Hi all,
I wonder if anyone can help me ?
I've been bending my brain all evening trying to work out what the transistor does in the following variable power supply circuit.
As far as I can figure, the circuit would work perfectly well without the transistor. Not only that, but I'm not convinced that the transistor will ever turn on.
This is an actual circuit that I purchase for a couple of euros from China. I only wanted the LM317 and a heat-sink to contribute to making a little variable power supply, but I got this little circuit delivered to me at a cheaper price than I could of purchased the heat sink and LM317 alone. Hey, I view that as free components added !
Out of curiosity, and as a learning exercise, I produced a schematic of it. In fact I wish I hadn't, because now I'm stuck thinking about it. Can anyone out there enlighten me on why it's designed with the transistor this way and just how it works ? Far as I can see, the transistor never turns on, because the base can never be at a higher voltage than the emitter as the wiper of the pot returns to ground. I can't be right though .. because then, why have the transistor ? Hence, I'm coming to you folks to see where I'm going wrong.
Incidentally, D5 is to protect the voltage regulator from inductive spikes from a load ?
I was also amused to find that the little PCB has two different capacitors identified as C4 (corrected by me in the schematic). You really do get what you pay for. But if I learn anything from this, it'll be two euros well spent.
I wonder if anyone can help me ?
I've been bending my brain all evening trying to work out what the transistor does in the following variable power supply circuit.
As far as I can figure, the circuit would work perfectly well without the transistor. Not only that, but I'm not convinced that the transistor will ever turn on.
This is an actual circuit that I purchase for a couple of euros from China. I only wanted the LM317 and a heat-sink to contribute to making a little variable power supply, but I got this little circuit delivered to me at a cheaper price than I could of purchased the heat sink and LM317 alone. Hey, I view that as free components added !
Out of curiosity, and as a learning exercise, I produced a schematic of it. In fact I wish I hadn't, because now I'm stuck thinking about it. Can anyone out there enlighten me on why it's designed with the transistor this way and just how it works ? Far as I can see, the transistor never turns on, because the base can never be at a higher voltage than the emitter as the wiper of the pot returns to ground. I can't be right though .. because then, why have the transistor ? Hence, I'm coming to you folks to see where I'm going wrong.
Incidentally, D5 is to protect the voltage regulator from inductive spikes from a load ?
I was also amused to find that the little PCB has two different capacitors identified as C4 (corrected by me in the schematic). You really do get what you pay for. But if I learn anything from this, it'll be two euros well spent.
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