R11 (damping-R) in coil-housing?

[Pilot68]

Hi together,

what about placing the damping-resistor R11 directly at the coil in the coil-housing?
Then it would be very easy to test different coils without changing something on pcb.

Or is there any effect that recommend R11 on PCB (Carl?).

I think in some commercial coils theres the damping-R also in the coil-housing...

My hammerhead will be µP-controlled, so that its easy to match all times with the different coils.

Thanks for answers.

[bbsailor]

Pilot68

Placing the damping resistor in the coil housing locks you into a final design of both your coil and PI circuit. Let me explain. The damping resistor value is based on the total capacitance that the coil sees in parallel with it in the following key areas:
1. The turn to turn capacitance of the coil based on the insulation thickness, dielectric constant and tightness of the coil bundle and final coil inductance.
2. Length of coax from the coil to PI circuit in the control box. Some hip- mount control boxes require about 2 to 2.25 meters of cable which translates into about 100 pF per meter or about 200 pFs total. A control box mounted on the shaft can get by with about 30 inches of cable with about 65 to 75 pF of capacitance.
3. The output capacitance (COSS) of the coil driver MOSFET can vary from about 700 to 1000 pFs for some older MOSFETS to a low of 35 to 70 pFs for some newer MOSFETS.
4. Small capacitance of the clamping diodes, about 4 to 5 Pf each.
5. The coil shield will impose what is called "coil-to-shield" capacitance in the order of from 125 to several hundred pF.

Add all these capacitances contribute to the value of the damping resistor. More capacitance needs a lower value damping resistor which also reduces coil sensitivity somewhat. Higher capacitance also limits the minimal delay that the PI circuit can potentially use. Higher capacitances usually mean longer delays are needed to keep the coil from locking up.

Reg made a suggestion a while back that makes good sense. Place a higher value damping resistor on the circuit board, such as a 1500 ohm metal film .5 to 1 watt. Then place a smaller .25 watt resistor in the coil connector that tunes that particular coil to the PI circuit. Typically this resistor will be between 1000 and 6000 ohms which will cover a combined parallel damping range of from 600 ohms to 1200 ohms. This way each coil can be induvidually damped or tuned when any capacitance contributor (1 to 5 above) is changed.

As an alternative, to Regs recommendation you could place a 1500 ohm, (.5 to 1 watt) on the main circuit with a small .5 watt 5K ohm pot next to the coil connection. Then, attach the damping adjustment pot to the two coil contacts with a 1K ohm(.5 watt) resistor in series with the pot to limit the lowest pot value. This combination will cover an effective damping resistor value of 600 ohms to 1200 ohms. Just do the parallel resistor math to see for yourself (1000 ohms in parallel with 1500 ohms and 6000 ohms in parallel with 1500 ohms). Then put a small numbered dial or dial plate on that adjustment pot and dial in the appropriate pot scale value for each coil. This allows you to continue to modify your PI machine, change coax cable length, change the MOSFET, wind the coil with a lower inductance or Teflon wire, for seeking low conductivy target where low delays and special coil construction techniques are more critical.

If you are happy with your Hammerhead design and coil performance then seal the damping resistor up in the coil but understand the consequences.

I hope this helps.

bbsailor

[Pilot68]

Thanks

bbsailor

Thank you for reply and i think i'll follow Reg's idea, placing one part of R11 on
PCB and one (Pot and R) at the coil for first tests.

[bbsailor]

Pilot68,

Putting the damping pot and resistor near the coil might not work well. Try putting it in the PI control box, right next to the coil connector so you can fine tune the damping resistance for each coil and smallest pulse delay. Make sure that the pot is large enough to not exceed it's wattage rating otherwise it's center contact may arc if you have a high peak EMF pulse.

bbsailor

[Pilot68]

First PI-Detector

Hi all,

i have completed my first PI-detector today, it's not my hammerhead but a project from a german website www.pulsdetektor.de. Name of the Detector is Minipuls 2. Only Pulstime, Threshold an Gain are adjustable (Delay and Sample-Timing is fixed and done by a µP) but i must say, for my first one it works!

With a selfmade Coil 22cm x 15cm oval shape i detect a 1 eurocent at about 22cm (in the air and inside home). The coil is made of enameled wire 0,25mm², 24 windings and shielded with self adhesive aluminium-tape.

Now i work a bit faster on my hammerheads...

@bbsailor

I did the adjustment of damping-R with splittet R's like Reg. Should be no problem, but i'll test also performance with Damping-R directly at electronic-board.

Again sorry for my (german-) english...