# Switching negative voltage by TTL signal

Discussion in 'The Projects Forum' started by movax, Nov 2, 2008.

1. ### movax Thread Starter New Member

Nov 2, 2008
2
0
Hi everyone!

I am working on a circuit that has a dual power supply (+/- 12V),
with an additional +5V TTL supply, and I need to
be able to switch the negative voltage (Vout) by a logic level voltage
Vin such that Vin=0V should give Vout=0V and Vin=+5V should give Vout=-12V. I have a power NPN between ground and -12V, but now I need "something" to hook up to the base of this transistor that takes the logic levels mentioned as inputs.

The job would probably be simply done with an op-amp, but since I
will need a large number of these switch stages, I would like to avoid
that if possible. So I'm looking to see if anyone can think of a nice,
simple solution without ICs. I have a feeling that the answer might be
a FET, but I can't seem to get that to work.

Any help deeply appreciated!

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The circuit depends on what you want to drive with this 0 and -12 logic levels. Give us more information. Also, if you use a quad op-amp IC (14 pins) the final design will be more compact than building it with discrete transistors.

3. ### Bernard AAC Fanatic!

Aug 7, 2008
4,231
414
Try this for relative slow repitition rate and about one amp. With more info. might come up with better values. Pnp transistor something like complement to 2N2222. FET to match load with low gate capacity.

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4. ### movax Thread Starter New Member

Nov 2, 2008
2
0
Thanks a lot for your help guys!!

Some further details. There is a pair of transistors, one supplying a 12V and one
supplying a -12V potential to the endpoint of a wire, in order to control the current
direction through the wire, which is connected by a resistor to 0V at the other end.
Currents from the transistors are up to 1 amp, and switch times are slow, on the order of 0.1 ms.
The transistors should be controlled by two TTL-level signals. The one giving +12V is no problem
but the one giving -12V I can't figure out in a good way. An opamp could be used to ensure that
the base is at > -12V when the input is at +5V and at -12V (the emitter voltage) when the input is at 0V,
but that seems clumsy to me. Another solution would be some sort of voltage division setup,
as I think Bernard's circuit suggests, but that would require a negative voltage below -12V, which
I don't have.

Another experiment I tried involved FETs. I have zero experience with FET transistors,
but since I know they're voltage-controlled, I tried to setup a circuit to exploit this,
i.e. to provide the correct base voltage, but this failed miserably

I think there is probably a simple solution, but my electronics skills are too lousy..
Right now I think I might go with the opamps and make it simple for myself.