Help with a DC motor speed controller circuit
Pretty good schematic for a newbie! I've re-drawn in a more typical arrangement to make it clearer what's going on. Typically you draw from left to right as power flows, and from top to bottom as voltage falls. I've left off the "interconnector cable" - all that would do is override the speed control and put it in full speed mode.







simulate this circuit &ndash; Schematic created using CircuitLab

Now we can see this wont work. The pot is designed to control the voltage at the base of Q2, which controls the current into the base of Q1, which through amplification, controls the current into the load. But the pot needs a voltage to control and where it is at the moment it is going to have practically no voltage across it once Q1 turns on.

If the motor was actually where I've indicated with the arrow, it may work. That would turn the speed control into a low side switch, which is generally how NPN transistors are used.

I suggest you double check your inspection of the original circuit with an eye to the connection of the motor.

Why the diodes burnt is anyone's guess because it's not clear what modifications you made. It's a bit of a worry though - they should be rated for the full current of the motor, so I suspect you've inadvertently introduced a short somewhere.