20 Mar
2007
20 Mar
'07
3:48 a.m.
--- 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