Constant Current mod to a DC Motor Speed Controller

[yamajason]

This is for a one-off project making a HHO gas generator for a small engine.  I would like to use an off the shelf DC Motor Speed Controller (like the one linked below) and modify it to provide constant current control.  I plan to install a current shunt in line with the load to feed a (current indicating) voltage to the controller.  What is the best way to do this?
like this one https://www.aliexpress.com/item/2251832777845127.html

[Terry Bites]

The regulator from Ali has no current limit. It might have a fusible reistor CW1.
It doesnt have any obvious feedback path, it appears to be unregulated, so not a simple mod.
The schematic maybe wrong of course. Some else one may have an idea of how to mod this piece of junk. I dunno.

[langwadt]

This is for a one-off project making a HHO gas generator for a small engine.

what's the point?

[Siwastaja]

You can see from the picture alone the design is total crap.

DC motor speed controller of that size must have current control, that's basically the definition of what it is. Modifying something else to become a decent motor controller is probably more work than designing one from scratch, especially if the lack of current control is not the only issue; you need to learn everything related, anyway.

Now, design of brushed DC motor speed controllers isn't rocket science so it's entirely possible to learn to do it. It boils down to:
* having a half bridge (two MOSFETs; in some cases, one can be a diode instead)
* using a gate driver to drive the MOSFET gates; bootstrap IC is often fine
* adding current sense path, usually low side shunt resistor + amplifier IC
* drawing tight layout for the power components and adding DC link capacitors
* connecting it all together with a microcontroller; almost any will do. On-chip analog comparator with DAC-generated reference level helps reducing external parts.

But of course, if you have very little experience in power electronics, it's not going to be a one-weekend project but a bigger learning experience.

[jwet]

You don't actually say how much current your motor draws.  If its and amp or two, the Ali circuit shown is marginally ok.  It isn't very good at all.  Its just a 555 pwm driving an oversized low side switch with rather poor drive.  For low currents, it might work but it has a lot of shortcomings.  The type of PWM controller do indirectly control "average" current but without measuring current, its all pretty pointless.  With PWM control, you are indirectly controlling average motor current by turning the motor inductance on and off.  The current rises when the switches are closed and falls in between, in order to keep the current flowing, you should have a half bridge or a diode to conduct in the off cycles.  An open loop controller can work but it must be tuned for the motor inductance in your application.  Most hobbyist motor controller circuits are pretty bad- its complicated.  I would go to a good IC driver supplier like Infineon  and look at their circuits for driving motors.  If you can't use their circuits directly, you can use some of the ideas.  Like the previous poster said, driving FET's with a 555 is a bad start- it can only sink and source a couple hundred mA, this will create a very slow turn on with big FET's.  You need a proper FET driver and a lot of thought.  Best would probably be to buy a descent driver like those used in RC model cars if you're driving anything beefy.

[Siwastaja]

You don't actually say how much current your motor draws.  If its and amp or two, the Ali circuit shown is marginally ok.  It isn't very good at all.  Its just a 555 pwm driving an oversized low side switch with rather poor drive.  For low currents, it might work but it has a lot of shortcomings.  The type of PWM controller do indirectly control "average" current but without measuring current, its all pretty pointless.  With PWM control, you are indirectly controlling average motor current by turning the motor inductance on and off.

No, a PWM circuit won't even control "average current"; it's average voltage control. PWM as-is is just equivalent to variable voltage control, and motor current is (Vsupply - Vbackemf)/Rmotor. Practically what this means is, assuming 100% efficient motor, apply just 10% duty cycle, and current initially (or to stalled rotor) is infinite. At 10% rated rpm, current is then zero.

Remember, in an ideal motor, voltage ~ speed and current ~ torque.

In other words, just controlling PWM duty does not even try to control current; it tries to (poorly) control speed, and current is whatever it is; acceptable near the chosen speed, excessive when rotation is slower than the set speed.

With very inefficient motors, R is large and current is limited by the motor R itself. With simple PWM, this just results in very crappy speed control, but no other harm because of the limited current. But enter higher power, and motors will be more efficient. Stall current will be excessive, and the only way to deal with this is to utilize the inductance of motor to limit current, by terminating PWM cycles before current reaches too high - but this requires active sensing circuit.

[macboy]

Lots of people chiming in to talk about controlling DC motors but that isn't what the OP is wanting to do. Read the question. He wants a constant current power supply to generate hydrogen and oxygen from water.

[Terry Bites]

HHO is not very common parlance, hence the confusion. The word motor stimulates engieers' motor glands isntantly.

Do you really need a current source or just a psu with a settable current limit?
The latter is more often than not a feature on any decent DC-DC module.

The simple answer is to just buy a DC-DC converter with CC mode. Search for "DC-DC CC" Battery chargers frequently have a CC mode.

eg www.alibaba.com//trade/search?fsb=y&IndexArea=product_en&CatId=&tab=all&SearchText=dc-dc+CC

[BeBuLamar]

A DC motor drive as a constant current source is a poor choice. It's better to build a simple unregulated power supply with transformer, rectifer bridge and a filter capacitor and then feed the output to a switching DC-DC converter with current limit feature. Something like the Ruideng DPS series which is inexpensive enough.

[Siwastaja]

Lots of people chiming in to talk about controlling DC motors but that isn't what the OP is wanting to do. Read the question. He wants a constant current power supply to generate hydrogen and oxygen from water.

Oh, sorry, I missed it because OP did not say this at all; in absence of any mention it would be modified for a totally diferent use, I thought it's a fair assumption that a motor controller will be used to control motors, and addition of constant current mode happens to be a pretty obvious requirement for a motor controller! I do know what HHO is, but assumed there are some other magical steps OP did not disclose, and somewhere in the process was the motor.

So it seems OP just needs a very low-voltage switch mode current source to drive the electrolysis, yes? For this purpose, forget about the modification of crappy motor controller, it's totally unsuitable for the job. I don't know why this mod seems to be a thing. If you want to simply try throwing some cheap Chinese Aliexpress crap at it, try the CC-CV buck modules, with adjustable current trimpot. But anything with diode in it (non-synchronous rectification) will be very lossy due to very low voltage of electrolysis, maybe 40-50% at most. And I bet cheap Chinese buck modules are non-synchronous, the ones I've seen and used are.