How about using a High Level Driver in the common +ve supply to limit the
current, detect fault conditions etc. Then use P channel Power Fets to
switch the individual outputs on and off.

I would also question the use of D25 connectors in this application. I
cannot remember seeing any current rating for these connectors, but then I
have only used them for carrying data signals. Also they have a limited
lifespan in terms of the number of matings. The cheap ones are particularly
bad in this respect.

Regards

David Duffy wrote:
> Dwayne Reid wrote:
> >David Duffy wrote:
> >
> >>Sounds simple(ish) & obvious!  Hmmm, why didn't I think of it?  :-)
> >>I may be able to get away with a power resistor in series with the
> >>supply line (between battery & drivers) to limit the current. A lamp
> >>would work even better but can't afford an O/C filament to stop the
> >>unit from working. The battery supply is 24Vdc and the unit will be
> >>for firing fireworks "electric matches" or igniters by remote control.
> >
> >The fireworks / pyro control systems that my company manufactures does
> >things a little differently: the channel 'arm' relay connects one side of
> >the igniter to the high side of the battery.  A current sense FET
(IRCZ24)
> >is turned on when the channel is to be fired.  The current limit is set
to
> >about 6 amps or so.
> >
> >Both sides of the ignitor are shunted by the normally closed contacts of
> >the arm relay while the channel is not armed.  In addition, both sides of
> >the ignitor are floated above ground by a few volts and monitored to look
> >for ground shorts or leakage.  Its kind of a bridge circuit - if
something
> >upsets the balance, that channel is locked out and a fault indication is
> >sent back to master control.
> >
> >Each firing channel uses 3 sections of LM2901 quad comparitor (2 sections
> >for the bridge and 1 section for continuity detector), 1 section of
LM2904
> >dual op-amp (FET driver / current limit), 1 section of NEC EPE series
dual
> >relay (great little dual 30 Amp SPDT relay), 1 IRCZ24 FET, and about 1/4
of
> >a EPM7032SLC-44 epld from altera.  The epld does all the timing and
serial
> >communications as well as the fault / arm / fire logic required.
> >
> >In a nutshell, that is how I did our system.
> >
> >Look over your requirements carefully - there may be no need to use a
high
> >side driver.  It is dead simple to build a current limited low side
driver
> >using a sense FET and a single section of op-amp.  As a bonus, the op-amp
> >takes care of level translation from your logic supply to the drive
> >required for the FET.  The caveat is that you must control what happens
> >with the high side supply.  My system monitors for faults before it
allows
> >the ignitor to be connected to the high side, but this requires an arm
> >relay for each channel.  This is a tradeoff that I was happy to accept.
>
> The reason I was going for a high side driver was that this unit is to
replace
> a manual switch firing unit that uses a common ground system on the
> firing rails (right name for them?). Each rail has 20 outputs on it with a
D25
> connector. I suppose I could use the existing rails but the +/- markings
on
> each channel would be back to front . (Actually red/black spring
terminals)
> Getting 100 channels in a small space is going to be difficult &
expensive.
> Regards...
>
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