12 Volt PIR to Timer 210925

Hello Pommie, @

Diver300​

rjenkinsgb @
Report

KeepItSimpleStupid​

and the ETO forum,

Thought I had completed this project. Was ready to install at closet and a puff of smoke came off it and it stopped working
Have posted on this project thrice:
Transistor for 12 Volt circuit May 26 2021

12 Volt PIR to Timer June 24 2021
KSS said it will blow

12 Volt PIR to Timer 210705

This the latest

Not sure what blew. By the time I could get the power turned off it was hard tell visually which component fried but it seemed like Q2 was warmer than anything else.
Thought I had solved the hyper current issue. First a signal transistor, a BC547 was used at Q2. It got hot and so the BC547 for Q2 was replaced with a AUIRF540Z.
Did not seem to have heat problem with Q2. The data sheet, attached, indicates (I think) that the transistor is good to 36 amps.

So trying to figure out
1. Why the circuit smoked?
2. If the problem is Q2, the AUIRF540Z, why it overheated?

Any insight would be greatly appreciated.

Thanks.

Allen Pitts, Dallas Texas

I cannot find a datasheet for the 1156, but notice there is no current limiting
in the Q2 circuit. may not be needed, but no datasheet to show what max
current will flow, eg. is the 440 mA a max value over T and V for each LED ?

The turn on of Q2 slow due to the Rg of 470 ohms and its input capacitance.
So it stays in active region for some time, when switching on, which means its
dissipating a lot of power. How long from trigger to Vgs of Q2 ~= 10V ? Scope
useful for this, a DSO if you have access and setup for a single trigger.

You could also put a 1 ohm R in emitter of Q2 to capture its current waveform
to see what it is. 1 ohm means 1V / 1A voltage to scope.


Regards, Dana.