FAQ
TL;DR: For 100 VDC at 21 A from 120 VAC, use a phase-angle controller sized for ≥25 A and a DC bridge; “Do not try to control induction motors.” [Elektroda, Anonymous, post #21685665]
Why it matters: This FAQ helps makers choose safe, working parts and values to drive high-current brushed DC motors from mains.
Quick Facts
- Triac/SCR current rating: choose ≥25 A continuous for a 21 A motor load. [Elektroda, Anonymous, post #21685658]
- Test loads: use a 60 W+ incandescent or a 200–300 W heat lamp to validate the dimmer first. [Elektroda, Anonymous, post #21685665]
- Induction motors: use a VFD; SCR/triac dimmers are for brushed motors. [Elektroda, Anonymous, post #21685662]
- Bridge rectifier: size for ≥25 A and full mains voltage; place before or after control as described. [Elektroda, Anonymous, post #21685658]
- No large filter capacitor with phase control; the circuit needs the zero crossing to turn off. [Elektroda, Anonymous, post #21685665]
Can I run a 100 VDC, 21 A brushed motor from 120 VAC with a triac controller?
Yes. Use a phase-angle controller sized above your motor current and add a suitably rated bridge rectifier. Builders reported success after correcting component values and using a DC bridge. Size the controller and rectifier for at least 25 A and mains voltage. Keep wiring short and heat-sink the semiconductor. Avoid adding a large filter capacitor with the controller. “These circuits can easily kill you” without proper precautions, so follow mains-safety practices. [Elektroda, Anonymous, post #21685665]
What ratings should I pick for the power semiconductor?
Choose a triac or SCR with at least 25 A continuous current rating for a 21 A load. Add a heat sink with thermal grease. A sensitive-gate device reduces the stress on the trigger network. If you use SCRs, remember they conduct one way; add a full-bridge for full-wave DC. For a triac, ensure the case style suits your heat sinking. Derate for enclosure temperature and airflow. [Elektroda, Anonymous, post #21685658]
How do I adapt a 220 VAC phase-control schematic to 120 VAC?
Keep the power device current rating high, but reduce the trigger-network values. One builder fixed a non-working 120 VAC build by lowering the potentiometer, resistors, and capacitors from the 220 VAC design. Start with smaller RC values and verify the diac type matches the trigger voltage. Validate on a resistive lamp before connecting a motor. [Elektroda, Anonymous, post #21685660]
Why won’t a small 120 V fan (induction motor) dim smoothly?
Induction motors are poor loads for triac/SCR dimmers. The controller chops the waveform, which upsets induction motor torque and can stall or overheat the motor. Use a Variable Frequency Drive (VFD) for induction machines. “Do not try to control induction motors — use a VFD.” [Elektroda, Anonymous, post #21685665]
Do I need a bridge rectifier and should I add a capacitor?
Use a bridge rectifier when driving brushed DC (treadmill) motors; many users report smoother operation on DC. Do not add a large filter capacitor with a phase-angle controller. The controller relies on the mains waveform reaching zero to turn off each half-cycle. A big capacitor can hold voltage, prevent commutation, and cause surges and noise. [Elektroda, Anonymous, post #21685665]
Where should I place the motor in the SCR/triac circuit?
Placing the motor in the cathode (return) leg of a thyristor stage allows back-EMF to feed back into the control circuit. This improves regulation under changing load. It is a known configuration in classic thyristor controllers and is recommended for precise control. Ensure wiring polarity and insulation are correct. [Elektroda, Anonymous, post #21685658]
What does the series inductor do in these schematics?
It reduces radio-frequency interference generated by the fast switching edges of SCRs/triacs. You can confirm emissions with a nearby AM radio. The inductor is not for inrush limiting on motors. Use appropriately rated wire and core to avoid overheating at high current. [Elektroda, Anonymous, post #21685665]
How should I test my controller before connecting a motor? (3 steps)
- Wire the controller to a 60 W or higher incandescent lamp as the load.
- Power from isolation transformer if available; turn the knob through its range and observe smooth dimming.
- Let it run for 2–3 minutes; check device temperature and for RF noise on an AM radio.
[Elektroda, Anonymous, post #21685665]
Is an “85 VAC output” from a dimmer the same as transformer 85 VAC?
No. A dimmer outputs a chopped AC waveform with a lower RMS value, not a clean sine at 85 VAC. Some equipment, including SMPS and certain chargers, may misbehave or not work when powered from chopped waveforms. Measure RMS with a suitable meter, but expect compatibility issues. [Elektroda, Anonymous, post #21685662]
Why did my treadmill motor “take off” when I added a 100 A bridge?
Full-wave rectification increases effective torque on brushed DC motors, so they can accelerate quickly at the same knob setting. A too-large filter capacitor can also cause surges and audible noise. Reduce the control setting, remove the big capacitor, and re-test with a resistive load first. [Elektroda, Anonymous, post #21685663]
What potentiometer and RC values worked when the first build failed?
One builder got a 120 VAC version working only after lowering the potentiometer, resistors, and capacitors from a 220 VAC design. If your dimmer does nothing, reduce the timing RC and pot value incrementally, then re-test on a lamp. Document final values once stable. [Elektroda, Anonymous, post #21685660]
Can I place the bridge rectifier before or after the controller?
You can place the bridge before the controller (mains → rectifier → load → controller) as noted by an experienced contributor. Do not add a large smoothing capacitor in this arrangement, because the controller needs the waveform to drop to zero to commutate off. [Elektroda, Anonymous, post #21685665]
Is there a simple way to verify the controller is working at all?
Yes. Swap the motor for a resistive lamp load. If the lamp dims smoothly across the knob range, the controller is functional and any issues are load-related. If it does not, revisit component values and the diac/trigger network. [Elektroda, Anonymous, post #21685662]
What basic safety rules should I follow with mains motor controllers?
Enclose all live parts, add strain relief, and earth metal chassis. Use fusing appropriate to wiring and device ratings. Keep one hand behind your back when probing live circuits. “These circuits can easily kill you if you touch the wrong thing.” Seek help from a qualified electrician. [Elektroda, Anonymous, post #21685665]
What are SCRs, triacs, and diacs in simple terms?
An SCR is a one-way latching switch for AC/DC control; a triac is a two-way version for AC; a diac is a trigger device that fires at a fixed voltage to start conduction. These are the core of classic light-dimmer and brushed-motor speed controls discussed in the thread and recommended book. [Elektroda, Anonymous, post #21685658]
Will phase control deliver full motor power?
Half-wave SCR control can give roughly 75% of motor power compared to full DC, which may suffice for tools. For maximum torque and smoothness, use a full bridge to deliver full-wave DC to brushed motors. Always size parts for current and provide cooling. [Elektroda, Anonymous, post #21685658]
