I try to build a 300V voltage regulator using a LM317, which should be designed for 100 mA, but the circuit does not behave as I thought.
I loaded the circuit with 27K which is a current of 11 mA, and set the voltage regulator to 300 volt over the planned 22K in parallel with 1K.
It took me a few tries until I found the right resistor.
Some heat development at the transistor (heat sink) is not noticeable.
Then I bridged the output with a 5 kΩ resistor parallel to the 27 kΩ resistor which gives 4.2 kΩ and loads the circuit with 71 mA.
But instead of the voltage remaining at 300 V, it breaks down by more than 100 V, and the resistor before the Zener diode planned 3 kΩ used 3.3 kΩ gives smoke signals.
Also with this load no heat development at the heat sink.
At the transistor, 0.6 volts drop between base and emitter.
Can someone give me an assistance there?
Best Answer
If the 3k (3.3k) resistor1 is getting hot, it means that all of the load current is passing through it.2 If the transistor is working correctly, only a tiny fraction of the load current should be passing through this resistor, so verify that all of the connections to the transistor are correct. It's behaving as if its collector is open-circuit.
1 In the future, please add reference designators to your schematics. It makes it a whole lot easier to talk about specific components.
2 It's dropping about 36 V (0.4 W) with the 27 kΩ load, which (just barely) allows the LM317 to regulate. But it's dropping more like 150 V (6.7 W → smoke!) with the 4.2 kΩ load.