01-03-2019, 01:35 PM
Mike, it might be a good idea to repeat the test but this time, check the voltages both before and after the Darlington pair. Also the voltage across R4/R5 - if you are using the same 1R3 value (0R65 total), it shouldn't reach 700mV (the voltage at which Tr3 starts to conduct) until the current reaches ~1A.
There is no reason for the circuit not to work, so there must be a fault somewhere.
There should be a constant 1.4V drop between the base of Tr1 and the emitter of Tr2 when the Darlington pair are conducting.
When the protection circuit cuts in, it should short circuit the input to TR1, reducing the output to nil, so your 1300mA @ 5.5v figure is doubly wrong!
Maximum voltage for the 555 timer is 18V, so your 7815 regulator will be fine but not the 7805! That will reduce the 555 output to ~5V which, with the 1.4V drop in the Darlington pair will reduce the maximum output to 3.6V peak!
As for those diodes, D3 prevents R5 from shunting the output of VR1 with Sw1 in the normal position but I can't see any logical reason for D1 and D2.
There is no reason for the circuit not to work, so there must be a fault somewhere.
There should be a constant 1.4V drop between the base of Tr1 and the emitter of Tr2 when the Darlington pair are conducting.
When the protection circuit cuts in, it should short circuit the input to TR1, reducing the output to nil, so your 1300mA @ 5.5v figure is doubly wrong!
Maximum voltage for the 555 timer is 18V, so your 7815 regulator will be fine but not the 7805! That will reduce the 555 output to ~5V which, with the 1.4V drop in the Darlington pair will reduce the maximum output to 3.6V peak!
As for those diodes, D3 prevents R5 from shunting the output of VR1 with Sw1 in the normal position but I can't see any logical reason for D1 and D2.
