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PNP switching negative voltage

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maca_404

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Hi

I have a 0/5v square wave and I need to level shift it, I can easily use a pnp transistor and shift it to a 0/12v wave. However I would like to use the same transistor to level shifted the 0/5v wave to -12/0 I have a -12v and a 5v source but no matter which way around I put the setup I cant seen to get it to work. Can this be done ?. The load on the -12 supply will only be around 1.2ma.

Thanks
 
Hi

I have a 0/5v square wave and I need to level shift it, I can easily use a pnp transistor and shift it to a 0/12v wave. However I would like to use the same transistor to level shifted the 0/5v wave to -12/0 I have a -12v and a 5v source but no matter which way around I put the setup I cant seen to get it to work. Can this be done ?. The load on the -12 supply will only be around 1.2ma.

Thanks
hi,
Do you mean something like this.??
 

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Spot on thanks for that works great, I am just trying to get my head around why it is works. Would you have an example of a 0/5v signal being shifted to say 12/-12 that might give me a better idea of the concept ?. Thanks Again.
 
Spot on thanks for that works great, I am just trying to get my head around why it is works. Would you have an example of a 0/5v signal being shifted to say 12/-12 that might give me a better idea of the concept ?. Thanks Again.

hi,
To get the +12V for your latest idea, you need a +12V supply as well as the -12V.

How it works.?
Its an PNP transistor, so that means when the base is approx -0.65v with respect to the emitter the transistor conducts and connects the collector and end of the resistor to 0V, so the output is close to 0V.
When the transistor is not conducting the end of the resistor rises to -12V.

As we dont have a minus voltage to drive the transistor ON, the Base is connected to 0V and the 0/+5V is applied to the emitter, when the input is at +5V, emitter becomes +0.65v with respect to the Base, which is at 0V, so it turns on the transistor.

There are other ways to get a 'better' -12Vout, but it requires a bit more circuitry....OK.?
 
Thanks that certainly makes more sense now, I suppose the only downside of the above configuration is that the voltage source need's to be able to carry the full emitter current.
 
Thanks that certainly makes more sense now, I suppose the only downside of the above configuration is that the voltage source need's to be able to carry the full emitter current.

That would apply to most circuits.
But the emitter current is only [5V - 0.65]/1K = 4.3mA and the collector current approx 12V/3K3 = 3.6mA

What is the 0/-12V output connected into.??
 
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I would like to pulse the signal at around 3khz, Then use it to drive the gate of an irf740. The source would then be connected to a -12 supply and the drain a +12 supply through a resistor, The output of this setup would be connected to one side of a coil and the other to +12 which should give me short 24V pulses through the coil. Basically I am trying to make a front end for a pulse induction metal detector.
 
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