Digital PWM speed controller for a brushed motor.

Another review of various things for DIY projects. This time I’ll tell you about a digital speed controller. It’s an interesting little device, but I was hoping for more.
If you’re interested, read on :)

I have a few low-voltage devices around, such as a small grinder and the like, and I wanted to improve their functionality and appearance a little. That didn’t quite work out, though I still hope to get there eventually—perhaps another time. Today, I’ll just tell you about the device itself.
The manufacturer of this controller is Maitech—or rather, that’s a name you often see on various boards and modules for DIY projects, although for some reason I couldn’t find the company’s website.

Since I didn’t end up making what I wanted, this review will be shorter than usual. But, as always, I’ll start with how it’s sold and shipped.
The envelope contained a regular zip-lock bag.


The package contains only the controller, with a potentiometer and a button. There’s no rigid packaging or manual, but everything arrived intact and undamaged.


There’s a sticker on the back that serves as the manual. That’s really all you need for a device like this.
The specified operating voltage range is 6–30 V, with a maximum current of 8 A.


It looks quite good: dark “glass” and a dark gray plastic case that looks almost black when switched off. The appearance gets a thumbs-up; there’s nothing to complain about. A protective film was stuck to the front for shipping.
Mounting dimensions:
Length 72 mm (minimum cutout in the case: 75 mm), width 40 mm, depth excluding the front panel 23 mm (24 mm with the front panel).
Front panel dimensions:
Length 42.5 mm, width 80 mm


The potentiometer comes with a knob. The knob is a bit crude, of course, but it’ll do the job.
The potentiometer is 100 kΩ, with a linear response.
As it turned out, the 100 kΩ resistance causes a glitch. When powered by a switching power supply, it’s impossible to get a stable reading. Interference picked up by the wires to the potentiometer makes the readings jump by ±2 counts—and if that were all, it wouldn’t be so bad, but the motor speed jumps along with them.
The potentiometer has a high resistance and carries very little current, so its wires pick up all the interference around them.
This problem disappears completely when using a linear power supply.
The wires to the potentiometer and button are about 180 mm long.


The button is nothing special. It has normally open contacts, a 16 mm mounting diameter, and is 24 mm long. There’s no backlight.
The button switches the motor off.
So, when power is applied, the display turns on and the motor starts. Pressing the button switches it off; pressing it again turns it back on.
When the motor is off, the display is off too.


The circuit board is under the cover.
The power and motor connections are brought out to the terminals.
The positive terminals are connected together; the power switch controls the motor’s negative wire.
The potentiometer and button connect via plugs.
Everything looks neat. The capacitor leads are a little crooked, but I think we can forgive that :)


I’ll hide the rest of the teardown under a spoiler.

First power-up. As it happened, I did the tests before removing the protective film, so it's still on in these photos.
The display has good contrast, is reasonably bright, and is easy to read.


First, I decided to try it with small loads, and was disappointed right away.
No, I have no complaints about the manufacturer or the store. I just hoped that a device that isn't exactly cheap would have motor speed regulation.
Unfortunately, it's just an adjustable PWM controller; the display shows the duty cycle from 0 to 100%.
The controller didn't even notice the small motor; its load current was laughably low for it :)


Attentive readers have probably noticed the wire gauge I used to connect power to the controller.
Yes, I decided to take a more ambitious approach next and hooked up a more powerful motor.
It's obviously much more powerful than the controller, but its no-load current is around 5 A, which let me test the controller closer to its maximum operating conditions.
The controller performed great. By the way, I forgot to mention that when you switch it on, it smoothly ramps the PWM duty cycle from zero to the set value, providing a soft start. The display immediately shows the set value, unlike a variable-frequency drive, which displays the actual value in real time.
The controller didn't fail; it warmed up a little, but nothing serious.


Since the controller is a switching type, I decided to poke around with an oscilloscope out of curiosity and see what was happening at the power transistor's gate in different operating modes.
The PWM frequency is around 15 kHz and doesn't change during operation. The motor starts turning at about 10% duty cycle.




Originally, I planned to install the controller in my old (probably ancient by now) power supply for small power tools (I'll write about it another time). In theory, it would replace the front panel, with the speed controller mounted on the back. I wasn't planning to add a switch, since the device conveniently starts up in the on state.
It should have turned out nice and neat.


But then I ran into a bit of a disappointment.
1. The display was slightly smaller than the front-panel cutout, but the bigger problem was that it wouldn't fit depth-wise: it hit the posts that hold the two halves of the case together.
And while I could have trimmed the plastic housing around the display, I wouldn't have done that anyway, because the controller board was in the way too.
2. But even if I'd solved the first problem, there was a second one: I'd completely forgotten how my power supply was wired. The controller switches the negative supply line, but further down the circuit I have a relay for reversing, switching on, and forcibly stopping the motor, along with the control circuit for all of that. Reworking it all turned out to be much more complicated :(

If the controller had speed regulation, I would have gone to the trouble of reworking the control and reversing circuits, or modified the controller to switch the positive supply line. As it is, I might still rework it, but without much enthusiasm, and I have no idea when.
In case anyone's interested, here's a photo of the inside of my power supply. I built it about 13–15 years ago; it's worked almost all that time without any problems. I had to replace a relay once.


Summary.
Pros
The device works perfectly.
Neat appearance.
Good build quality
Everything you need is included.

Cons.
Doesn't work properly with switching power supplies.
The power transistor has little voltage headroom
The price is high for such limited functionality (though that's relative).

My opinion. If you overlook the price, the device itself is quite good: it looks neat and works well. Yes, it isn't very resistant to interference. I think that's easy to fix, but it's still a bit disappointing. I also recommend keeping the input voltage below 25–27 V.
What's more disappointing is that I looked at quite a few ready-made speed controllers, but couldn't find one with speed stabilization. You might ask why I need that. Well, I once got to use a sander with speed stabilization, and it's much nicer to work with than a regular one.

That's all. I hope you found it interesting :)

The product was provided by the store for review.