ASR33 selector magnet driver card

Mr Ian Primus ian_primus at yahoo.com
Tue Mar 20 05:48:26 CDT 2007


--- Chuck Guzis <cclist at sydex.com> wrote:

> On 18 Mar 2007 at 19:11, Mr Ian Primus wrote:
> 
> > So, I need to replace that varistor - it evidently
> was
> > there for a reason, and it kept me from blowing up
> the
> > transistor 

> Take a look here:
> 
> http://www.littelfuse.com/data/en/Data_Sheets/MA.pdf
> 
> You can toast a varistor if you apply a voltage and
> current that's 
> nigher than its specified ratings.  That 100A rating
> is only good for 
> something less than about 25 usec; if an overvoltage
> persists for 
> longer periods, you can take the MOV out with
> substantially less 
> current.

That makes sense. I blew this one up with a 12v DC
plugpack. I was running it through a big resistor, but
what I thought was a 680 ohm resistor was a 6.8 ohm
resistor (that'll teach me to work on stuff when I'm
tired)... I've since built a better loop current
supply, and have it working on the transmit line.
Pressing keys on the teletype blinks an LED with
signal - different keys produce different blinks, but
even 110 baud is too fast to see, really. So all I
need to do for that one is swap in an optocoupler and
run it into a MAX232. Haven't even started working on
the recieve side of the interface, since I am hesitant
to try connecting stuff to RX with that varistor
blown/missing.

So - in replacing this component, what should I be
looking for? The only specification givin is 100A, but
I see varistors in various places rated in terms of
Joules or Volts. I don't want to replace the varistor,
only to have it not give me any protection for that
transistor.

Thanks!

-Ian



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