555 Timer for Motor Pulsing - Fire!

You seem to be exceeding the output current rating of the chip. Use the chip to turn on/off a power transistor or optoisolator which is rated for at least 1.5x the motor current.

Hello, scott:

This was covered a little while ago...

http://cr4.globalspec.com/thread/43874/555-Timer-for-Motor-Control

You will find some pretty good ideas there.

[Edit] As stevem advises, the best idea in thread (I think) uses opto-isolator.

Thanks for the reply guys. I will definitely make the changes suggested, and give her a go. I grabbed a handful of chips today, so, as long as I don't catch the drapes on fire, I should be able to get it going.

Would you recommend sizes for the decoupling capacitor and the cao across the motor?

47uF for the decoupling capacitor seems fairly common, it isn't to critical but you may need more depending on your motor current and power supply (what are these by the way?) especially if you are running of battery(s) where battery voltage droop becomes a problem.

As for the snubber diode (cao?) across the motor, a standard and cheap 1A 400V (or better) diode is probably going to be fine (eg-1N4007), but again it depends on the motor current.

What's the application?

Make sure the motor snubber diode can handle the kickback current. Its repetitive surge current rating should be at least as great as the full-load current of the motor at turn-off.

For power supply bypass, use at least 10 µFd paralleled with 0.01 µFd. The smaller capacitor makes up for the relatively poor high frequency response of the electrolytic unit (it may not be necessary with a new capacitor, but it is if you use an older technology electrolytic cap out of a junk box). Keep the distance between the cap and the IC power and ground leads as short as possible.

Your frequency is a little bit low and you may want to use a bjt or mosfet instead of a relay.

Hi The back EMF from the motor is killing your IC. Use a good quality Zener diode of 13 Volt in parallel with IC supply, close to IC on Pins 1 and 8. This should solve your problem. Or use an independant supply for motor and timer circuit.

Seetharaman

Please add freewheeling diode across dc-motor.

Yes it is the correct solution may be a 1N5404 diode connected in reverse bias condition across motor will take care

seetharaman

Preferably a Schottky diode which is high speed and low resistance.

So you have failed the smoke test. Time to re-evaluate

I would add a reverse diode across the motor.

I would also either use a MOSFET instead of a relay or buffer the 555 to the relay with a transistor.

Check wich 555 the circuit was designed for, the old NE555 could handle more current if I remember correctly than modern equivalents....also, I do believe (from memory) that the chips all sink current better than sourcing (as shown here).

Thanks, again. With these wonderful suggestions I know I can get it going. Thanks for everybody's time.

I was asked what this was being used for - just pulsing a small pump motor (bilge type pump) off and on.

TAKE RA=10K INSTED OF 1K AND CAPACITOR 10MF INSTED OF 100MF.