PWM and chips!

Hi, I recently built the PWM circuit as shown on the web site. To test the circuit, I used a 12v motor and 12v power supply. I turned the speed adjust potentiometer from min to max and the motor only increased in speed a small amount, which puzzled me. Then I spotted smoke coming from the potentiometer and quickly turned the circuit off. However, I wasn't quick enough and fried both my 555 chips (good job they're cheap)!
I wonder if anyone could explain why this might of happened, as I am now very reluctant to try and use the circuit again! Cheers, Dave.

Posted by Dave Terry (david.terry2@ntlworld.com) on 08/11/2003, at 23:01 GMT

Some general points to check

It is difficult to give detailed advice, but here are some general points:

If you built the circuit on stripboard, did you remember to break the tracks where necessary, including under both chips?

Check your circuit carefully to make sure every connection goes to the right place, one way to do this is to work round each 555 from pin 1 to pin 8 checking all the connections to each pin in turn. Check too for short circuits of solder between tracks - a common problem on stripboard.

I assume you are using a smooth DC supply, AC will certainly destroy the 555s!

Posted by John Hewes on 10/11/2003, at 16:15 GMT

Hi John

Thanks for your reply. I was only experimenting, so I was using breadboard at the time. I firstly tried the circuit with a 9v battery and the motor only turned very slowly. It was then that I thought I'd connect my Meccano transformer and try again. The motor still opnly turned very slowly, smoke arose and I'd had my chips!!!
I was under the impression that the gaugemaster range output was 12v smoothed, so didn't see any problem, but then I am very new to electronics and obviously missed something along the line!!!

Posted by Dave Terry (david.terry2@ntlworld.com) on 10/11/2003, at 17:29 GMT

PWM and chips

There are a few points to note when using 555 or CMOS 4000 ics on what is highly likely to be an unsmoothed supply that uses a 15v ac transformer.1 Max voltage present when circuit is in stopped position is likely to be upto 21v peak when only using rectifier and smoothing capacitor.This is unfortunately too much for the above ics whose max voltage is 18v. Therefore either use cmos op amps(CA3140/CA3240) (30v) or operate the ics through a 7812 regulator/there is the LM2940-C12 regulator if the dropout voltage headroom is a problem.

Posted by R W Neill (varient412@yahoo.co.uk) on 18/03/2011, at 16:17 GMT