View Full Version : DC motor drive circuit?

01-02-2012, 08:01 PM
I think I broke my PWM speed controller :rolleyes:

I found that when I ran the spindle the keyboard of the computer would act unreliably which makes CNC kinda tricky. I figured it was probably EMI from the motor so I tried putting a capacitor across the motor terminals to short the high frequencies (also known as a snubber). However I didn't realise my motor speed control was PWM based, and so it basically shorts all motor drive and the circuit blew. Doh!

It will cost me about 15 in parts to replace most things that could be the problem (not clear what blew).

On the other hand can you guys recommend an alternative affordable DC motor speed controller?

I have also considered just making an alternative mounting bracket and breaking apart an affordable hand router for the AC motor and speed control. Why not??

Robin Hewitt
01-02-2012, 08:11 PM
A snubber is a resistor and capacitor in series :heehee:

What kind of Volts and Amps are we talking?

01-02-2012, 08:21 PM
Stop giggling like a girl Robin :p Actually a capacitor on it's own is able to 'snub' out unwanted transient spikes, as it forms a simple 1st order filter. The problem is that PWM supplies the drive voltage in high frequency packets so it gets shorted by the cap where pure DC wouldn't.

I'm not sure what voltage to be honest as I never measured it. It is the supplied motor with a CNC3040.

01-02-2012, 09:36 PM
Anyway to follow up, the motor shows 5 Ohms across its terminals so taking the 300watt rating on the eBay site (I know!) that needs about 40V and 8 amps.

EDIT: That's all wrong! It seems the resistance when the motor is idle is different than when spinning. It actually seems to be about 180 Ohms when working. I found 37V made it draw 0.2A so that is 180Ohms. If the motor really takes 200-300watts then I need about 200V of drive.

02-02-2012, 09:32 PM
Hello! I have an update for you..

I built a 140V linear DC supply with a beefy transformer, rectifier and smoothing cap. It drives the motor quite nicely with around 100 watts. This supply should have no high frequency spikes but interestingly it still interferes with the PC, and makes the keyboard stop working.

Sometimes the supply lead going to the motor can act as an aerial transmitting noise so I changed the cable to a shielded one and grounded the shield. This improved matters a little but I still had occasional problems.

I noticed that although the stepper drive leads are shielded, they were not connected to ground so obviously provide little protection from interference! I grounded each of the leads but to be honest didn't notice much change. The keyboard still stopped responding sometimes for a few seconds.

Finally, I bypassed the motor near its terminals with a 1uF poly capacitor. This seems to have done the trick and everything is reliable now :)

So for anyone with a chinese CNC3040 router I advise using a shielded lead to supply power to the motor, and bypassing the motor with a capacitor or snubber network. I can use a simple capacitor because I am using a linear supply, but note that if you have the standard one in the control box of the CNC3040 or other pulse width modulated supply you need to be vary careful not to blow it up like I did with a simple capacitor bypass! Apparently the capacitor and resistor 'snubber' network can be used but part values must be carefully chosen not to cause problems.