Component Ratings for Adjustable AC Power Supply for 100VDC 21A Motor from 120VAC

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#1 21685655 12 Dec 2021 18:31

Ali mowahed Ali mowahed

Anonymous

Post #1
21685655 12 Dec 2021 18:31

Hello,
I like to make an adjustabale AC power supply that I can then use a full bridge rectifier to run a DC motor which needs 100VDC and with a 21A current rating.
I like to use 120VAC as the power source. Possibley 220VAC if I have to.

In my understanding I need the following:
1. A triac. BTA41-600 or?
2. A potentiameter with a rating of 100-500K?
3. A Diac DB3?
4. A resistor 500Kohm?
4. A capacitor 100nF?

I have the wiring schematics but I don't know the ratings of the components and what I like to know how the rating is determined which is what I really like to know.
I also have got the impression that a capacitor can be of larger value ( has no harm) than under value.
Thank you in advance for any feed back.
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#2 21685656 23 Dec 2021 08:13

David Ashton David Ashton

Anonymous

Post #2
21685656 23 Dec 2021 08:13

Not an adjustable AC power supply but a phase control circuit commonly used for speed control of electric drills, light bulbs etc:

https://www.electroschematics.com/motor-speed-regulator-with-triac/

This is for 220V but it should be possible to adapt it for 120V. You would need a beefy triac for 21A.
#3 21685657 24 Dec 2021 08:52

Ali mowahed Ali mowahed

Anonymous

Post #3
21685657 24 Dec 2021 08:52

That looks like a nice circuit.
I can think of a more robust triac such as BTA -800B version.
Aside from that why does all these circuits are 220VAC version.
It appears to me that these cicuits must be designed and provided by non=western countries. I can not think of other reason.
Nonetheless I still need to know all the other values that need to be changed so 120VAC would work as the supply.
Thanks in advance if you happen to stumble for those values.
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#4 21685658 24 Dec 2021 14:56

David Ashton David Ashton

Anonymous

Post #4
21685658 24 Dec 2021 14:56

Ali this is a fairly huge subject and I would recommend that for a better understanding of the principles involved you download and study this book:

http://www.introni.it/pdf/Marston%20-%20110%20Thyristor%20Projects%201972.pdf

I got this book when I was about 20- something (about 40 years ago now) and it has been invaluable.
Most motor control circuits will work just as well on 110V as on 220V, but in your case the one big factor is the large current (21A) that you will be switching. Make sure your triac or thyristor is well rated for this current (at least 25A) and get one as sensitive as possible so it does not overload a circuit designed for smaller components.

I hope this will notice that many of the circuits use Thyristors (SCRs) which only conduct one way so effectively you are supplying your motor with DC - albeit half wave rectified. Most motors will give at least 75% power under these conditions but if that's not good enough then you could put a full wave rectifier before the circuit. Your Bridge Rectifier will also have to be rated at least 25A and the full voltage of the mains input.

You will also notice that some circuits place the load (motor) in the cathode lead of the triac, rather than in the anode lead. This lets the motor Back EMF give some feedback to the circuit and makes for very precise control under varying motor loads, and I have tried this and would recommend it. This is covered in the book.

Hope this helps... David
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#5 21685659 25 Jan 2022 08:02

Ali mowahed Ali mowahed

Anonymous

Post #5
21685659 25 Jan 2022 08:02

Hello David,
I finally put together the circuit and I got basically nothing.
I just made a breadboard version just to run a 120V fan motor, just a small fan. In fact when I unhooked the the potentiameter and even the triac, the motor ran. In fact just as long a I had one of the capacitor the fan ran, like either the one on the first left or the last capacitor in the schematic.
In the link you sent me, in one of the comments there a gentleman had placed a slight modified version of it so one can use 120V for it.
The only thing different I have done is I just used regular resistor and not one with higher wattage, just to see if it works.
You can see everything in the picture of the breadboard, they are all in the right place. Of course the fan and power input is missing, but that is not part of the circuit itself. I installed the power and the fan just as the schematic shows. All components are brand new except the Triac. The triac I used is the 800C version, it tested fine.
All right value capacitors and the one inductor.
#6 21685661 26 Jan 2022 09:29

Sekemiat KE Sekemiat KE

Anonymous

Post #6
21685661 26 Jan 2022 09:29

I am working on a similar project and the only difference is the AC output that am focused on. Mine is to get 85 VAC output and I have not come across a transformer that can do the same. Am worried if this could be achievable too.
#7 21685660 26 Jan 2022 09:29

Ali mowahed Ali mowahed

Anonymous

Post #7
21685660 26 Jan 2022 09:29

After my last message, I got it going.
I had to change them specs.
The values on that improved version was wrong.
I lowered the value of the potentiometer and resistors and the capacitors.
#8 21685662 26 Jan 2022 09:57

David Ashton David Ashton

Anonymous

Post #8
21685662 26 Jan 2022 09:57

Hi Ali, glad you won, sorry I was away for a bit.
A couple of comments
These circuits are intended for brush type motors so a small fan motor (they are usual induction motors) would probably not give good results. Use an electric drill (one without a built in speed control if possible) or a high wattage incandescent lamp (60 watts or more). Heavily inductive or capacitive loads may give problems (but most brush motors should be OK).
With amateur designs you often do have to experiment with the values, good work for getting them working!

