Add a resistor between the collector of Q1 and base of Q2. As it
stands you will be shorting the power supply via Q2 base  - probably
blowing either or both transistors. I don't know the current levels
involved but something like 3k3 may be appropriate as a guess.

And, as mentioned by Andre (below), a base-emitter resistor on Q1 will
ensure it turns off in the event the input is open circuit.

RP



2008/10/12 Andre Abelian <andre@ditechnology.com>:
> You need to put pull down resistor on npn transistor. That's why it doesn't
> Work correctly. You didn't mention how are you turning it on? Are you using
> pic?
> If yes then the schematic needs to be changed you may need to add opto
> coupler instead
> Of npn transistor other wise pic will get higher then 5 v.
>
> Andre
>
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
> Info
> Sent: Saturday, October 11, 2008 4:44 PM
> To: 'Microcontroller discussion list - Public.'
> Subject: Relay replacement (High side switch)
>
> 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.
>
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