Looking for a good test rig for power transformer testing
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Nate
Power transformer test rig
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martin manning
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Re: Power transformer test rig
What are you wanting to test? A multimeter is usually sufficient to figure out what windings, voltages, and DC resistances are. Shorted turns can often be found using a neon tester. If you want to test under load that is more complicated.
Re: Power transformer test rig
To build on what Martin says about testing under load, unless you build a rather sophisticated (and probably expensive) rig, one problem is that you essentially have to test for an intended application and hope you don't let the smoke out.
The primitive version that I've used calls for a plank of wood or an old chassis. Make a FW solid state diode rectifier with a couple of caps, and add some terminal strips to tack on high wattage sandblock-type wire wound resistors as the load across the rectified secondary. The lower the ohms, the more you tax the transformer (more mA). First make an educated guess at the mA limit, start with a higher ohm load and work your way down to your target. All while stay very alert for the burning smell, which will always precede the smoke. You have very little time to shut off the power once you smell the burn, so don't hesitate. If your nose is clogged with a cold, that's the wrong time to do this. I've left out many details, like using a fuse and a switch, etc. It's useful to employ screw-type terminal blocks that all you to change out parts. Essentially, build the power section of an amp, but no tubes.
Finding the required load is a simple application of Ohm's law. For example, you have a PT that puts out 300-0-300 with no load. You want to use it in a circuit that requires 150mA. I guess with FW SS rectifier 1.4*300*95% = 399, so I round to 400VDC. 95% is a guess at voltage drop under load and this can very a bit depending on how hard or soft the secondary winding is. 400V * 150mA = 60W. 60W is a constant that will guide you from here in choosing resistors of adequate rating. Apply Ohm's law, R=V/I; 400V/.15A = 2667Ω. Now you have your target load. I'd probably start at 10K. You can measure the voltage across the 10K and by moving the decimal point you get a very quick mA read on your first test. (From here, you may want to adjust your test plan is it is too far off from the expected result; 400V/10000Ω = 40mA.) Then, for example, 5K should show you 80mA, 3K should show you around 133mA, If it seems good so far and the PT isn't burning hot to the touch, you can go down to your target load and then you know you can use it for your intended circuit.
A modest selection of 25W and 10W sandblock resistors should allow you to find a combination of values that give enough voltage protection and the ohms you want. You'll be combining them in series and parallel for the load.
If you want to know the full capacity, you've got to keep reducing the load in small increments until you smoke the PT. Then, the one before you smoked it, that's the answer. Of course, you'll need to have it rewound, so what's the point? IOW it's really hard without sophisticated test gear to get to the maximum load and preserve the PT.
A point and shoot type thermal detector that gives you an immediate readout on the PT's external temperature can be a very useful monitoring tool. I'd say, as a rule thumb, you should be OK up to around 200F or nearly 100C. Both of these are too hot to touch, but your PT can probably operate that hot without melting anything. Personally, I don't think I'd want anything besides the tubes at such a high temperature, but you'll have to choose and there's no guarantee.
Keep a fire extinguisher nearby just for good measure. Better safe than sorry. Are you sure you want to do this?
The primitive version that I've used calls for a plank of wood or an old chassis. Make a FW solid state diode rectifier with a couple of caps, and add some terminal strips to tack on high wattage sandblock-type wire wound resistors as the load across the rectified secondary. The lower the ohms, the more you tax the transformer (more mA). First make an educated guess at the mA limit, start with a higher ohm load and work your way down to your target. All while stay very alert for the burning smell, which will always precede the smoke. You have very little time to shut off the power once you smell the burn, so don't hesitate. If your nose is clogged with a cold, that's the wrong time to do this. I've left out many details, like using a fuse and a switch, etc. It's useful to employ screw-type terminal blocks that all you to change out parts. Essentially, build the power section of an amp, but no tubes.
Finding the required load is a simple application of Ohm's law. For example, you have a PT that puts out 300-0-300 with no load. You want to use it in a circuit that requires 150mA. I guess with FW SS rectifier 1.4*300*95% = 399, so I round to 400VDC. 95% is a guess at voltage drop under load and this can very a bit depending on how hard or soft the secondary winding is. 400V * 150mA = 60W. 60W is a constant that will guide you from here in choosing resistors of adequate rating. Apply Ohm's law, R=V/I; 400V/.15A = 2667Ω. Now you have your target load. I'd probably start at 10K. You can measure the voltage across the 10K and by moving the decimal point you get a very quick mA read on your first test. (From here, you may want to adjust your test plan is it is too far off from the expected result; 400V/10000Ω = 40mA.) Then, for example, 5K should show you 80mA, 3K should show you around 133mA, If it seems good so far and the PT isn't burning hot to the touch, you can go down to your target load and then you know you can use it for your intended circuit.
A modest selection of 25W and 10W sandblock resistors should allow you to find a combination of values that give enough voltage protection and the ohms you want. You'll be combining them in series and parallel for the load.
If you want to know the full capacity, you've got to keep reducing the load in small increments until you smoke the PT. Then, the one before you smoked it, that's the answer. Of course, you'll need to have it rewound, so what's the point? IOW it's really hard without sophisticated test gear to get to the maximum load and preserve the PT.
