Variable DC Motor Control

[tuttuttut]

Hi,

To start with, I know this circuit doesn't include an Arduino, however I hope you can help / enlighten me some...

I'm basically using this pretty standard MOSFET circuit:

(I also have a capacitor and flyback diode included, my MOSFET is an IRF540, with 10k pot and resistor)

The motor runs, however there is no change when adjusting the potentiometer??? I've tried different pots and MOSFETS, all appear to be functioning, yet I get no variance in output!

Any ideas please...

Many thanks

[cedarlakeinstruments]

Put the motor on the Drain side of the FET instead of the Source and that should help.

[tuttuttut]

I will try but now you say it, if I'd drawn the circuit out myself I may have noticed that, cheers

[tuttuttut]

No... As you said, put the motor Drain side didn't appear to work... I looked at the following (which shows it done as said), and set up a 5v input and tried that too.

MOSFETs and How to Use Them | AddOhms #11

Again the motor runs but no variance, so I tried the following, which puts it back down on the Source side.

Anyway, time ran out for today.

The AddOhms video is what I used when did this using an Arduino, so I'm a bit miffed at the moment... It'll probably come down to my foolishness in the end...

[cedarlakeinstruments]

IIRC, an IRF540 (if that's what you're using) will not conduct much with a 5V input so if you want to control the motor with a pot like this, you should have the high side of the pot at 12V.

[MarkT]

We don't do power control that way (analog) (perhaps back in the 1970's we did, but not for 50 years. You would need a sizable heatsink on the MOSFET and it would get very hot.

You use PWM to switch the MOSFET, have the motor on the drain, add a free-wheel diode across the motor, and probably not need any heatsink. This is switch mode motor control, universally used these days.

[tuttuttut]

Ah, that's helpful...

I was doing this for some tools which didn't warrant a microcontroller or separate 5v supply.

So, instead I'd use a 555 IC, any other alternatives from those in the know?

Cheers

[tuttuttut]

I set up a quick circuit using a nano to drive the gate... It all worked fine, but then the potentiometer didn't do anything and the motor ran at full... This has now happened 3 times today... No unplugging, yes there is a heatsink, flyback diode, pull down resistors...

Have I bought a cheap bad batch of MOSFETS or why has this happened three times?

How do I protect the MOSFET?

Cheers

[tuttuttut]

Upon further investigation, I read here:

1N4007 or MBR340 for Flyback ?

You didn't mention if the motor is being PWM'd or on continuously. If PWM'd I would use the MBR340 + a 0.01µf ceramic capacitor. Reasoning: Assuming your switching times are fast (i.e. the time it takes to transition from On to Off and vice versa. The 1N4007 is not good because it will conduct in the reverse direction for a short period of time when switched quickly (Google reverse recovery time and /of look here Reverse Recovery Time of a diode) The 0.01µF capacitor is to reduce voltage noi…

Not fully understanding... I believe this to be a snubber network (???)

I found this to look useful:

RC snubber circuit design and calculations for inductive loads

Any idea what a typical 775 frame motor has for inductance please?

Got lots to learn tonight...

Any suggestions for typical values used?
24V DC supply, 2.5A believed load

[MarkT]

@tuttuttut

Any idea what a typical 775 frame motor has for inductance please?

Datasheet for the motor is the place to find this - the frame size is not sufficient, this is a property of the motor and the winding.