Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!zaphod.mps.ohio-state.edu!mips!hal!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: Re: Using computer to switch high current circuit
Message-ID: <34039@mips.mips.COM>
Date: 2 Jan 90 17:00:43 GMT
References: <1990Jan2.005157.12780@uokmax.ecn.uoknor.edu>
Sender: news@mips.COM
Reply-To: mark@mips.COM (Mark G. Johnson)
Distribution: na
Organization: MIPS Computer Systems, Inc.
Lines: 57

In article <1990Jan2.005157.12780@uokmax.ecn.uoknor.edu> blpike@uokmax.UUCP (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.  
>


Here's one approach.  Another, infinitely simpler and less elegant,
is to use a "mercury wetted" relay; its contacts don't bounce.  Or,
use 3 independent relays in parallel, which reduces the probability of
contact bounce.

The trick to any circuit is to find a way to recharge your 0.11 Farad
capacitor between firings.  The circuit below uses a resistor (R4) to
charge the cap.  Probably R4 should be large enough so the recharge time
is at least 10X the discharge time.



   +5V LOGIC                          V+ OF CAPACITOR
    SUPPLY                                 SUPPLY
      |          +----------------------------|
      |          |                            |  +----+
      |          |                            +--| R4 |---+---------+
   +-----+    +-----+                +--+     |  +----+   |         |   YOUR
   | R1  |    |  R2 |               2|  |5    |1          |       ----- BIG
   +-----+    +-----+              +-------------+     +-------+  ----- CAP
      |          |                3|             |     | YOUR  |    |
      |          +-----------------|  CMOS 4050  |     | LOAD  |    |
      |1         |5                |HEX INVERTER |     |ELEMENT|    |
   +----------------+              +-------------+     +-------+    |
   |  OPTOISOLATOR  |                |8    |4             |         |
   |      4N35      |                |     |            D |         |
   |                |                |     |          |---+         |
   +----------------+                |     |        | |  HEXFET     |
      |2   |4    |6                  |     +--------| |  IRF-Z40    |
      |    |  +-----+                |            G | |             |
      |    |  | R3  |                |                |---+         |
TTL   |    |  +-----+                |                  S |         |
------+    |     |                   |                    |         |
INPUT      +-----+-------------------+--------------------+---------+
                 |
                 |
              V- OF CAPACITOR
                SUPPLY

R1 = 1.5K  (LED current limiting resistor)
R2 = 3.0K  (collector load resistor for optoisolator)
R3 = 100K  (base bias resistor for optoisolator)
R4 = 47 ohms or more  (trickle charge path for your big cap)

All parts above are in the current Digi-Key catalog.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}