I am working on a small project which requires the switching of a 13Vdc power supply. The max. current will be 25 Amp (limited by the DC power supply). I would like to use AC contactors to reduce cost and standardise parts with stores but am aware of the arcing issue. Could the arcing issue be overcome by the use of diodes or MOVs accross the contactor?

 

You will need to properly spec the contactor for use with this high DC current. Most contactor manufacturers give an AC and DC rating for suitable contactors, so I guess the first thing to do is check if the contactors you normally use
are given a DC current rating. DC rating will probably be 25%ish of AC rating.

 

I work with contactors on a daily basis and in my opinion the arcing of contact points would not be objectionable since the contactor will close so rapidly and forcefully. A definite purpose Square D brand contactor rated 40 amps for
inductive loads as found on page 469 in the current Grainger catalog should work very well in your application and be inexpensive as well.

The issue that does need to be addressed is coil generated transients if the contactor will be near solid-state equipment. In this case a Square D brand transient suppressor that is designed to be installed directly on the contactor
coil terminals can be found on page 495.

 

Diodes or MOVs will help with with arcing caused by inductive loads when properly installed.

You haven't said what type of power supply you are using, whether it is a fixed output type, or if it is more sophisticated model. Many power supplies have a remote enable/disable feature which can be controlled by a dry contact. If it doesn't have this, but is analogue controlled (i.e., you can control the output voltage via an analogue command signal) then you can also "disable" it by setting the command signal to zero.

Standard practice in these cases is to disable the power supply, switch the contactor, delay to allow the contactor to switch (100 msec or so), and then enable the power supply. This extends the contactor life by ensuring you are not switching under load. AC contactors have been used quite successfully in this fashion.
If you are using remote sensing, then disabling the power supply will also prevent it from tripping on over-voltage when you switch the contactors.

You should investigate the DC rating of your AC contactors. Many contactors have a much lower DC rating than AC rating, but at low voltages (e.g. at the 13 volts you mention), the DC rating may be the same as the AC rating. You may have to ask the manufacturer about this point, as it is not always listed in the catalogues.


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Michael Griffin
London, Ont. Canada
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Assuming the power supply is hardwired, and has no real logic/enable, the cheapest thing would be to gate paralleled logic lvl fets. 0.006 Rds-on fets are commonplace. With 4 in parallel, the total power lost is minimal. Very simple to implement also.