What is the function of TR1 in this circuit

[ku637]

Hello,

I came across a circuit . I believe the Node 1 will be in steady state charged to 48V approx which will drive the Nchannel MOSFET TR2 .
But im not clear with function of TR1. I think it will be OFF in all case.. Can anyone tell me function of the red colored components?

Thanks,

 

[d123]

Hi,

Is that drawn right (it looks upside down)? What's "control"? The MOSFET diode will always be conducting the 24V with the MOSFET in that direction. 100k/22k divider put ~4V and I guess 240uA into NPN base, so it should be on all the time, but not if the NPN is meant to be like that. I guess it's/they're there to put a constant voltage into the MOSFET gate.

The MOSFET feeds the basic voltage doubler to increase its own gate voltage? Is it supposed to have ~1mA into the gate? Is the other 100k supposed to separate the 24V from whatever comes out of node 1?

 

[FvM]

Looks like a reverse polarity protection circuit. TR1 is on for positive input voltage and off for negative input.

 

[d123]

Hi,

So it seems to be. I simulated with positive polarity, and then saw your post so simulated with VS1 and ground reversed. Why doesn't the MOSFET gate get protected with positive polarity, or is that something to do with my simulation?

 

[ku637]

I

Is that drawn right (it looks upside down)

No i have drawn actual circuit.

What's "control"?

Control is a square wave oscillator.

Looks like a reverse polarity protection circuit. TR1 is on for positive input voltage and off for negative input.

FvM, i feel like TR1 will be always off as for +ve input base of TR1 (through 100k) and emitter is both at 24V. So TR1 should be OFF right?

When reverse polarity comes at the input side.The GND point of 22k will be 24V i suppose,while the input 24V terminal will be 0V.
In that case current flows from 24V -> through 22k -->diode-->TR1 base-->0V at the input terminal.This will cause the base to be forward biased with approx base current (24V)/22k=1.05mA. So in that case transistor will be ON and it will short out the N-channel FET gate, turning OFF the power to "Other CKT''

 

[FvM]

FvM, i feel like TR1 will be always off as for +ve input base of TR1 (through 100k) and emitter is both at 24V. So TR1 should be OFF right?

Right, I should have written TR2 instead of TR1.