Interfacing a coil to the C64?

From: Marko Mäkelä (
Date: 2002-09-10 21:30:20

Since I'm not an expert in analog electronics, I thought I'd pass the 
following question on to this list.  Please CC: to cbmnut, as he is not 
(yet) a subscriber.


This is pretty simple to someone with knowledge.  So here I go:
I would like to control solenoid with c64.  So, I use
of IPS511 mosfet switch, taking TTL in.  So now I just need
some good power.  For this purpose, it is employed the LMD18400.
I produce 32v@500mA, which is sufficient in most cases, however
I'm having trouble starting the movement of a stiff mechanics.
Thus I propose to use a capacitor, I guess 6800uF, across output.
Thus: IC2-reg. out---IC1 (high side switch)--coil (16ohms)---gnd
          \--gnd    \--+C1 (6800uF)---gnd
                     \--(-) D1 1N4004 (+)---gnd (for coil kickback)
The purpose I intend, is that C1 charges slowly until ready, then
when MOSFET switched on, coil sucks huge current from C1, for some
100ms maximum (it is wanted to be).  This is sufficient to un-stuck
coil, thus the remained is subsisting on 500mA only.
The duty cycle is low, on a for a 1sec, off for some sec.

Do you have any ideas about this?  I calculate C1 because 32V/16Ohms=2A, 
but IC2 only does 500mA, so I need "boost" with C1 to 2A for 100ms, and 
T=RC=.1088, about 100ms.  But I can't calculate
how long it takes to charge.. and also if this is safe to regulator.
Also, can gnd be shared with C64, or does some kickbacks leaks through?
Thanks man

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