Here's how you could use back-emf for feedback to control motor speed.
Voltage is picked off at the FET drain and, provided the power supply is stable, will vary directly with back-emf. Works ok in simulation. Rather than leave the FB input floating I've connected it to Vref via 100k. Not sure what the optimum arrangement is.

 

Here's how you could use back-emf for feedback to control motor speed.
Voltage is picked off at the FET drain and, provided the power supply is stable, will vary directly with back-emf. Works ok in simulation. Rather than leave the FB input floating I've connected it to Vref via 100k. Not sure what the optimum arrangement is.

Cheers man. I will test this out later in the week. what did you use to simulate it? LTspice?

 

Yes, LTS. Note that I modelled the motor simply as a resistor and didn't include the freewheel diode across the 'motor'. The sim was just to show an example of a feedback arrangement.

 

Not sure about the tl494, but the outputs of the sg3525/lm3525, which is a similar chip, have a dead time between the output pulses. When used together, at low PWM percentage, they cause a problem like your describing.

 

Yes, LTS. Note that I modelled the motor simply as a resistor and didn't include the freewheel diode across the 'motor'. The sim was just to show an example of a feedback arrangement.

I will test the circuit tomorrow.

Not sure about the tl494, but the outputs of the sg3525/lm3525, which is a similar chip, have a dead time between the output pulses. When used together, at low PWM percentage, they cause a problem like your describing.

I do have TI's UC3638 PWM motor controller http://www.ti.com/lit/ds/symlink/uc3638.pdf

and UC3637 http://fribotte.free.fr/bdtech/Drivers/18332.pdf

Thinking they would be much better yet i have read through the datasheets many of times but dont fully understand how to use the chip because they both do look similar to the TL494 but for motors with a couple of cool features.

 

Here's how you could use back-emf for feedback to control motor speed.
Voltage is picked off at the FET drain and, provided the power supply is stable, will vary directly with back-emf. Works ok in simulation. Rather than leave the FB input floating I've connected it to Vref via 100k. Not sure what the optimum arrangement is.

That circuit doesn't vary the pwm/speed. Just tested it and it approx 200khz at 87% duty. Doesn't vary regardless of what pot you use.

 

well so far i went back to my circuit here (http://forum.allaboutcircuits.com/attachment.php?attachmentid=63584&d=1389876516)

changed it a little and put that 4.8v feedback loop in alec_t mentioned (http://forum.allaboutcircuits.com/attachment.php?attachmentid=63644&d=1389999697)

and it has certainly improved it when i was running on a 12v smps. However changing back to the 12v battery, it started to fluctuate but not as much as it was doing the first time round. So i guess now i will have to use a buck-boost converter to stablise the supply better in order for it to compensate that bit more.

Any suggestions on decent buck-boost converters rated at 12V 3A minimum output?

I have seen the ones on ebay as its pre assembled but i don't actually know if its worth using. I would prefer not to build one as i need to get this project sorted very soon but i am not entirely sure if there are any different converters other than the ones on ebay. (http://www.ebay.co.uk/itm/3-5-28V-t..._Measurement_Equipment_ET&hash=item51ab7d061a)

Any suggestions wouldn't go a miss.

Cheers once again guys.

 

Here are the sim files. Had to rename the symbol file as a .txt to get it to upload.
The .txt file extension should be changed to .asy