A point and shoot type thermal detector that gives you an immediate readout on the PT's external temperature can be a very useful monitoring tool. I'd say, as a rule thumb, you should be OK up to around 200F or nearly 100C. Both of these are too hot to touch, but your PT can probably operate that hot without melting anything. Personally, I don't think I'd want anything besides the tubes at such a high temperature, but you'll have to choose and there's no guarantee.
Keep a fire extinguisher nearby just for good measure. Better safe than sorry. Are you sure you want to do this?
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Stevem
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,,,,
Do you mean to just test if it's not shorted?
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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stretch2011
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Re: Power transformer test rig
Phil:
That's some great stuff. I think it would be a little simpler if you use a 50-100 watt variable resistor though.
That's some great stuff. I think it would be a little simpler if you use a 50-100 watt variable resistor though.
Re: Power transformer test rig
Thank you but it's not original, so I can't take much credit. A variable resistor is a great idea. However, I think we are talking about a very expensive part because it needs to be rated for at least 50W. I'm thinking $80-$100 for a wire wound 100W rheostat. Do you have any idea who stocks such a thing and what it actually costs? As usual, I'm having trouble with of Mouser's filter system. IME you can buy a handful of generic 25W sandblock resistors for $1-$2 each, so that's what I use.stretch2011 wrote:Phil: That's some great stuff. I think it would be a little simpler if you use a 50-100 watt variable resistor though.
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stretch2011
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Re: Power transformer test rig
They are around the $20-30 mark. I get mine from a liquidation outlet. Got 2. 200W for the air brake for $8 a piece.
Re: Power transformer test rig
A good idea maybe an electronic load. The schematic is for low voltage but can easyly be changed to high voltage. The opamp needs approximately +9 volts. You need a LM358 however, a TL072 or RC4558 wont work. If you take a high voltage mosfet you can use it for a HV powersupply. A few diodes and a few electrolites and voila. You can see the load in mA by measuring the voltage across the 1 ohm resistor. The mosfet needs to be cooled with a heat sink and/or a fan.
There is a catch however: high voltage + mosfet = oscillation. So you may need a few capacitors to kill this. An oscilloscope might come in very handy.
I built this a few years ago and if you need more details I have to open up my loadbox since I didnt make a schematic.
Greetz,
Gait.
There is a catch however: high voltage + mosfet = oscillation. So you may need a few capacitors to kill this. An oscilloscope might come in very handy.
I built this a few years ago and if you need more details I have to open up my loadbox since I didnt make a schematic.
Greetz,
Gait.
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martin manning
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Re: Power transformer test rig
Nice Gait, thanks for posting. I think adding a gate stopper for the MOSFET would be a good idea, and might be enough to prevent oscillation.
Re: Power transformer test rig
The best response here is "what are you testing?"
In general, you can see if a transformer is dead with an ohmmeter and a neon tester. All windings continuous where they should be, open where they should not connect, and no internal shorts as tested by the inductive dump test with a neon tells you it's almost certainly not defective.
If you want to test voltage ratings, it gets tricky. You have to measure/compute the voltage ratios, and to do a good job run one winding up to the edge of saturation by watching the instantaneous magnetizing current on a winding for when it starts up. That lets you detective-ize an unknown.
If you want to test overall loading power on a single-secondary one, you run it up on a load until the external temp is no more than 130F, that corresponding to the external temp that, on average, keeps the internal temps under Class A/105C insulation.
It is POSSIBLE that the internal insulation materials are class B/130 or even up to class H/180C and the thing would be happy at temps that would - literally! - fry an egg on the exterior surface. But you can't know that unless it's printed on the outside of the trannie or you tear it down.
If you want to wring the last watt out of a multi-secondary trannie, you have to load each winding to a reasonable temperature.
That's not impossible to figure out. The resistance of copper increases a bit with its temperature, so you can use the internal copper for a thermometer, by measuring winding temps very accurately with the unit is dead cold, then heating it up til it stabilizes (which may be hours; the're massive) and reading the temps from each winding, jiggering the loading until the hottest one is just at 105C, or whereever you think you can get away with it.
In general, you can see if a transformer is dead with an ohmmeter and a neon tester. All windings continuous where they should be, open where they should not connect, and no internal shorts as tested by the inductive dump test with a neon tells you it's almost certainly not defective.
If you want to test voltage ratings, it gets tricky. You have to measure/compute the voltage ratios, and to do a good job run one winding up to the edge of saturation by watching the instantaneous magnetizing current on a winding for when it starts up. That lets you detective-ize an unknown.
If you want to test overall loading power on a single-secondary one, you run it up on a load until the external temp is no more than 130F, that corresponding to the external temp that, on average, keeps the internal temps under Class A/105C insulation.
It is POSSIBLE that the internal insulation materials are class B/130 or even up to class H/180C and the thing would be happy at temps that would - literally! - fry an egg on the exterior surface. But you can't know that unless it's printed on the outside of the trannie or you tear it down.
If you want to wring the last watt out of a multi-secondary trannie, you have to load each winding to a reasonable temperature.
That's not impossible to figure out. The resistance of copper increases a bit with its temperature, so you can use the internal copper for a thermometer, by measuring winding temps very accurately with the unit is dead cold, then heating it up til it stabilizes (which may be hours; the're massive) and reading the temps from each winding, jiggering the loading until the hottest one is just at 105C, or whereever you think you can get away with it.