Author Topic: Highside Switching Q?  (Read 1264 times)

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Offline Things

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Highside Switching Q?
« on: August 01, 2013, 02:34:15 pm »
Hey all, I'm trying to highside switch 24V from a microcontroller, and I'm spec'ing out some transistors. However I'm having a bit of a mental blank (Why I am designing a PCB at midnight is beyond me), but does the base of the PNP need to be rated for the voltage it's switching? For example I'm looking at the BC817 and BC80 pairs, which have +5 and -5V emitter-base voltages respectively.

Dan
 

Offline ignator

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Re: Highside Switching Q?
« Reply #1 on: August 01, 2013, 04:25:07 pm »
Do you have a schematic of your circuit?
A PNP, I don't see how this works. A NPN (emitter follower), with the collector connected to the +24, the emitter to the load, A resistor from the collector to the base (this turns it on) and the base through a load resistor to an open collector to ground switch.
But this configuration is tough if your wanting more then a few milliamps. I've used a self resetting fuse for the collector to base turn on source power (the smallest one Polyswitch had), and a FET from the base to ground, that turns this off. This was intended to act like a normally closed switch. And the FET was used to turn it off.
 

Offline Things

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Re: Highside Switching Q?
« Reply #2 on: August 02, 2013, 02:01:13 am »
Like this:

 

Offline Kremmen

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Re: Highside Switching Q?
« Reply #3 on: August 02, 2013, 07:44:12 am »
This could work. The PNP switch (Q2) base never sees less than V+ - ~0.7 V or so. Q1 and Q2 Vcemax must be rated for the full voltage.
Nothing sings like a kilovolt.
Dr W. Bishop
 


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