I want to build a low noise linear bench PSU with an electrostatic screen. I have a Cotswold Electronics 240V, 330VA toroidal with a burnt out primary which I intend to rewind; I know it's not economically sensible but a) nobody is paying me for my time, b) screened transformers are hard to find, never mind at a reasonable price and c) I fancy the challenge.
I have removed the 880 turn, .8mm dia. primary winding which was on top of the secondary. At 127m the primary would have been 4.2ohms which is reasonable for a 330VA 240V transformer. Assuming there are 5% additional primary windings to compensate for the voltage drop due to winding resistances (regulation) that gives 3.5turns/V.
The core dimensions are approximately (I haven't removed the secondary yet) 118mm outer dia., 62mm inner x 35mm depth giving a core area 9.8cm^2. At 3.5 turns/volt that implies a flux density of 1.3 Tesla. That seems a bit low by current standards but this is a late 1970's transformer. It weighs 2.8kg
The 2x35V secondary are 1.85mm dia. and measure .206 ohms each giving 2.7% voltage drop at max load, compared to 2.4% for the primary.
So the questions:
I intend to use .95 or 1mm dia wire for the primary to give me headroom to increase the output power if needed (>= 358VA for 2 x 100W outputs). There is sufficient winding space to accomodate this although it will make the winding a bit more difficult. The wire comes in 2 x 500g reels so it's tempting to do 1 x 115V + 1 x 115/125V windings. But I haven't managed to find out how these are typically wound on a toroid - one on top of the other minimises the layer to layer voltage stress (5 layers in total for 240V) but the two windings will have differing resistances. I guess the difference would be small enough to ignore. Bifilar winding would mean an unacceptable (?) 115V wire to wire stress all the way through the primary. Otherwise I guess the two windings would each have to be wound back and forth over 180 degrees of the core with insulation between the two windings where they meet. This would be awkward.
1) Primary windows on top of each other?
2) Is it worth considering any interlayer insulation in the primary (I have 50micron kapton tape)? I have never seen this done but I've seen it suggested to minimise the stress on the magnet wire insulation, especially given the increased risk of mechanical damage when hand winding.
3) What rating thermal fuse(s) should I use? I have 2 and 3 amp fuses to hand which should be ample - except that I cannot find any surge ratings for thermal fuses. Chances are that I will need inrush current limiting anyway but it would be a shame to install thermal fuse(s) that could be at risk of failing from the stress of a few hundred power-on surge events.
4) Is there any disadvantage to using 212C rated grade 2 polyester magnet wire compared to 155C enamelled wire? There isn't any price difference.
5) Commercial transformers typically use 180C rated wire and 130C insulation - but why use PVC insulation sleeve for the leadout wires only rated at 105C? The cost saving on the latter must be negligable but it reduces the overall rating to 105C - which seems odd given that 130C thermal fuses are used (Vigortronix).
6) I have an old Farnell XA35-2T supply (dual 35V x 2A) which hums loudly under load. The toroidal is probably around 400VA. It's quiet at no load so I assume the hum comes from loose windings rather than the core. So how important is it to keep the windings tight to prevent this? Typical toroids that I have don't have impregnated windings so I guess they rely on the insulation tape and /or the mechanical rigidity of thick secondary windings in the case of higher power transformers?
Apologies for so many questions; thanks for any useful insights you can provide.
I have removed the 880 turn, .8mm dia. primary winding which was on top of the secondary. At 127m the primary would have been 4.2ohms which is reasonable for a 330VA 240V transformer. Assuming there are 5% additional primary windings to compensate for the voltage drop due to winding resistances (regulation) that gives 3.5turns/V.
The core dimensions are approximately (I haven't removed the secondary yet) 118mm outer dia., 62mm inner x 35mm depth giving a core area 9.8cm^2. At 3.5 turns/volt that implies a flux density of 1.3 Tesla. That seems a bit low by current standards but this is a late 1970's transformer. It weighs 2.8kg
The 2x35V secondary are 1.85mm dia. and measure .206 ohms each giving 2.7% voltage drop at max load, compared to 2.4% for the primary.
So the questions:
I intend to use .95 or 1mm dia wire for the primary to give me headroom to increase the output power if needed (>= 358VA for 2 x 100W outputs). There is sufficient winding space to accomodate this although it will make the winding a bit more difficult. The wire comes in 2 x 500g reels so it's tempting to do 1 x 115V + 1 x 115/125V windings. But I haven't managed to find out how these are typically wound on a toroid - one on top of the other minimises the layer to layer voltage stress (5 layers in total for 240V) but the two windings will have differing resistances. I guess the difference would be small enough to ignore. Bifilar winding would mean an unacceptable (?) 115V wire to wire stress all the way through the primary. Otherwise I guess the two windings would each have to be wound back and forth over 180 degrees of the core with insulation between the two windings where they meet. This would be awkward.
1) Primary windows on top of each other?
2) Is it worth considering any interlayer insulation in the primary (I have 50micron kapton tape)? I have never seen this done but I've seen it suggested to minimise the stress on the magnet wire insulation, especially given the increased risk of mechanical damage when hand winding.
3) What rating thermal fuse(s) should I use? I have 2 and 3 amp fuses to hand which should be ample - except that I cannot find any surge ratings for thermal fuses. Chances are that I will need inrush current limiting anyway but it would be a shame to install thermal fuse(s) that could be at risk of failing from the stress of a few hundred power-on surge events.
4) Is there any disadvantage to using 212C rated grade 2 polyester magnet wire compared to 155C enamelled wire? There isn't any price difference.
5) Commercial transformers typically use 180C rated wire and 130C insulation - but why use PVC insulation sleeve for the leadout wires only rated at 105C? The cost saving on the latter must be negligable but it reduces the overall rating to 105C - which seems odd given that 130C thermal fuses are used (Vigortronix).
6) I have an old Farnell XA35-2T supply (dual 35V x 2A) which hums loudly under load. The toroidal is probably around 400VA. It's quiet at no load so I assume the hum comes from loose windings rather than the core. So how important is it to keep the windings tight to prevent this? Typical toroids that I have don't have impregnated windings so I guess they rely on the insulation tape and /or the mechanical rigidity of thick secondary windings in the case of higher power transformers?
Apologies for so many questions; thanks for any useful insights you can provide.
A static shield, to be effective, would want a broad and short foil, but a toroid does not allow for that, it dictates a quite narrow and long foil.
If you really need a static shield better forget toroid, look instead for SU, SE, or some laminated core where a static shield will be much more effective and is comparable easy to implement.
If you really need a static shield better forget toroid, look instead for SU, SE, or some laminated core where a static shield will be much more effective and is comparable easy to implement.