The best location for the damping resistor?

[Tim]

Very good info again thanks gents.

[Ferric Toes]

I checked some other resistor sizes that I frequently use. The test apparatus is basically a low power PI that has a peak TX current of 0.3A into a 40mm diameter solenoid type shielded winding of 200uH. The pre-amp is a dual type followed by differential gates and integrator. Then follows a dc amplifier into a LCD voltmeter module scaled to read 2000mV max. Sample delay was an accurate 10uS from TX switch off as this is what is aspired to in many designs for good sensitivity to small objects. Each resistor was lowered vertically on the axis of the solenoid to get the point of maximum reading. Even the smallest resistor gives 20mV. This response for all the resistors is due to the metal end caps which are clearly seen in KT315's picture.

The picture shows the resistances used and the reading on the LCD module in millivolts. The figure of >3500mV for the 2W resistor represents the saturation level of the dc amplifier and was measured on an external testmeter as it was out of range on the LCD module on the test rig.

Eric.

[Tim]

So, Eric. Does this mean we are best to use the smallest damping resistor wattage possible for detecting small/low TC objects? Perhaps it is best to sacrifice a few measly resistors at the risk of not finding a burnt one from time to time, rather than miss all the little bits of gold? Also is 4x1K5(1/4W) better than 1x330R(1W) ??
I am gathering the lowest practicable wattage would be best-this is just an example, as I realise coil current will limit the size of wattage reduction practicable.
A very informative experiment, thank you.

[Ferric Toes]

So, Eric. Does this mean we are best to use the smallest damping resistor wattage possible for detecting small/low TC objects? Perhaps it is best to sacrifice a few measly resistors at the risk of not finding a burnt one from time to time, rather than miss all the little bits of gold? Also is 4x1K5(1/4W) better than 1x330R(1W) ??
I am gathering the lowest practicable wattage would be best-this is just an example, as I realise coil current will limit the size of wattage reduction practicable.

Not really, as it depends on the characteristics of the detector i.e. coil current, pulse rate, pulse width, back emf amplitude. The damping resistor can get quite hot and be subject to 500V, or more in some cases. In most cases I would use a metal oxide film resistor of at least 1W and mount it on the circuit board. If it gets too hot to touch, go for a larger wattage. Alternatively, the wattage can be calculated if you know the previously mentioned characteristics.

A trimming fixed parallel resistor could be housed in the coil connector if there is room. My view on PI's is not to put resistive damping in the coil as it can cause problems. On the circuit board they give no measurable signal even up to 5W size.

Eric.

[Tim]

Thanks for the clarification Eric.

[nonkapo]

So shuld i read this as either on the box coil connector or the board it self it does not get detected?
Thanks

[dbanner]

That seems to be the gist of it.

[beiduo]

https://www.youtube.com/watch?v=pLSXdXRjsz8
How White V3i connects Garrett's 2500 Hawk Eye part