01-03-2019, 09:59 PM
Mike, my initial thought was that Tr2 was dropping an excessive voltage as the current increases but now I'm not so sure.
Look at the difference between the two voltages and calculate the effective resistance of Tr2.
At 240mA it is 23R16, at 450 mA it is 14R, and at 750mA it is 9R06, so it is trying to compensate for increasing current by reducing its effective resistance, as it should
However, your PSU isn't stabilised so, as it sags, the control voltage will also sag. You haven't said how you are providing this voltage but, if it is from the 10k pot, the upper portion of the track will be supplying the base - emitter current of the two transistors, so this will further drop the control voltage.
I would suggest that you repeat the test but also measure the voltage at Tr1's base and see how that varies.
Your Tr3 base - emitter voltage suggests that it is limiting at 1A2 whereas 700mA would indicate a limit of 1A5.
However there must be a sort of yo-yo effect because when Tr3 turns on because of an overload, Tr2 and Tr3 are turned off, so there is no output and thus no overload so it must turn back on again! Not something I've ever investigated but it might be interesting to put a 'scope on the output!
As for the diodes in the other circuit, depending on the state of the discharge pin either one or other of the diodes will conduct and determine which bit of R3 is in circuit to vary the mark -space ratio - at least that's my interpretation!
Look at the difference between the two voltages and calculate the effective resistance of Tr2.
At 240mA it is 23R16, at 450 mA it is 14R, and at 750mA it is 9R06, so it is trying to compensate for increasing current by reducing its effective resistance, as it should
However, your PSU isn't stabilised so, as it sags, the control voltage will also sag. You haven't said how you are providing this voltage but, if it is from the 10k pot, the upper portion of the track will be supplying the base - emitter current of the two transistors, so this will further drop the control voltage.
I would suggest that you repeat the test but also measure the voltage at Tr1's base and see how that varies.
Your Tr3 base - emitter voltage suggests that it is limiting at 1A2 whereas 700mA would indicate a limit of 1A5.
However there must be a sort of yo-yo effect because when Tr3 turns on because of an overload, Tr2 and Tr3 are turned off, so there is no output and thus no overload so it must turn back on again! Not something I've ever investigated but it might be interesting to put a 'scope on the output!
As for the diodes in the other circuit, depending on the state of the discharge pin either one or other of the diodes will conduct and determine which bit of R3 is in circuit to vary the mark -space ratio - at least that's my interpretation!
