>
>
> On Oct 11, 2008, at 5:44 PM, Info wrote:
>
> Hi, I am trying to build a high side switch as a replacement
> for an old relay using a PNP transistor. The load must be driven
> from 24vdc comming from the relay then back through GND. The
> coil was driven by 24v then to GND.
>
> See attached schematic.
> I got the PNP switch working. If the base floats or
> is connected to GND the output is off. When its connected
> to 24v the load is switched on. So far so good.
>
> Unfortunately that is the opposite of what is needed so
> therefore I added the NPN inverter which works fine by itself.
> The problem comes when the two circuits is connected toghether.
> The PNP doesnt switch off when its connected to the NPN inverter.
>
> Why doesnt this work, what is needed to make the PNP switch
> off by the NPN ?
>
> If there is some major problem with my approach I am open for
> other solutions. This is a one off btw.
> <HS-switch1.gif>--
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1) The PNP will be off if the base is floating or the base is  
connected to 24V.  ( not as you said to gnd )
Perhaps you do not have a PNP for Q2.  That might explain the problem.
2) You may want to consider limiting the current flowing from the base  
of Q2 by using a series resistor.  This would protect Q2 ( and not  
change the operation of the circuit ).
3) You may want to put some voltage limiting circuitry across the  
relay inductor.

ma


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