# Need help on this.

Discussion in 'The Projects Forum' started by paulepc, May 7, 2014.

1. ### paulepc Thread Starter Member

Apr 2, 2014
55
0
I have a supply of 1.4 to 3.2 volts. If the voltage drops below 1 volt I need to
switch off both +/- 12 volt rails

Looking for an easy solution.
Thanks

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2. ### wmodavis Distinguished Member

Oct 23, 2010
739
151
Toggle switch!

3. ### John P AAC Fanatic!

Oct 14, 2008
1,748
260
You could have a transistor with a diode in series with the base connected to the variable voltage (and a resistor too), so it would turn on when the voltage is enough to drive current into the base, then the collector of the transistor operates a double-pole relay which turns on the output lines.

That's easy, but it won't work at exactly 1V. If you're really fussy about an accurate switching point, you'd need something like a comparator.

4. ### paulepc Thread Starter Member

Apr 2, 2014
55
0
Thanks, I have looked up comparators it seems like it's ideal. but as my input is going to be 3.2 max to under 1 volt and I need the output to be +12 volt for comparator 1
And -12 volts for comparator 2 I can't see how I can get the correct + & - values into the op amp.
if I want to switch the 12 volt supply off and on, based on my comparator voltage It will always be on. Hope that makes sense.

5. ### wayneh Expert

Sep 9, 2010
16,102
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Option 1: Use a comparator to control the power to the "sub circuit". Power the comparator from the same 24V supply as the sub circuit. Provide a reference voltage to the comparator of +13V. If I understand correctly, you want to shut off the sub circuit at any voltage below that.

Use the comparator output to control an N-channel MOSFET placed between the sub circuit and the -12V pole. A pull-up resistor on the gate keeps the MOSFET (and the sub circuit) on for all voltages above +13V. Below that, the comparator can pull the gate voltage down and shut off the MOSFET and the sub circuit.

Unknown: It's not good to have input voltages on an IC that is not connected to the power rails. And it's not defined what your outputs might be if the sub-ciruit is unpowered but connected to V+ and those inputs.

Option 2: You could switch the outputs on and off using a similar technique, but that requires knowing ore about what the outputs are used for.

6. ### paulepc Thread Starter Member

Apr 2, 2014
55
0
Thanks I;ll read it a few times to get my head round it. This is where I am at the moment trying to set up the comparator.

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7. ### wayneh Expert

Sep 9, 2010
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Since the reference voltage in that circuit is below both test voltages, the state won't change with the switch.

I see you're planning to power the comparator from only the +12V rail and ground. That's fine, but you could also use the -12V rail for a 24V supply. The LM339 quad comparator, for instance, is fine at up to ±18V, or 36V supply. It's not good for a comparator to ever have an input voltage outside the power rail voltages, so if you use both, you avoid that possibility.

I'm not sure what you mean about the power rails.

8. ### paulepc Thread Starter Member

Apr 2, 2014
55
0
I am using the comparator to switch the +/- 10 volt rails off, therefore shutting down the op amps they are feeding. The reason is the op amps are feeding a motion controller, and I have instances where the controller has moved off on is own when the signal has been switched off.
So I am trying to make certain if there is a loss of signal the comparators will switch off the +/- 10 volt rails feeding the next stage.