Relay contact rating on DC 2

[tonypdean]

If a relay contact is used to switch raw (unsmoothed) DC obtained by full-wave rectifying AC line voltage, do I use the AC or DC contact rating. Since the voltage still drops to zero, does this mean I can use the AC rating, or is there a derating factor I can apply.

I am finding it almost impossible to find a relay to switch 10amps @ 120V.

 

[xyzz]

Actually, it is the load inductance that makes difference between AC and DC for the contacts.
For pure ohmic resistance there is no difference, at least in first approximation.
For inductive load I would rather use AC rating.

For unrectified DC you can try solid-state relay.

 

[alehman]

I think DC is harder to interrupt and requires sturdier contacts because there is no periodic zero current point as there is with AC. AC/DC rated circuit breakers usually have lower voltage ratings for DC than for AC. That would be the case for inductive or non-inductive load.

 

[advidana]

check with the relay manufacture ac and dc contact are rated and tested differently.

 

[kvkev]

Tonypdean,
Alehman is correct, DC tries to maintain the arc across the contacts, where AC passes through zero and helps extinction.
One method is to use DC contacts wired in series, which effectively increases the contact gap. Have a look at the information for the 700SP with four contacts in series, which may match your requirements, or use similar.

http://www.ab.com/en/epub/catalogs/12768/229240/229266/229661/3087128/229739/print.html

Regards,
KVKEV

 

[Haldir]

Look in to these products; they can switch 10A@120V

TI 2CR Motor Starting Relays

http://www.ti.com/snc/products/controls/motor-2cr.htm

Power-IO H Family SSRs

http://www.power-io.com/products.htm

 

[Laplacian]

Mercury displacement relays have high DC interrupting ratings in a small package; and are very reliable for many operations. Look at Durakool (AMF) or Magnecraft/Struthers-Dunn for more technical information about their mercury diplacement lineups.

 

[Laplacian]

Here is the link to Durakool Relays: (up to 30A @ 120VDC)

http://www.aecsensors.com/indust.html

 

[Mike2468]

While you can use mercury displacement relays, these have a substancial quantity of mercury inside. They may be regulated in several states (CA, MN, WI, IL, MI, ...), can not be used where food is processed, and can not be used in many applications overseas.

For these reasons, we prefer to use solid state relays.

We had an EPA company quote the cost of hauling away an old control cabinet that had a bunch of MDRs. Wow.

 

[sreid]

You are likely to be able to use the AC rating. The DC rating is based on creating and sustaining a metalic arc when the contacts open (which can happen at surprisingly low voltages and currents). You are likely to get to voltages and currents that will extinguish the arc.

 

[CarlPugh]

Is this for a production unit or is it for only one unit?

What is the effect of a relay failure? Do you only have to replace the relay or could someone get injured?

USE THE DC RATING UNLESS A FAILURE WOULD HAVE MINIMUM CONSEQUENCE OR TESTING IS PERFORMED.

As has been stated in some of the other posts, if there is inductance in the circuit, the contacts will (if you are lucky) be good for the DC rating.

 

[buzzp]

I would use the DC rating since there is no zero cross for the reasons already mentioned. If you want a big relay then check out the MG-6 relays from ABB.