Well, an interesting and expensive outcome follows!

I connected the Mosfet to my circuit as described in the video posted above by "cmartinez", and did omit the zenor diode as mentioned by "crutschow". Stupidly however I didn't install an inline fuse, to my expense...."doh"!! But never again.

When I connected in the Mosfet modified circuit to the battery, and the correct way round to make sure the circuit worked as before, there seemed to be a larger spark momentarily when I connected the second crocodile clip to the battery. The circuit didn't work.

I then removed the Mosfet part of the circuit so it was as before, but the circuit still doesn't work. The power LED does light up but that's it, the pulsing is "dead".

I am no expert and have just basic knowledge of circuits. I also have no idea how to troubleshoot this problem, even if the circuit is recoverable.

One thing I would love to hear is, what could have happened?

Cheers




connected the battery the correct way round

 

Sounds like something was misconnected.
Since the transistor is almost always faster than the fuse, adding a fuse to your circuit may not have helped.

Post the exact circuit configuration you used.

 

Thanks "crutschow" I'll post what I can.

One thing I forgot to mention in my last post: I connected the Mosfet body to the aluminium chassis ( V- ) as a heat sink. Could this have been the problem?

 

I connected the Mosfet body to the aluminium chassis ( V- ) as a heat sink. Could this have been the problem?

Possibly.
The drain of a MOSFET (unless it's an insulated case) is normally connected to the package tab, so that would connect the drain to V-.
Thus, if you used a P-MOSFET in the positive line, you shorted the V+ to V-.

The MOSFET shouldn't need to be on a heatsink if it is a power type with a typically low on-resistance.

 

The MOSFET is an FQP47P06 P-Channel -60 V, -47 A, 26 m ohms, so from what you said, yes I must have shorted +12v directly to -12v. I connected the Pulse circuit to the MOSFET circuit where the load is indicated. Then with the negative lead connected to the battery, I momentarily connected the positive lead to the battery which is when I noticed a larger spark than usual.

So it would appear that I've damaged the MOSFET, but what puzzles me is why the pulse circuit now no longer works with the MOSFET disconnected?

Any ideas would be appreciated?

This is the protection circuit:

The pulse curcuit:

Parts list:

 

A lot of fence chargers generate the high voltage pulse by discharging a capacitor through a transformer primary. This provides an intrinsic safety benefit in that the shock is only momentary, which is all that is required to startle cattle and horses. It also is good for keeping rabbits out of gardens. At this hour it is not obvious to me as to how to add that feature to the circuit shown. Perhaps when I wake up it would be.