Re: Solid-State Marx (fwd)

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Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Mon, 18 Jul 2005 14:08:20 +0930
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Solid-State Marx (fwd)

Thanks to all who replied on this thread; all comments considered and
taken on board.

1) I like the diode idea from the defib.  Is this the Fitch
configuration that Jim mentioned?
2) The repetition rate is low - triggering is manual.
3) I don't know what the instantaneous current is because I don't know
what the load resistance will be; I'm a little concerned about the ESR
of the caps - would a heavy discharge through a string of them cause
them to 'pop'?

I suppose I should have explained myself better in the first place, so I
will now set out what I'm trying to do and why:

I have a problem with snails; this area gets lots of small, white
snails.  They come in plagues and cause all sorts of problems for the
grain farmers, getting in the crop and damaging augers and other
transfer equipment.  My problem is snails in my vegetable garden.  I
can't use poison - we have dogs.  My most effective way of dealing with
them is picking them up one by one and whacking them with a half-brick -
not very elegant, time consuming and requires a lot of bending over.

My idea (and I'm sure to get hate mail now from the Mollusc Protection
League) was to have an HV source built into a wand (PVC conduit) with a
couple of electrodes, about a snail's width apart.  A manual trigger at
the top would dump a few Joules through the snail, cooking it.  (Sadly,
these aren't the sort that can be served with garlic and herb butter.)

Idea #1 was to use just a disposable camera flash mechanism and dump the
250-300V from the cap straight into the snail, but I thought that there
would be insufficient voltage to puncture the shell, even if one of the
contacts was on the "wet end".  That's when I got the idea of the
photo-flash Marx bank.

You know, I had never actually pondered the workings of solid state
relays before and certainly didn't realise that they used a photovoltaic
device (I always assumed that gate drive was acquired from the line
somehow.)   The photovoltaic solution is certainly one way to get power
to the gates - the biggest problem in my mind.

I have two or three SSRs on my bench - I may do a test on this.  They
must be rated up to 340V DC if they are running on 240V AC, so would be
of roughly the same voltage rating as the caps.  1N4007s can handle the
charging for me so I could conceivably run the whole thing off a 4 cell
NiMH pack.  I would be at least 10 seconds between snails (note: must
wear goggles in case of flying shell shrapnel), so charge rate wouldn't
need to be that high.

Time for some experimentation next weekend.

Cheers

M


-- 
Matthew Smith
South Australia
http://www.kbc.net.au