The best location for the damping resistor?

[nonkapo]

Good evening to all. Would like to ask for your oppinion where is the best place for the damping resistor electric wise(ignore the practicality)?
The coil, the socket or the board? Should it be close to the coil or close to the FET?
Thanks

[waltr]

Good question.
Most common is on the PCB near the MOSFET/Pre-amp input R & clamp diodes.

In theory at the Coil would be the best place but I have a feeling that it does not matter. Reason is the R damps oscillation in the coil 'system' which includes the cable & MOSFET.

I place one damping R on the PCB near the MOSFET and a second one on the coil cable connector. The first one is large and the second in parallel allows damping of each coil. This way each coil's total damping resistor can be different.

Each of my coils work well, A 27cm mono, a 20cm concentric and 30cm DD.

[nonkapo]

Thanks. I've noticed somewhere in the treads that someone mount it in the coil, and on some pictures i spot it in the DIN connector. The points to ask about for me are first - how likely the resistor can me "sensed" by the coil? Second is the practicality as if i'm planning few coils for different purposes it will be easier to have the resistor on the coil rather than switching between or resoldering on the board. So your point about the coil system may reveal it's better to solder the FET at the coil as well (hypotetically)which will probably bring another issues. I'm stopping here for now and will come back to the topic when i reach the point of building the coil.

[kt315]

I've noticed somewhere in the treads that someone mount it in the coil
---
are they specialists of Whites? of Garrett? of Tesoro? of First Texas? not. you see only anti-specialists with low
level in electronics and do the conclusions.

[Andi68]

And you kt315 are one of the lighthouses with specialized knowledge

[kt315]

guys

before to get conclusions read INSIDE THE METAL DETECTOR. this book not for me. was done for you. specialized knowledge are always in specialized
books. you know this fact best from yours university course.

[Andi68]

[Qiaozhi]

There are many places where you can fit the damping resistor, and each one has its pros and cons.

1. Inside the coil.
2. Inside the coil plug.
3. On the back of the coil socket in the electronic enclosure.
4. On the PCB.
5. Or combinations of the above.

If you fit the damping resistor inside the coil, this has the advantage that coils can be easily swapped. However, it may prevent the coil from being used on another detector.

Some people have put the resistor inside the coil plug, but quite often this is not practical due to the limited space available.

If you use a multi-pin coil connector, one flexible solution is to put different value damping resistors on the back of the connector. By wiring the appropriate pins together inside the coil plug, different damping resistors can be used with different coils.

Putting the damping resistor on the PCB is probably the least flexible solution, as each coil would need to have similar characteristics.

The solution posted by waltr uses a combination of points 2 and 4 above, and may prove difficult for begineers to implement.

Personally, I've tried all the variations above over the years, and tend to go for point 3 as the easiest and most flexible solution. Keeping the damping resistor out of the coil and the coil connector, allows the same coil to be used on other detectors.

[waltr]

Is the conclusion that where the damping resistor is placed does NOT effect performance but only construction/adjustment and coil swapping?

I have one detector and three coils, two of which are IB (TX/RX coils) so combination of 2 & 4 works best here.

[kt315]

IB?? PI coil with splitted windings does not balanced.

[waltr]

IB?? PI coil with splitted windings does not balanced.

Two windings- one TX, one RX. Adjust to Balance by watching Pre-amp output on a scope- move coils until you get a NULL.
Explained here: http://www.geotech1.com/forums/showt...237#post238237

[Carl-NC]

Is the conclusion that where the damping resistor is placed does NOT effect performance but only construction/adjustment and coil swapping?

I suppose it can. RD dampens not only coil parasitics but also cable & FET parasitics. Which side of the cable you place RD may slightly affect performance, but I would expect this to be minor. The biggest issue is coil swapping, and putting RD in the unit instead of the coil means a one-size-fits-all damping experience. If you are trying to run the sample delay on the hairy edge of too fast then putting RD in the coil may be better.

[sinclairuser]

The benefit of putting it in the plug or the coil itself is that each coil can be critically dampened, as carl said above (in so many words).

For most of the basic projects i suppose it does not matter since winding coils to within a certain inductance and ohms region is not that hard especially for PI, its one of those cases where not all PI coils need to be critically dampened but all PI coils benefit from being critically dampened.
some projects need it, while others its "gilding the lilly" but its never a waste of time.

[homefire]

For simple practicality I like and do the Connector Idea. I can easily tune the coil for another detector in minutes. And in the Same logic the Detector is not Locked to one coil.

[kt315]

For simple practicality...... if i have three coil with 520 550 560 kHz resonance i can set one damp resitor on board and forget.
if i have a coil with different resonance.... i have to think that coil is bad.
board position is also good for heat removal.