Replies to Minipulse Plus

[Davor]

Your probe is ~1M impedance, so you may put a divider to go on a safe side. If the probe says 300V, it is probably because of the potential arcing that could damage things.

[Qiaozhi]

Well my FET avalanche voltage is 500V and the scope probes say 300V so I wasn't sure
if I needed a voltage divider or high voltage probe to see what's going on (as it isn't working
so good). The scope is a loaner, I got it because it's previous owner kept blowing channels
up on it, I don't want to do the same...

As it's a loaner, I can understand why you're being cautious, especially since the previous owner has already had similar problems.

In that case, is there any reason why you need to probe the TX voltage directly? Is it just that you want to check if the MOSFET is breaking down?

[Silver Dollar]

Well I built the simplest PI detector and it doesn't work. I want to experiment with different FETs
and adjust the damping resistor so it would be nice to see what I'm doing...

[Qiaozhi]

Well I built the simplest PI detector and it doesn't work. I want to experiment with different FETs
and adjust the damping resistor so it would be nice to see what I'm doing...

Adjustment of the damping resistor is usually done by monitoring the pre-amp output. With a 10k pot in series with a 270R resistor, and both in parallel with a 1k2 resistor; connect this across the coil, and adjust the pot until you achieve critical damping (no ringing in the pre-amp signal). Disconnect the resistor network and measure the resistance, and then fit a damping resistor of the nearest preferred value.

[greylourie]

In step 5 of construction, what if, instead of a drop in voltage at TP3 there is an increase of voltage when bringing a metal object close to the coil ?

[Qiaozhi]

In step 5 of construction, what if, instead of a drop in voltage at TP3 there is an increase of voltage when bringing a metal object close to the coil ?

Eddy currents are generated in any metal that is close to the coil. This has the effect of lengthening the decay time, and thereby causing the voltage to move in one direction only. The result is that all metal targets produce a decrease in voltage at the pre-amp output.

[greylourie]

Sorry, my thinking often skews reality. With improvised coil am getting 30mV difference when a pair of mini side-cutters are brought close.

[Qiaozhi]

With improvised coil am getting 30mV difference when a pair of mini side-cutters are brought close.

Please be aware that the +1V DC offset I measured at TP3 was taken from a single build. The second MPP I've constructed has an offset of -30mV, so it appears that the absolute value will vary from unit to unit. The important thing is that you actually see a voltage drop at TP3 when a metal target is placed near the coil.

[greylourie]

I tried using an oscilloscope. But I could not get a dancing decay like in 6666's video. The curve "shrinks" when a metal object is brought close. Does the attached image look ok ?

[Qiaozhi]

I tried using an oscilloscope. But I could not get a dancing decay like in 6666's video. The curve "shrinks" when a metal object is brought close. Does the attached image look ok ?

Is your scope timebase set to 250us, and the channel volts/div set to 2V?

If so, the amplitude, shape and frequency of the TX part of the pulse looks ok, but the decay curve doesn't start until about 425us. What are the details of your coil?

[KingJL]

Is your scope timebase set to 250us, and the channel volts/div set to 2V?

If so, the amplitude, shape and frequency of the TX part of the pulse looks ok, but the decay curve doesn't start until about 425us. What are the details of your coil?

I would bet that it is not a mono... probably a high inductance dual coil like a dd (which would explain his rising voltage at tp3 with a ferrous target) that was designed for a vlf.

[greylourie]

The coil is 24 turns of 0.5/0.6mm wire. I used some cat5 cable. Had to strip it to remove the mylar /7 strand wire shield, and then reapplied the pvc outer. Made three laps of roughly 20cm diameter, and soldered up and heat shrinked the 7 joints. Getting a very low reading of 0.6 ohm. Checking with online calculators for copper/aluminium, my reading is way off what it should be. I will try to redo the soldering/splicing and see if that makes any improvement.

Oscilloscope settings for the image in post 249 were 0.2mS/Div and 0.2V/Div. Well worn probe with dodgy switch set to x10.

[Qiaozhi]

The coil is 24 turns of 0.5/0.6mm wire. I used some cat5 cable. Had to strip it to remove the mylar /7 strand wire shield, and then reapplied the pvc outer. Made three laps of roughly 20cm diameter, and soldered up and heat shrinked the 7 joints. Getting a very low reading of 0.6 ohm. Checking with online calculators for copper/aluminium, my reading is way off what it should be. I will try to redo the soldering/splicing and see if that makes any improvement.

Oscilloscope settings for the image in post 249 were 0.2mS/Div and 0.2V/Div. Well worn probe with dodgy switch set to x10.

I was close with the scope settings.

With 24 turns of 0.6mm wire on a 100mm radius, the inductance should be 325uH. But it certainly looks like your coil is the culprit.
I assume you don't have an inductance meter?

[6666]

The coil I used for the video was a 6inch 285uH (measured) using .45mm solid copper wire with thin pvc insulation as used in phone cable, with short leads connected to pcb via a short cable.

[greylourie]

Thanks for the replies. The coil leads are approximately 34" long. And the conductor/insulation is very similar to the kind found in phone cable. Unfortunately I have not yet got round to making/buying an Inductance meter. Have re-soldered the "joints", in the coil. Now reads between 1.2 - 1.3 ohms. But have not re-taped up the "weave", so whatever the inductance is - will change when I get round to doing that. With connection of scope on TP3, the image is identical to the one I posted earlier - although there is more deflection when my mini side-cutters are presented.

With scope connected across R1, and after twiddling knobs I have the following images. I had to turn the intensity right up on the scope in order to capture the vertical bit displayed in the pictures.