Wiring a diode to a NC limit switch for two way motor control


I need help with choosing what current diode I need to use for the following circuit diagram I have below.

I am currently using 60V 1 A diodes and they seem to be working just fine as a current pass through for when the switches are activated. However, since the motor is rated up to 21 Amps, I am unsure if this 1 A diode will work in the long run even though the system only passes electricity through the diode when the limit switch is open which happens for a split second when the switch is actuated.

The motor above will be used to drive a linkage that will have a varying load on the motor between 100 - 250 lbs. upon using an ampmeter on where the diode goes, the maximum ampere readings i seem to be getting seem to be at 5 Amps.

Here is some additional detail on the motors we are using.

Thank you

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Hello hobyist, Welcome to the DigiKey TechForum.
If you want to use a little higher rated diode, you can look at P600A-E3/54GICT-ND, which is rated for 50V, 6A.
Maybe one of the Engineers can add to this post, on what they feel would be good in this circuit.

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Hello hobyist,

This is an interesting question as we are working to balance long term reliability with cost.

As an example, consider the 1N1200A diode. Provided it is properly cooled, it has a continuous forward current of 12 A. It also has a surge current of 240 A. I suspect the diode you are using has similar characteristics allowing it to absorb the motor’s 21 A starting current.

Here is where you must make a judgment call. If your design enters production, there will be a time when a customer accidentally blocks the mechanism and then activates the motor. For example, the mechanism may be fully extended, and a safety pin installed to prevent movement. In this situation your motor will be at full current for an extended period of time. In this situation, something is going to break. It could be the diode, a melted switch or relay contacts, the power supply, or the motor.

What I’m trying to say is that you need to design for the worst-case situation. May I recommend you start by installing a 10 to 15 A fuse or circuit breaker. This would generally accommodate the initial motor starting surge while provide protection for the other components.

With that said, how robust are your rocker and limit switches? Can they handle the high current associated with a mechanically blocked system? If not, consider using a pair of relays to activate the motor. You can then use smaller wires for the control circuitry. This would eliminate the need for high current steering diodes.

Please let us know if we can assist in the future.

Best Wishes,


P.S. Note that there are two distinct diodes with the same specification: 1N1200A and 1N1200AR. The difference is the polarity. With A has cathode as stud while the AR model has anode as stud. This flexibility may help simplify your design.

P.P.S. In this note I have assumed your power supply can provide 20 A for a brief amount of time. If you are using a fuse or circuit breaker, be sure that the power supply can provide enough current. It’s important that the fuse open in a high current situation. If not, we will find ourselves in the undesirable situation where the power supply protects the fuse.

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Considerations of continuous vs momentary carrying capacities are relevant and worthy of study, but in this case might a more expedient solution be available by omitting the diodes and changing the placement of the limit switches?


That’s a much better solution.

Thank you, Rick.

HI APDahlen,

Thank you for the suggestion and explanation on the potential issues down the road. The diode we are currently using is actually just a 1amp diode and it somehow seems to be working fine with the starting surge of the motor.

it looks like the solution of eliminating the diodes all together would be a much better solution in terms of price so I will try look into doing that first.

Thank you.

Yes, @rick_1976 had the better solution - I was so excited about explaining diodes that I never thought to simplify the design. At the same time, I really like your thinking as the diode / switch placement is a good way of illustrating the current flow in the circuit.

Please let us know if you were successful. Pictures are especially welcome.



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