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I want to pass a current of 20mA through the overwind, as suggested many times on this forum. The overwind measures 136ohm so I calculate it to need 2.7vdc. Is this right, it doesn't seem like a lot compared to what I've read others have used
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Hi Dave,
Your calculation is correct but, alas, I don't think 20ma will do you much good!
The object of the exercise is to generate some heat in the winding to drive out the moisture. Therefore, it's
power that's important, not current.
In the various LOPT's that I've done this to, I have found that about 2 Watts produces a comfortable heat in the winding.
All of my transformers, including the V410 one, have much lower overwind resistances than yours, so require quite large currents to get 2W of heat.
I did my V410 one recently and that had a resistance of 42R when cold. I passed a current of 220ma through it which caused it to get to probably 40-50 deg C. That works out at 2W.
So to get 2W dissipation in your winding of 136R, you need 121ma to pass through it. That equates to a voltage of 16.45V
You could start a bit lower than that and see what temperature it attains after half an hour. Hand warm is what you are aiming for.
Have you decided whether to order proper transformer oil ( as used by Mikey 405) or veg oil? As a short term measure, I'll probably use veg oil in mine for testing purposes.
All the best
Nick.