Self running coil?

[Hoppy]

Hi Gyula,

Thanks for expanding on my observation. I should have added that I have a freewheel diode across the coil, as in my application the coil is operating a heavy duty DC contactor coil with a resistance of 20R. My pulse rate is slow at about 1 Hz. The mosfet is an IRFZ48N (Crss - 200-270pf), which as you say is high enough to account for the effect.

Hoppy

Hi Hoppy,

This is an interesting observation and you nicely described what you found,  however, some further notices may be in order.

The energy coming from the drain side back to A is caused by the drain gate interelectrode capacitance and depending on the type of the power MOSFET, this capacitance can be quite high to be able to cause the situation you described.  IF you choose another MOSFET type that has a few pFs only between the drain gate, then this situation cannot really happen as severely as you found, of course this also depends on the working frequency too. In Luc's circuit the frequency has been only as high as 50-60kHz or so and the IRF640 drain source cap value, C_rss is about 53pF at 25V drain voltage, this gives about X_L=40-50 kOHm capacitive reactance so high that significant current cannot really flow through it. Especially when there are no voltage spikes at the drain side because of tuning for resonance there, I think of the lack of high voltage difference between the drain gate -- this is the second reason why in Luc's circuit this is not experienced. (of course if he did experience it, it would be an unwanted phenomena because it would steal energy from the output resonant circuit.)

So the energy coming back via the C_rss also depends much on the instantenous drain voltage wrt the gate, and also, the higher this cap value, the bigger the energy coming back, at a given frequency.

You surely can recall other members finding that the body, (the casing) of the 555 integrated circuit sometimes gets heated up or suddenly simply go wrong after working for a few minutes, when directly drives the gate of MOSFETs that switches a coil via the drain, especially if the coil at the output has a resonant frequency that is far from the input drive frequency, hence big, nasty voltage spikes can couple back to the gate to find the output circuits inside the 555, beyond its Pin 3. 
While the amount of the unwanted energy coupled back can really supply a 555 as it were a separate power supply, I think it is an unwanted situation in most of the applications and you would have to make sure to limit the amount of the current coupled back, afterall it steals from the output energy, and make sure the 555 or the 15V Zener for instance will not overdissipate.  Also, when you wish to switch ON such a A circuit that gets its supply from the drainside, it is not at all a 100% sure it can start again after a switch OFF because it would have no or very little supply voltage due to the initial lack of the big spikes at the drain side (a static 30V at the drain cannot couple back much current via the drain gate capacitance to start up the 555).

Sorry for the long 'rambling' I felt it interesting to discuss.

rgds,  Gyula

[MeggerMan]

Hi Rob,

Re on common mode choke: you are correct the winding technique Luc used for his toroidal coils is really the one as the so called common mode chokes are made BUT the big difference is the way how they are connected: Luc connected the two coils in series aiding i.e. the MUTUAL inductances of the two coils add to the sum of the individual inductances in series,  so that the resultant inductance is nearly the 4 times of a single coil, L_resultant=(L₁+L₂+2M where L1=L2=L in the equation (the two coils are assumed to have the same inductance which is nearly true and M is nearly L because in ring cores the coefficience of coupling is nearly 1 ).  (A useful link on this is here: http://www.daycounter.com/LabBook/Mutual-Inductance.phtml )

I hope this helps clarify your doubt above.

rgds,  Gyula

Hi Gyula,
yes, I did a drawing on paper and showed the current direction and the field direction and it makes sense now, its because the  windings are wound in different directions that threw me, but in essence the you can wind the coils around the core in either direction. Its direction of the current flow for a piece of wire laying on the core that determines the field direction.

On the low power front, I have some ICM7555 low power timers to test out.
http://www.intersil.com/data/fn/fn2867.pdf
Around 40uA supply current then its up to you to pick and choose the right timing capacitors and resistors to keep any additional power required to a minimum.
Also I have a LTC4446 mosfet driver to test out.

Rob

[gotoluc]

What do you get if you take out the pickup coil in the above test (still -.000006 or less or more), and how does the scope shot looks like then?

--
NextGen67

Hi NextGen67,

I removed the pickup coil and the meter went to -.000025 and the scope shot is the fist one below.

I attached the previous scope shot below it (with pickup coil) for comparing.

Luc

[gotoluc]

@ everyone,

what do you think of the following mosfet specs.

www.irf.com/product-info/datasheets/data/irlml2502.pdf

www.irf.com/product-info/datasheets/data/irlml6402pbf.pdf

www.irf.com/product-info/datasheets/data/irf7700.pdf

I'm looking for a 20v mosfet that I can buy on ebay and the above are available

Luc

[gotoluc]

Hi Luc,

Did you add the capacitance in parallel with the toroidal coil?

Yes Gyula, the capacitance is in parallel with the toroidal coil (480pf)

Did you have any capacitance in the pickup coil circuit to make it resonant at the toroidal coil's lower resonant frequency?  The two freqs have to be the same to get the most output.  Hint: at the best setup as you wrote above (150mV across 100Ohm etc), try to place capacitors in series with the 1KOhm load to reach a resonant pickup coil situation (or as Mark suggested, connect it in parallel and use an audio transformer to step down the output power from the pickup coil), to approximate best a matched condition between the pickup circuit as a whole and the 1k load.

Thanks,  Gyula

No, I did not add capacitance on the pickup coil. I tried quickly but could not find something that worked. This needs to be experimented with and something I'll look into.

Luc