Motor Spike Suppression?

[alexb1406]

Hi Folks, can someone advise the best way to deal with this. This is a switching circuit for an intermittent wiper delay system - a little complex but I want to use the original switch without adding anything that shouldn't be there. The switch on the diagram is part of the complex original switch that I have to use so I'm stuck with it. This is the theory - S1 (normally closed to earth) is opened and closed (springs back to closed). This activates monostable that sets pin 1 of the 4001 to low for a few seconds. Wiper switch is then flicked the other way to close S2 that momentarily brings pins 5 and 6 to high, and hence pin 2 to low that makes pin 3 high. This makes the transistors latch on and switches the relay on. This then activates the intermittent delay circuit.

The problem is that it keeps blowing the 4001 - it works ok till I connect the motor, and then after a few operations it stops working. The fault is pin 11 does not go low when S1 is opened and I suspect this may be due to spikes from the motor. Although I have shown the motor in the diagram that is not the whole picture - by means of another relay and more components to the switch the motor is reversed and I guess this compounds the problem of spikes. Arcing at the switch contacts may also add to it.

Is this just a matter of putting a cap across the motor? If so, what size? Or does anyone have a better idea?

 

[Reloadron]

Not seeing any attached diagram/schematic?

Ron

 

[alexb1406]

Sorry, realised after I posted - it is there now!

 

[dougy83]

you can insert a 47-100k inline with pin 13 (to the left of the 10k pullup - I'd also put a 1uF cap across the pullup and/or a schottky diode [cathode to 12V] across the motor but that's optional). The resistor should reduce the current spikes going into the 4001

 

[alexb1406]

you can insert a 47-100k inline with pin 13 (to the left of the 10k pullup

Don't you mean pin 12?

- I'd also put a 1uF cap across the pullup and/or a schottky diode [cathode to 12V] across the motor but that's optional). The resistor should reduce the current spikes going into the 4001

Do you mean in parallel across the 10k? I haven't got any 1uF caps here - would a 0.1uF be useless?

A diode across the motor won't work as the polatiry reverses to make the motor park - the motor part of the diagram isn't complete - there is another relay that reverses the motor when park is activated from the switch (S1 does that, and at the same time unlatches the transistors) - I put the motor there merely to show where it's power comes from.

I'll try what you suggested tomorrow though, thanks.

 

[dougy83]

Don't you mean pin 12?

Yes

Do you mean in parallel across the 10k? I haven't got any 1uF caps here - would a 0.1uF be useless?

A 0.1uF should do something. You can put it across the motor or across the 10k pullup. Yes, I meant parallel

 

[alexb1406]

Thanks, I've decided to go shopping and get the correct cap for this so I'll have to wait till next week.

Something I hadn't thought of earlier, the 12v signal from S2 is also used in the delay circuit to pause the astable if the wipers are switched fully on when the delay is on - that also uses a 4001 so I guess that will need a similar protection.

My thought is that it would be better to suppress the spikes as much as possible at the motor before they get a chance to go anywhere near the IC's, so would a 1uF cap across the motor be sufficient? What voltage rating? - I'm struggling to find anything above 50v locally and I think that is a little low for a 12v motor - or is it? I will still use the cap and resistor at the ICs.

 

[Mosaic]

A bidirectional TVS will solve it. I had a similar prob with cmos logic in a car alarm I built about 22 years ago. The spike from the car horn was killing the cmos after a little while.

 

[alexb1406]

A bidirectional TVS will solve it. I had a similar prob with cmos logic in a car alarm I built about 22 years ago. The spike from the car horn was killing the cmos after a little while.

Ok, I just had to google this, didn't know such a thing existed, it sounds like this may be just what I need.

Can you, or someone, point me in the direction of a suitable part. I'm looking at Fairchild PKE15CA, reverse standoff voltage 12.8v, breakdown voltage 14.3-15.8v, and clamping voltage 21.2v. I don't know if I understand the terms correctly but I think it means the voltage could go as high as 21.2 volts before it is fully capped, and that may be a little high for the cmos. If I go for a lower voltage there is one with clamping voltage 15.6v - still a little high for a cmos, but it starts to conduct at 9.4v - too low!

And, would it need a heatsink?

Would this work together with the resistor and caps as described above?

 

[cachehiker]

Reverse biased 1N4001 between each motor lead and VCC. Reverse biased 1N4001 between each motor lead and GND.

 

[alexb1406]

I don't understand how that would work - the motor is driven by Vcc and ground, and polarity can be reversed.

 

[ronv]

If you hook them up like this it will keep the voltage from going higher than 12.6 or lower than -.6. The input will probably survive that.

 

[alexb1406]

Thank you, a picture paints a thousand words! I will probably use through-hole diodes instead of the MBRS360's as this is all assembled on veroboard. One advantage I can see with this is that it will allow full charging voltage to get to the motor.

I suspect you'll probably say it isn't necessary but would the tvs and resistor/cap discussed above help together with this? - just in case?!?

 

[ronv]

What you might do is add a .01Uf to .1Uf ceramic cap across the motor terminals and from each terminal to the frame of the motor. This will help reduce brush noise.

 

[alexb1406]

Ok, I'll try that.

Would these be suitable diodes? And would they need a heatsink? It would be easier if they didn't as the flanges are cathodes and would have to be mounted separately.

 

[ronv]

More than enough. Since it only turns on and off every few seconds a 1 amp schottky diode would be ok. No heat sink required.

Like a 1N5819