The best location for the damping resistor?

[beiduo]

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

[dbanner]

That seems to be the gist of it.

[nonkapo]

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

[Tim]

Thanks for the clarification Eric.

[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]

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]

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]

Very good info again thanks gents.

[kt315]

in reality a resistor is spiraled track done around a ceramic base. it is inductivity, but with very low value.
and yes, it can receive EMF and retransmitted it back.

[Ferric Toes]

I mostly put the damping resistor on the circuit board and only occasionally in the connector on low power PI's. I don't put them in the coil housing for the reason shown in the plot. The fast decaying trace is for no object in the coil, while the top trace is for a 1 watt metal oxide resistor, which is the type I generally use for damping. As you can see the resistor generated quite a substantial signal due to its internal metal endcaps. Higher wattage resistors are considerably worse. I like to start off with as flat a response as possible out of the preamp, without any superimposed metal signals. The Vallon VMH3CS detects this 1W resistor a couple of inches off the coil.

Eric.

[dbanner]

I used damping tool circuit on my home built surf and just left it in as permanent. So I can vary from 381 to 1.6k depending on coil I use. I'm always making new coils for experimental purposes, trying to find what works and what doesn't work best, so variable damping is convenient when intending to use multiple coils. I can return to a preset position on tool for any previous coil.

[Tim]

Another fabulous insight from BB Sailor - Thank you Joseph. Although I get good results with my SMpro(DD Tacoma) with a ML commander DD, it was fitted with a 390R. I think it was Homefire or 6666 that alerted me to the fact that the Commander has an internal damper, so it will be under-damped with both connected. Therefore next time I dismantle it I will fit a switch on the 390R, so I can use the ML/Coilteks as well as Homemades.
I think I remember reading here somewhere that a wirewound pot would be no good as an inbuilt variable damping resistor because of changing inductance, but a 2(?) watt wiper type - would that work? Perhaps 1K in series with a 1K2x2W resistor to reduce current through the pot??

[nonkapo]

Here's the Minelab patent ->

So it sounds like shielding the soldering joints?!?!? I was expecting something more sofisticated;-).

[dbanner]

Incidentally, the Fisher impulse schematic shows damping resistors (162R fixed plus a 500R trimmer) located on pcb at loop connector. There are also internal resistors in the
Search head. I suppose this is the best of both worlds.

[Qiaozhi]

Here's the Minelab patent ->