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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: Using computer to switch high current circuit
Message-ID: <5170065@hplsla.HP.COM>
Date: 2 Jan 90 17:51:56 GMT
References: <1990Jan2.005157.12780@uokmax.ecn.uoknor.edu>
Organization: HP Lake Stevens, WA
Lines: 26

blpike@uokmax.ecn.uoknor.edu (Brian Lee Pike) writes:
> I am trying to build a circuit that allows my Commodore 64 to connect a
> 110,000 microfarad (that's not a typo, I mean 0.11 farad), 18 volt capacitor
> across a load for a brief period of time (somewhere between about 0.1 second
> and 0.75 seconds) and then disconnect the capacitor.  
> ... 
> 1) There must be NO WAY for the capacitor's current to reach the computer.
>    I have heard of 'opto-isolators', but I do not completely understand how
>    to use them, particularly in a high-current situation.
> 
> 2) The switching should be as 'clean' as possible, i.e., should approximate a
>    square wave.  I suspect that this cancels out using a large relay, although
>    please correct me if I am wrong.

It doesn't matter much how big the capacitor is; what we really need to know
is how much current you expect to switch:  maximum "on" current; maximum
allowed "off" current.  From the looks of it, the maximum voltage across
the switch when it is "off" is 18 volts; what is the maximum you can allow
in the "on" state across the switch?  Also, "as clean as possible" is
awfully open-ended!  I can get all carried away providing nanosecond
switching times for something that really doesn't require them.  Can you
be more specific?  (A relay just _might_ be the best solution, but an
optoisolator is indeed another way to get good electrical isolation, and
a power transistor or FET, driven via an optoisolator, could handle 10's
of amps.)  If you don't know the numbers for sure, let us know just what 
you are trying to do...