Mauriscivic's Pulser

[gyulasun]

Hi Garry,

That is ok but can you extinguish fully the bulb when the gate-source DC voltage is 2V or less?  That is important. USe a DC voltmeter to check the Vgs voltage level

Gyula

[garrypm]

Hi Gyula,

Have setup as you recommended plus an mA meter inline.

Bulb begins to glow at around 3.6v then at 3.8 or higher is seems to be full bright (same current draw as direct connect 320mA)

What can we deduce from this?

Thanks, Garry

p.s. I was test with the IRF840's in parallel and just using the drain of the one under test. Interesting that at full pot the circuit draws twice
as much even with the opposite drain disconnected. weird ?

[gyulasun]

Have setup as you recommended plus an mA meter inline.

Bulb begins to glow at around 3.6v then at 3.8 or higher is seems to be full bright (same current draw as direct connect 320mA)

What can we deduce from this? 

The test shows your MOSFET behaves normally and its threshold gate-source voltage is around  +3.5V.

p.s. I was test with the IRF840's in parallel and just using the drain of the one under test. Interesting that at full pot the circuit draws twice
as much even with the opposite drain disconnected. weird ?

Sorry, I do not fully get this.  So you paralleled 2 IRF840s except their drains, right? On "full pot" you mean the wiper is at 12V, so the common gates and common sources get 12V, right?  Do you have the lamp as the load in the single drain which is used?
Normally, if you build the circuit as per Fig. A with your 5W, 12V bulb with a single IRF840 (or with any other power MOSFET) and the wiper of the pot is set to 12V, and then you connect a second IRF840 in parallel with the first FET, the current draw, 320mA must not change, perhaps less than a few %. In fact the current draw is influenced mainly by the lamp: its resistance is 12V/0.32A=37.5 Ohm so that a single IRF840 having say 1 Ohm ON resistance in series with it does not really count much, AND two IRF840s in parallel would have say 0.5 Ohm ON resistance which would count even less than a single IRF840. Now you found that NOT connecting one of the drains of the otherwise paralleled IRF840s draw twice as much current (640mA)?  This is weird indeed and I suggest using a "dead bug" wireing style if you are using a plug-in circuit board during the tests. OR one of the IRF840s (that has the unconnected drain pin) has a fault between its source-gate at 12V gate-source voltage level.  Or I do not know....   LOL   Do you happen to have any other type of power MOSFETs?

Gyula

[TinselKoala]

@Tinselkoala,

Any chance you caould spare a couple of minutes and take a look at that video for me.

It may be that N-Channel IRF840 are not suitable.

Many Thanks. Garry

I looked at the video and I spent some time this evening fooling around with the circuit.
I didn't have a pair of IRF840 mosfets, but I tried IRF830 and a bunch of different coils and transformers and could not get it to work.
Also tried IRFP450 and IRF530n, no luck.
But finally with 18N50 mosfets from a PC power supply, and one particular coil set I wound for a JT, it worked, exactly like shown in the video, but with somewhat different waveforms. It's hard to interpret what's being shown on the scope in the video. The oscillation is audible, a high-pitched whine, and shows up well on my scope.
Unfortunately I wound up blowing out all 4 of my 18N50 mosfets.... the gate spike from my coil set is too high I guess. But the interesting thing is that once it started oscillating, I could reduce the input voltage from 12V down to around 2 volts and it would continue working... the only way I could stop it was to disconnect the power supply.
Also, I found that it works with just the first mosfet oscillating, with the second one removed and a jumper from D to S (turning it into a regular one-transistor JT, I think).
I think if I do this again with mosfets, I'll probably try to incorporate gate protection zeners. I think my mosfets blew from high voltage spikes on the gates.

[gyulasun]

Hi TK,

From earlier setups shown by mariuscivic, the active components he mostly uses are scavenged indeed from PC power supplies or from other appliances which are around.  So chances are he used MOSFET types found in PC supplies. 

Regarding the scope it seems he shows the waveforms across his L1 coil and those waveforms could be ON spikes when both his MOSFETs conduct and they momentaraly switch the battery across L1, say these are the negative spikes under the zero line and the positive spikes just seen next to the very right of the negative spikes are the inductor kick-back spikes when current is switched off in L1.  (We can see single positive spikes too but I assume those are due to false triggering.)

Did you use separate coils in loose mutual coupling?

I agree your FETs were ruined by gate-source junction "puncture" and a zener diode protection is needed.  Mariuscivic may have higher losses in his coils (together with maybe less turns ratio than you had) so his FETs did not get ruined.

Mariuscivic moved his L2 coil away from L1 by at least 3-4 cm to kill oscillations, this maybe analog to your reducing supply voltage down to 2V (though you have to reduce it it zero to kill it).  However in both cases, say when he moves L2 away by 2cm or you reduce supply voltage to say 4V or less and you both do a switch off - switch on sequence on the supply voltage it is questionable your setups have a chance to oscillate under those circumstances.  This is normal behaviour for blocking or other types of oscillators (involves loop gain and phase shift as usual).

Thanks, Gyula