4.0v 0.7a

[Armando Herjim]

Hello. I got a little problem here. You see, I need to control two dc motors. Each one consumes .7 A so I had a variable reguator for each of them. I used the LM317 and it is supposed to give 1.5 A, but when I used them, after a while the motors stop and the variable regulator are so hot I can´t even touch them... Does anybody can give me any other idea of how to get 4 V for the motors?? My power source is a battery (11.7V, 10A). I need this motors working because they "are" the wheels of a vehicle we´re building.

Thaks a lot to all.

[mister_e]

any heatsink? don't forget... it's a motor, not a simple resistive load. there's no chance they consume ONLY 0.7 A straight.

[skimask]

One word - PWM - ok, three words...Pulse Width Modulation.
Keep it simple, use a PIC10Fxxx or something very simple. Use 2 of the outputs to run the motors, use 2 of the inputs to control the speed and/or direction of each motor. Write a simple program to use Timer0 to set the frequency of the PWM, use the 2 outputs going to the motors to turn on a couple of N-channel MOSFETs. Control the duty cycle of the pulse width with 2 buttons and a little ingenious programming. Do a Google search. You'd be amazed what you'll find.

If I've lost you on that part, you probably need to do some reading up on how regulators work, and for that matter basic electronics.
11.7v @ .7a = 8.19 watts, that's what you're drawing at the input.
4v @ .7a = 2.8 watts, that's what you're drawing at the output.
Where is the extra 5.39 watts going? Heat...all heat. Look at the datasheet for the LM317. You'll see that you can't possibly dissipate 5+ watts without a HUGE heatsink.

If you really must stick with a couple of LM317's, try preregulating that 11.7v down to 6v first, then re-regulate that down to 4v.
And for godsake, put some heatsinks on them regulators. You're killing them!!!

(climbing on soap box)
$20 says this is some sort of 'graduate' project...and it's gotta be done soon!!
(climbing off soap box)

[mister_e]

PWM may work... but if those motor are cheap and the PWM signal is not correctly filtered... it may just burn the motor. Think about those peaks!!

[skimask]

True...but at least it's better than the straight up LM317 option. How about a regulator with a soft-turn on? Just maybe not so soft, but softer than a PWM circuit.

[mister_e]

mmm, i never tried it but... how about PWM at the LM317 inputs?

[skimask]

I guess you'd still have to PWM the high current input to the 317. That just sounds rough on the chip, not to mention noisy to the rest of the circuit. Think of the EMI generated by that! It'd be ugly...

[Armando Herjim]

Hey thanks o lot!! I´ll trie those hints. Anyways if there is any more, will be welcome.

Regards