Hello,

The 10X scope probe has a very high input impedance.
Try to put a resistor as load on the RPM connection to ground.
A value of 1 - 10 K and see if there is any difference.

Bertus

Thanks but still getting the spikes. I think I am going to have to clamp it with a 5V zener.

 

I think it is a 2 pole motor, controlled by halls. So you get 2 pulses per revolution.
You can either clamp it with a 5v zener to ground or a diode to +5.
The faster the motor spins the higher the back emf (think of it as a generator when not being driven) so if you slow it down the voltage will go down.

 

I think it is a 2 pole motor, controlled by halls. So you get 2 pulses per revolution.
You can either clamp it with a 5v zener to ground or a diode to +5.
The faster the motor spins the higher the back emf (think of it as a generator when not being driven) so if you slow it down the voltage will go down.

That did it. Thanks for your help.



P.S. I used a Schottky tied to 5V.

 

Yay! Nice going Ron.

Ron

 

Yay! Nice going Ron.

Ron

I am guessing in actual practice, it will be safer to use a zener? If my 5V would drop out for some reason then the spikes would occur again frying my PIC.

 

I am guessing in actual practice, it will be safer to use a zener? If my 5V would drop out for some reason then the spikes would occur again frying my PIC.

Well figure it this way, a 5 volt zener diode cost how much and is how difficult to include? Following due thought I vote to include the zener.

Ron

 

I don't think it matters as long as you have some resistance in series before the diode.

 

Well figure it this way, a 5 volt zener diode cost how much and is how difficult to include? Following due thought I vote to include the zener.

Ron

But if you don't have one one hand.... Time to hit up eBay. Can't believe I am out of these.

 

I don't think it matters as long as you have some resistance in series before the diode.

Actually it does. I tried it. Disconnected 5V and the spikes came back. A resitor is only going to limit the current.

 

For a belt and braces approach you could combine the two methods Ron described, i.e both A and B as below:-

 

For a belt and braces approach you could combine the two methods Ron described, i.e both A and B as below:-
View attachment 79063

Thanks Alec

 

Was there any other load in the circuit when you disconnected the +5 volts??
It's true it will try to power up your circuits thru the resistor but usually a few ma. won't do much. Without the clamp diode it will do the same thing except thru the micro input protection diodes. Of course it will only go to 5 volts with the zener there.

 

Will you be using the micro to measure the amplitude of the tacho pulses (minus the spike), or only their timing/frequency? If the latter, then a simple potential divider could bring the volts way down, e.g. to 1V, to be absolutely sure of no ill effects.

 

Will you be using the micro to measure the amplitude of the tacho pulses (minus the spike), or only their timing/frequency? If the latter, then a simple potential divider could bring the volts way down, e.g. to 1V, to be absolutely sure of no ill effects.

No I was planning on counting the pulses.

 

Was there any other load in the circuit when you disconnected the +5 volts??
It's true it will try to power up your circuits thru the resistor but usually a few ma. won't do much. Without the clamp diode it will do the same thing except thru the micro input protection diodes. Of course it will only go to 5 volts with the zener there.

There was not. I think I will toss in the zener anyway. It won't hurt.

 

If you get a chance could you slow it down with your finger and see if the amplitude of the square part of the wave gets lower. Just curious.
(has to be done without the diodes)

 

If you get a chance could you slow it down with your finger and see if the amplitude of the square part of the wave gets lower. Just curious.
(has to be done without the diodes)

Real hard to tell if it has an effect on amplitude. If it does then it is not by much.