NI 6514 to drive Relays

[MarcoMauri]

Hi!

I would like to use a NI 6514 board to drive some relays.

I've seen on "NI 651x User Manual" (Figure 4.3) that the driver has a built in clamp diode.

Here is the question:

do I need to provide a flyback diode to my relays or the on-board diodes are enough?

Thanks in advance for your help,

Marco

[Buechsenschuetz]

Technically, the clamp diode integrated in the module will be sufficient, as long as you operate the relay from the same supply voltage source as the clamp diode is connected to. 

However, in case you use a long cable between the relay and the board, you should use a clamp diode located to the relay since the backlash voltage created by the relay may cause EMI as it is running along the cable.  

[MarcoMauri]

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@Buechsenschuetz wrote:

[..]

However, in case you use a long cable between the relay and the board, you should use a clamp diode located to the relay since the backlash voltage created by the relay may cause EMI as it is running along the cable.  

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A cable of, let's say, 50 cm can be considered long enough to require an extra diode?

Marco

[Buechsenschuetz · Accepted solution]

As mentioned, a diode close to the relay is not necessary but recommended. I would use a diode with a cable length of 50cm. A diode costs a few cents but searching for the source of EMI may take hours, so why not try to avoid this?