More DC-51 motor speed controller reverse engineering

EinW DC-51 possible circuit 1421

Approx circuit of DC-51 motor speed controller

I like it, and will probably change R28 from 25mΩ to 100mΩ to better match my 0.9A motor – the controller is set up for 4A max motors and the gain does not seem to be high enough as the motor resistance compensation cannot be moved into instability to fine-tune it, and the current limit works right up one end.

Safety note for folks not used to working with the mains: All electronic parts of this DC-51 motor controller circuit are connected to live mains and could kill you. This blog and its diagram may contain accidental errors. 

The main power circuit is a simple two-thyristor two-diode controlled bridge rectifier powering the motor via a current sense resistor. There is no output capacitor, so the positive output voltage to the motor is thyristor-chopped full-wave rectified mains and bounces up and down from the mains neutral voltage to +380V (plus transients, the negative or which are eaten by D7).

It is to this very lively rail that the analogue control circuit is referenced to (which would be ‘0V’ nearly all other circuits). Despite its dynamic nature, I have labelled it ‘240Vdc’ in the diagram for simplicity.

The transformer with its associated bridge rectifier and four capacitors create plus (‘240Vdc+Vcc’) and minus (‘240Vdc-Vee’) rails around 240Vdc to run the quad op-amp and the rest of the circuit. The extra diode D1 is a small modification from standard – more of this later. I don;t know the values of the two Zeners. I think they are nearer 5V that 15V, but don’t know,

Neatly, the current sense resistor is positioned between the controlled rectifier and the motor,  so it provides a small positive voltage proportional to motor current for the control circuit.

The current limit circuit (see diagram) compares this voltage to an adjustable negative voltage using an op-amp wired as a comparator (naughty with most op-amps).

The speed sag compensation circuit, which can compensate the way motor brush and armature resistance slows rotation under load, amplifies the current sense voltage modified by the ‘IR comp’ pre-set pot.

Speed feedback R23 and R24 are another feedback path, providing a signal proportional to the motors terminal voltage, which is roughly proportional to motor speed. Because of the high resistance and where it is connected, it provides a negative current with respect to ‘240Vdc’ that is near zero at zero revs and gets more negative as speed increases.

The speed set point is provided by the 10kΩ speed control pot on the front panel of the unit.

All four preceding circuits are bought together (via D3, R20, R24 and R17) at the ‘virtual earth’ (actually ‘virtual 240Vdc’) at pin2 of the LM324 op-amp, where their currents are summed to make a control voltage for the thyristor timing circuit. Its output voltage (pin1) rises to ask for less power and falls to request more power.

Because of diode D3, the current limit circuit can only pull this control loop in one direction – up to cut current to the motor if it gets too high.

The circuit around the pnp transistor is a zero-crossing detector, with D1, R5 and R7 keeping the pnp ‘on’ the whole time except when the mains cycle is very close to it zero crossing when, twice a mains cycle, it turns ‘off’ briefly allowing R9 to charge C13 via the diode – resetting C13 to ‘240V-Vee’.

After this, C13 discharges through R26 over the half cycle towards 240Vdc – this is the ramp generator for the thyristor timing circuit.

The timing comparator (another abused op-amp!) compares the ramp with the summed-and-conditioned control voltage via R12, turning on the npn current amplifier when the ramp passes the control voltage. Feed to the thyristor gates is from the npn emitter via R1 and R2 (which might be swapped in the diagram compared with the pcb).


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