Sekemiat KE - bear in mind that these designs do NOT put our pure AC, rather an AC with part of each half-cycle missing. So you can get 85V RMS (Use a meter to measure the output) but many commercial devices (eg battery chargers,and anything that used a switch mode power supply) may not work or may do strange things when connected to one of these controls.
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#9 21685663 27 Jan 2022 08:41

Ali mowahed Ali mowahed

Anonymous

Post #9
21685663 27 Jan 2022 08:41

I seriously thought that this circuit did what these chinese scr speed controller does.
Not even close.
I have used these speed controllers and have installed a full bridge rectifier and they work fine if you don't except too much from them.
In fact they ran them Treadmill motors just fine.
I guess they are different circuits.
Treadmill motors although can run on AC but they run much smoother with DC.

So I went to the garage and installed a full bridge rectifier with what I had. The rectifier is high end version that can do 100A.
The instant I plugged everything in, the Treadmill motor took off.
So I figured I will use a higher value potentiometer like a 500K. It made no difference.
Plus the motor acted like it was running on AC.
And I didn't like the high voltage capacitor I had there after the rectifier, it was making sound. I took it out. It never made any sound when I had it hooked up with them chinese supplies.

I basically was on a plight that I would have easy AC to DC adjustable power supply.
That shows how much I know.
If it was that simple, why would those power supply circuits look so complex. I guess there is reason for it.

The two picture files attached are far apart while both have the same name.
#10 21685664 27 Jan 2022 08:41

Ali mowahed Ali mowahed

Anonymous

Post #10
21685664 27 Jan 2022 08:41

So are you saying that I if I install a full bridge rectifier and maybe even a capacitor I can comfortably run a brushed DC Treadmill motor just for light usage such as a bench top sander or maybe even a grinder? I won't be climbing on it.

Also what is the purpose of that inductor?
If it is to control an inrush current, I wonder if it will be toast with hardly any current passing through it first before it even has a chance to control it. Or is it that it can not deal with a load in the form of an induction motor?
Thanks in advance for all your advice and support.
#11 21685665 27 Jan 2022 18:27

David Ashton David Ashton

Anonymous

Post #11
21685665 27 Jan 2022 18:27

Ali the circuits I have suggested are very similar to what the Chinese speed control / Dimmer circuits use.
If you have your components right then test your circuit with a 60 watt or bigger incandescent (filament) lamp. This is a resistive load and if the circuit works with that you at least know that any further problems are to do with the load. (You may find trouble finding incandescent bulbs these days but you should be able to find a heat lamp - probably about 2-300 W)

Do not try to control induction motors - these SCR circuits are not made for them. For induction motors you need a Variable Frequency Drive (VFD).

Any series inductors are for limiting interference. Thyristors (SCRs) switch very fast and can generate lots of RF interference, try putting an AM radio nearby to check this.

You can put a full bridge rectifier in front of the control circuit - ie mains - rectifier - load - controller, but don't put any capacitors, these circuits depend on the voltage going right down to Zero wo switch the load off. The capacitor may be causing some of your problems.

Lastly take great care here. Get help from an electrician, preferably one who knows power electronics. These circuits can easily kill you if you touch the wrong thing.

Also you referred to pictures but there were none attached that I could see.
#12 21685666 03 May 2022 21:02

zuritusty zuritusty

Anonymous

Post #12
21685666 03 May 2022 21:02

Ali this is a fairly huge subject and I would recommend that for a better understanding of the principles involved you download and study this book:

This book is very helpful. But I'm using a different website for reading because it allows me to listen to it. It's so comfortable when you are going somewhere of your eyes are tired after long work. This book helped me so much, so I recommend it too.
#13 21685667 03 May 2022 21:03

Maurizio Di Paolo Emilio Maurizio Di Paolo E...

Anonymous

Post #13
21685667 03 May 2022 21:03

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Topic summary

✨ The discussion centers on designing an adjustable AC power supply using a triac-based phase control circuit to power a 100VDC, 21A brushed DC motor from a 120VAC source, with possible adaptation to 220VAC. Key components considered include triacs (BTA41-600, BTA-800B), a potentiometer (100-500K), a DIAC (DB3), resistors (~500KΩ), and capacitors (~100nF). Emphasis is placed on selecting a triac with a current rating exceeding the motor's 21A load, ideally 25A or higher, and ensuring sensitivity to avoid overloading control circuits. The circuit type discussed is a phase angle controller commonly used for speed regulation in resistive or brushed motor loads, not suitable for induction motors or heavily inductive/capacitive loads. Full bridge rectifiers can be added after the AC control stage to supply DC to the motor, improving performance for brushed motors such as treadmill motors. Component values, especially potentiometers, resistors, and capacitors, often require empirical adjustment for proper operation at 120VAC. Inductors in series serve to limit radio frequency interference generated by rapid switching of thyristors. The discussion references a classic thyristor project book for deeper understanding. Testing with resistive loads like incandescent lamps is recommended to verify circuit functionality before connecting motors. The use of high-wattage resistors and appropriate component ratings is critical to avoid circuit failure. Capacitor values can be increased without harm but may affect circuit timing. The overall approach is a cost-effective, experimental method rather than a pure adjustable AC power supply, with limitations in waveform quality and load compatibility.
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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]

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