Diode sizing

I size my flyback diodes so that their maximum voltage & current ratings are like 5-10x the expected current & voltage of the coil. So for 24VDC relays and contactors I just end up using 1N4007 diodes off digikey:


They're cheap and always in stock. Also, 1N4001 is basically just as good, but has a 50V rating (never had any issues with it, but i like my maximum voltage rating to be a little higher).

 
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The current and voltage ratings of the diodes there are maximums. So them being higher than what the expected current and voltage won't be an issue for operation (just means they will hold up against any unexpected crazy voltage/current spikes.)

The other important thing to take into account is Forward Volage. You need the forward voltage value to be lower than your expected voltage (i.e. it needs to be lower than 24VDC) in order for the diode to allow current through. The 1N4007 & 1N4001 have forward voltage of 1.1 volt.
 
Also of note: remember that utilizing a flyback diode WILL cause your contactor to open a little slower. So this may lead to slightly accelerated wear on the contact pads from arcing/sparking, and there will be a slight delay before the load is de-energized.

So while using a flyback diode is important to protect your controller's transistor outputs when being used to control inductive loads (i.e. relay/contactor coils), you may want to consider utilizing a controller with relay outputs instead. This way you can have the relay outputs drive any contactor coils, then you don't have to worry about using diodes and having a delay.
 
More details.

The problem is a counter on an auger that adds three counts when the power to contactor is cycled off. Low speed so the the auger is not coasting. 12v DC Control Power to the motor contactor coil and the counter dry contacts are in the same cable. I’ll post a drawing when I get one.

Will contact bounce have any affect? I always thought of that being when energized.

I’ll try to get a drawing.

Manual switches. No solid state other than the counter.
 
What is the periodicity of the auger being turned on/off? I can see a few ways to solve this issue with varying cost: Replace contactor with a different one less prone to bounce (ABB comes to mind).

Move the input for the counter from the contactor's aux contact to a separate relay who's coil is energized by the same switch that energizes the contactor's coil.

Add on-delay relay between aux contact and counter input. The diagram was made using a Klemsan Z1T-NDX from Automation Direct:
(This option would not be optimal if the auger is going to be cycled on & off at rates faster than 1/2 of a second)

Both examples are shown in the attached pdf.
 

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And also you are correct to assume that contact bounce is usually an issue only on closing. The force of the armatures slamming shut can make them bounce open ever-so-slightly a bit on closing, but it would make sense that the same thing could happen when they also spring open. It also wouldn't be outside the realm of possibility for the aux contacts to be more susceptible to that opening bounce, since they would probably have a much smaller distance to open and close since they have a much smaller voltage rating (manufacturer is assuming only control voltage will be used on them so they require less of an airgap). Since the aux contacts are mechanically linked to the main power contacts, then the force of their larger springs might be able to impart enough force on the aux contacts to cause them to excessively bounce on opening (enough to actually bounce into the closed position a few times before coming to rest open).
 
Ah I misunderstood. What is the auger being used for? It could be process related. I have seen powder handling augers forced to spin a time or two after being deenergized. I know you did say it is very low speed, but with enough mass it could still be a problem (but I'm not so sure about making multiple full revolutions). And is the counter's method of detecting a revolution just like a mechanical switch somewhere on the auger end where it gets depressed by a cam?
 
The flyback diode is in reverse polarity when the relay coil is activated. So a 50 to 70V PRV diode in a 24V DC system is conservative and acceptable. When the coil is de-energized, the flyback voltage is clamped at about 0.7V.

Using a normal diode can slow the relay response because the time constant is L/R where R is the forward biased diode impedance and L is the relay coil inductance. In some applications, where the fastest response is required, back-to-back Zener diodes are used. The Zener inpedance is quite high until the break down voltage is exceeded.
 
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