Criticall damping

[Vladimir Minkov]

Anybody knows how to choose damping resistor for PI detector?

[Robert Hoolko]

Re: Criticall damping

Vladimir

To calculate the required damping resistance you would have to know the inductance of the coil and the total capacitance in parallel with it. You probably will not know exactly what the capacitance is.

You might use a variable resistor of a few hundred ohms and adjust it to give the best damping while watching the output of the amplifier. Then you can measure the variable resistor and replace it with a fixed resistor.

Robert

[simeon webber]

Re: Criticall damping

use a simple pwm to voltage converter to drive a npn power transistor
to adjust the damping in RT

[Vladimir Minkov]

Re: Criticall damping

Thanks,Robert!
I have mesured first inductance and after with voltage generator, resistor 47 kOhm,coil and oscilloscope,changing frequency I discover a self resonant frequency.Now is easy to find total capacitance without forgeting for osciloscope input capacitance.
Vladimir

[Eric Foster]

Re: Criticall damping

Just a word of caution when using variable resistors. The high voltage back emf on switch off can damage the contact between wiper and track. I have observed arcing taking place at this point, which destroys the contact and everything becomes total noise. OK for low power TX's, but for high current high voltage types, it is better to swap fixed resistors in and out. Even here, I have known carbon film resistors fail when a repetitive 500V spike appears across them. Use metal film of adequate voltage rating, even two or three resistors in series if need be.

Eric.

Eric.

[Robert Hoolko]

Re: Criticall damping

Vladimir

If you know the ringing frequency then the required damping resistance is approximately 4 pi L f0. Or if you know the capacitance R2 = 4 L / C.

As you say, you have to take into account the effect of the probe capacitance on the ringing frequency. You also have to take into account the capacitance of the drive transistor and any other snubbing device as well as the wiring.

Robert

[Robert Hoolko]

Re: Criticall damping

Eric

I am glad you mentioned that. I was thinking of asking you if the wiper ever gets burned.

I also wondered if you adjust the damping for the coil in the air or on the ground, or is the difference so small that it does not matter.

Robert

[Carl]

Re: Criticall damping

>Even here, I have known carbon film resistors fail when a repetitive 500V spike appears across them. Use metal film of adequate voltage rating, even two or three resistors in series if need be.

Thanks, I hadn't thought of carbon film being a problem. I do use pin sockets on this and other components, to make swapping easy while tweaking.

- Carl