Transistor as voltage regulator.

Thread Starter

martinktm

Joined Dec 21, 2013
2
Hi,
I'm repairing electronics that has npn transistor as voltage regulator. The circuit is resistor from transistor collector to base and Zener diode from base to gnd. The input voltage at collector is 20V. The problem I have is that there is no voltage at output. At transistor base is 0V, I expect 12 because Zener is 12V. I replaced resistor R4P and Zener diode D9P jet no voltage at base.

I'm attaching image sorry for poor quality.
I can't figure out why there is no voltage at base?



transistor_regulator.png
 

Irving

Joined Jan 30, 2016
1,006
Suspect q4p is short circuit. If you probe c-b and b-e with a multimeter on diode test setting do you get a junction voltage ~ 0.4 - 0.6v or a short?

With q4p out of circuit does zener/resistor junction give you 12v?
 

Thread Starter

martinktm

Joined Dec 21, 2013
2
I tried to replace q4p with bd911 for test jet no voltage on output. Before soldering new transistor I checked voltage at base and it was 0V.
 

dendad

Joined Feb 20, 2016
3,602
Is it 0V or 0.7V?
Have you measured the resistance to the 0V line to see if there is a short on the output? If so, check the capacitor.
And also measure the resistance of the base to 0V.
 

Audioguru again

Joined Oct 21, 2019
1,908
The datasheet for the MJD122 shows that it is an NPN darlington that already has the R5P resistor in it.
The datasheet for the BZX55C16 shows that it is a 16V zener diode that needs 5mA but here it gets only 1.6mA to 2.2mA.

The darlington has a base-emitter voltage loss of 1.2V for no load current to 2.8V at 4A so the voltage regulation of this circuit with the 16V zener diode is from less than 13.2V to 14.8V. That is terrible voltage regulation.
The four 10 ohm paralleled resistors produce a voltage loss of 10V at 4A.
 

Irving

Joined Jan 30, 2016
1,006
That's just a weird regulator... what useful purpose do the 10ohm resistors achieve other than adding to global warming... without knowing what the official load is, nothing obvious... ah, maybe a poor man's 12v SLA trickle charger for battery backup (which might explain the TS' belief it was a 12v zener?), protecting against a completely discharged battery? but regulating the float voltage at low charge currents? In which case that regulation is probably OK.
 
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