope, this might well be very easy to you. But, as you raised the question, I'll assume that it's worth spelling this out - but apologise in advance if it's stating the obvious. Also, it's worth stating to anyone else who might be in a similar position.
If you connect them in parallel, as per Al's third drawing, but get the phase wrong, there will be a "fight". With luck, the primary fuse will fail. But it could be much worse...
One approach to determine phase without using an oscilloscope is to wire the secondary windings in series. If you get 240V that's great - add the dots as shown is Al's first diagram, then proceed to diagram 3. But, if you get 0V, you know that you've wired them out of phase - but luckily, with a series circuit, the windings will dodge each other rather than fight it out
And, even when you're sure you've got everything right, can I suggest that this really is a good use for a lamp limiter. If the lamp remains on at full brightness, you've probably got a phase error on the secondary side.
Hope this helps,
Mark
