At 05:59 PM 7/26/2003, Rob Peacock wrote: >Think of the 5 ohm resistor as a big heater that I want a minimum of 3A >flowing through when the transistor is turned on. I'm guessing from your other messages that you are building a controller for Pyro effects or fireworks - am I close? If so, there are some other things that you need to take into account: 1) the squibs often go short circuit when fired - that means that your drive circuit has to be able to handle a dead short without self-destructing. 2) you have to ensure that your continuity check circuit draws no more than 2 or 3 mA. 3) If you have bypass capacitors anywhere on the leads going to the squibs, you need to ensure that the open circuit voltage is very low so that stored charge will not set off your squibs when you connect them to the controller. 4) You need a mechanical air gap (relay or switch) somewhere in the system - when the system is not armed, a failed transistor will not cause un-intended initiation. This can be in the battery line and protect all channels - you don't necessarily need one relay per channel. 5) you should also have some provision for shunting the channels when they are not armed. 6) you really need to think about failure modes - what can fail in your circuit and what effect will it have? dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 19 years of Engineering Innovation (1984 - 2003) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu