What we are trying to accomplish is digitally controlling the speed of a 18VAC motor, which can easily pull 1A (specifically old H0 Märklin locomotives, which can reportedly pull 5A at peaks). The supply power in our case is fixed at 18VAC.
Ideas we considered:
- Voltage divider with digital potentiometer which is probably not feasible due to the high current requirement.
- Using a DIAC+TRIAC dimmer circuit (with a digital potentiometer), but the existence of low-breakover-voltage DIAC's seems to be a myth. I found one 15V SIDAC but that still seems to high for our 18VAC motors.
Dimmer circuits for AC with voltages below 30V don't seem to be popular so I hope one of you can come up with/knows a circuit able to do this. Or perhaps you know an alternative for the DIAC? Not sure if this helps but a microcontroller with PWM capabilities is available.
Best Answer
simulate this circuit – Schematic created using CircuitLab
Figure 1. Modified circuit.
Try leaving out the DIAC. It's main purpose is to give a well defined breakover voltage to even out the differences between the positive half-cycle switching point ('first quadrant' in datasheets) and the negative half-cycle switching point ('third quadrant'). Without it you may see a difference in trigger point on a scope but it's unlikely to cause trouble with your model train. You will also be able to get much closer to 0° trigger angle than you could with a DIAC in circuit.
Figure 1. Triac phase control for dimmer application.
Figure 2. Triggering modes. Quadrants, 1 (top right), 2 (top left), 3 (bottom left), 4 (bottom right). Source: Wikipedia TRIAC.
simulate this circuit
Figure 2. Onboard triac control.
For onboard control you can trigger the triac directly from the micro-controller (through a suitable current limiting resistor).