I need to use a relay in order to do field sweeps with my electromagnet, which is hooked up to a couple very beefy monopolor power supply. (My magnet's impedance is ~2 Ohm, and I am putting ~40 Amps through it at maximum power).

Initially, I figured the Potter & Brumfield T92 Series Two-pole 30A PCB Relay would work for me.

A cursory glance at the contact data revealed:

  • Rated Current: 30A
  • Limited Continuous Current: 40A
  • Limited Making Current: 40A
  • Limited Breaking Current: 40A

I assumed I would be pushing the limits, but this relay is $13 and I had one laying around. However, then I looked at the contact ratings, and I saw, under the 'Load' header:

  • 40A, 277VAC, resistive
  • 30A, 120/277VAC, resistive
  • 10A, 600VAC, general purpose
  • 20A, 28VDC, resistive

Where I have italicised the rating that concerns me. Is this to say that I should not use this relay? Does anyone have any idea what kind of relay I should use? When I look at "power relays," they all seem to have just two contacts, rather than the eight that I would expect... Also, how do I tell if the relays are make-before-break or break-before-make? I know that the one I have now will work, only because I know someone else who is using it for the same thing...

Thank you for your help!

  • \$\begingroup\$ You might want to search for "contactors" rather than "relays". Some of these bad boys would probably do the job; @DwayneReid's suggestion of an automotive starter solenoid is also a good one (and probably cheaper) \$\endgroup\$ Commented Apr 2, 2015 at 18:24

2 Answers 2


The DC voltage rating of relays and switches is roughly 10% of the AC rating. When you start getting to higher currents the DC rating is even worse.

This is because of arcing when the contacts are broken. With AC the arc is automatically extinguished when the waveform crosses the zero point. Of course, DC doesn't do this, so the arc doesn't extinguish by itself. So you can only support much lower voltages for the same contact distance. And of course, because of this the arcs can last longer, which means the maximum current must be reduced as well.

In general relays are break-before-make since they work by a single leaf moving between two contacts. To get a make-before-break you effectively have to have two completely separate contact systems which operate at different times. It is more normal to use two separate relays and control them at different times if you really need make-before-break.

"Power" relays which handle very high currents are often just on-off, since they have very large contacts which travel over a greater distance.

Another consideration with the relays is the type of load. You have an electromagnet. That is an inductive load, not just a resistive load. The rating is for purely resistive, and doesn't take into account any back-EMF (back electromotive force) from an inductive load. The EMF induced by the collapsing magnetic field can be many times higher than the voltage you are operating at, and this EMF can cause more arcs in the relay contacts further degrading the relay. So for an inductive load you need a higher rated relay than for a resistive load.

  • \$\begingroup\$ So, just to make sure that I understand this correctly, you would recommend getting two power relays, and then put some logic into my controller to have them both made simultaneously when I do my switching? \$\endgroup\$
    – nosirrahcd
    Commented Apr 2, 2015 at 13:28
  • \$\begingroup\$ Also, you say: "This is because of arcing when the contacts are broken." If I am always switching at zero current (which I plan to), is this an issue? \$\endgroup\$
    – nosirrahcd
    Commented Apr 2, 2015 at 13:29
  • \$\begingroup\$ The critical time is when you break the connection. That is the worst arcing time. If the power is already "off" at that point then there can be no arc. What is your circuit? How does it work? \$\endgroup\$
    – Majenko
    Commented Apr 2, 2015 at 13:31
  • \$\begingroup\$ I have two power supplies connected in series in constant current mode. I sweep the current from 40A down to zero. Then, at zero, I have been turning off the supplies, manually switching the leads to the magnet, turning back on the supplies, and continuing my sweep. That way I can sweep both positive and negative fields. I do not have a good understanding of why it needs to be make-before-break, but several others who have done this kind of thing before have told me that it must be. \$\endgroup\$
    – nosirrahcd
    Commented Apr 2, 2015 at 13:38
  • \$\begingroup\$ Make-before-break would cause a short across the magnet. That may be desirable to fully discharge any field and EMF in the magnet before starting the negative sweep. There is no logical difference between one relay with multiple poles and multiple relays with one pole. \$\endgroup\$
    – Majenko
    Commented Apr 2, 2015 at 13:42

Majenko tells you why most relays have such limited DC current ratings. However, there are other options.

You mention 2 Ohms, 40 Amps. I assume that means that you are running from about 80 Vdc.

1) Automotive starter solenoids are rated for very high DC currents with very inductive loads. You will want to look at 24V solenoids used for starting big-rig diesel engines, not those used for 12V passenger automotive vehicles.

2) I think your better option is to look at MOSFETs. This also allows you to implement your make-before-break timing in your control circuit.


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