Replies to Minipulse Plus

[Qiaozhi]

With 10nF cap on in place of coil, i get the following image. Approximately 11.5Volts.

That looks good to me.
Conclusion: your coil is the problem.

I would suggest that you go to your local electronics/electrical store and get a small reel of hookup wire. Then wind 20 turns on a 10" diameter former, leaving the tails long enough (about 12") so that you can connect it to the PCB. Simply wrap the wound coil with electrical tape, and don't bother about shielding. Also, do not twist the wires that connect to the PCB, and make sure you keep the coil away from any large metal objects.

[greylourie]

RE the CRO, connect a second CRO lead to the output of the 555 then set trigger source to CH2, then the full waveform should show up.

Hi mick, all these knobs are doing my head in. I have a port for connecting an external trigger, haven't learnt how to use it yet. When I try running both channels at the same time, I find I don't have enough space on screen to display all information.

When I mentioned my images in #255 - not looking like the one posted by Qiaozhi, I meant mine does not show all the signal (TX off and start of flyback) when scope connected across R1. All I can see in my images is what looks like the pinnacle of flyback, and the drop back down, and the small oscillations on the tail. Counting squares get 275V, but that is only what is shown on the drop down, and I cant count from start and use as a proper baseline because it does not show.

[greylourie]

With 10nF cap on in place of coil, i get the following image. Approximately 11.5Volts.

[mickstv]

Hi Mick, thanks for the tip. A continuous length probably my best bet. I only have about 1.2 m of cat 5 left - don't want to be joining lengths up. Will go see what the local hardware store has.

Those images I posted in #255, are bothering me. I can't get the earlier part to display - like in Qiaozhis image. I am failing to get the scope to display the on-period and then where switch-off occurs.

RE the CRO, connect a second CRO lead to the output of the 555 then set trigger source to CH2, then the full waveform should show up.

[greylourie]

Hi Mick, thanks for the tip. A continuous length probably my best bet. I only have about 1.2 m of cat 5 left - don't want to be joining lengths up. Will go see what the local hardware store has.

Those images I posted in #255, are bothering me. I can't get the earlier part to display - like in Qiaozhis image. I am failing to get the scope to display the on-period and then where switch-off occurs.

Sry Qiaozhi, just seen your reply. Can I use the poly cap from the kit marked 10n ?

[Qiaozhi]

I cant get an image like that. What could be wrong ?

The problem definitely appears to be associated with the coil.

BUT - just to be certain, let's do a simple experiment to test if the circuit is working ok.
Disconnect the coil, and replace it with a 10nF capacitor. Connect your scope across the damping resistor (R1) as before, and look at the waveform. Set your scope probe to x1, as the flyback will not be present.
Here's the result:

[mickstv]

Hi greylourie, if you have heaps of cat5 cable, spend the time and remove the outer insulation and split into the 4 pairs then get one length and undo it so
you have a single strand then wind your coil with it ( I have done this before, but be aware it could take 1/2h or so to do).

[greylourie]

I cant get an image like that. What could be wrong ?

[Qiaozhi]

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.

Here's what the waveform should look like when measured across the damping resistor (R1).
Note how the decay happens over a period of about 5us (even with the original [slow] Minipulse coil). Note the use of a x10 probe. The flyback voltage is measured as 316V.

[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.

[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.

[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?

[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.

[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.

[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?