simple voltage switch

[scrag]

Ive got a system which switches 240v when on and 68v when off. I need to shut the voltage off in it's "off" state. I was thinking of bridge, smooth, potential divide then using this lower dc voltage as a threshold switching voltage. now I was thinking of using something like UJT or a BJT to switch a relay to disconnect the supply.

My analogue electronics is a bit rusty, can anyone comment or give a better solution to this problem.

THANX

 

[DLwalsh]

Try a window comparator that monitors for the 240v and 68V. It can be designed to turn on or off at either potenial.

 

[scrag]

cheers mate, one question though... what's a windows comparator????

 

[maximka]

Any transistor with appropriate voltage divider ( two resistors) at the base would help. But, if you are not comfortable with that, use a relay that operates at 240V.
Let me know if i understood your problem correctly.

 

[shuggie]

Scrag,
You have not made it clear where your control voltage is from or at what level it would be. Your simlest solution could be to use a solid state relay, these are available to handle large currents (50 amp common) and work at various voltage levels including 240V.

 

scrag,
1. Properly done, I don't see why your way
wouldn't work just fine. Probably more
rugged with a relay/more fun with an triac.
2. DLwalsh's solution is more elegant. To
answer your question, a window comparator is
a circuit that uses two comparators to sense
an upper and lower threshold, like a physical
window blocks you from seeing above and below
certain levels wrt the floor. A comparator
is an open loop differential amplifier with a
very high gain. A comparator has a plus and
minus input and the comparator amplifies the
difference between the two inputs by a factor
of between 10,000 and 10,000,000, thus forc-
ing the output, effectively, to swing between
positive and negative supply voltages
depending on which input is more positive.
3. Be aware that you have to design
hysteresis into the circuit so it won't
"chatter" in the vicinity of 68 V.
4. By the way, how much current are you int-
errupting and what type of load is being
protected from low voltage? I believe you
can find circuit breakers with under voltage
protection in electrical supply houses that
might suit your need.

 

[nbucska]

Could you please explain in detail what do you need ?
Are the voltages AC or DC ? If you switch using some
sensor ( i.e. not manually ) what are the data of
the circuit measured/sensed ? <nbucska@pcperipherals.com>