Model train controller

[Crackle]

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.

Hi Terry,
I am using a .47 ohm resistor for the current sense R in the first circuit.

There is no reason for the circuit not to work, so there must be a fault somewhere.
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!

Not sure what is happening there then, when I said the protection circuit had cut in, I was going by the fact the LED was alight.

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!

I was only intending to use the 7805 regulator for the mosfet circuit, as I believe the gate voltage needs to be less than the 15v supply. (as per second circuit.)

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.

Terry, I was confused by the 2 diodes, D3 & D5 in the second circuit (with mosfet) around R3, the 50k pot .

I will check the voltages in the first (darlington) circuit again and before the darlington as well.
Thanks
Mike