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Switching DC load via AC rated relay ?

Discussion in 'General Electronics Discussion' started by Lei Reyes, Mar 11, 2015.

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  1. Lei Reyes

    Lei Reyes

    Jul 2, 2014
    Hi !

    I'm just wondering , what would happen if I switch a 12VDC lamp using the alarm relay output of this temp controller? with the following parameters below in the uploaded file . Here it says that it has a 1A 250VAC capacity , it does not indicate its DC capacity or is it only for AC loads ?


    and also, if switching DC loads to a relay with AC drive capacity indicated is possible, what would be the hazards or possible consequences of doing this besides failure if I exceeded the relay's drive capacity ..?

    the operation would be if the temp controller reaches the assigned limit value, it will trigger the 12VDC lamp ON using its relay output .
    Last edited by a moderator: Mar 11, 2015
  2. davenn

    davenn Moderator

    Sep 5, 2009
    That's just the relay contacts rating
    it should be able to handle an amp or so at 12VDC

  3. PAFMElb


    Mar 20, 2017
    Relay contacts have different current ratings for AC and DC, or conversely, different voltage ratings for the same current for DC and AC. Reason has to do with drawing an arc on contact opening. If the contacts are handling an AC current, any arc tends to extinguish twice in a cycle. Not so with DC. An arc keeps alive until the contact separation is such that the arc cannot be sustained anymore for whatever the current flowing happens to be.

    As a result relays are rated for much less current for DC loads for a given voltage. Alternatively, at the maximum rated contact current (which depends mostly on the surface area and surface material of the contacts) a relay is rated to switch a much lower DC voltage for the reason above.

    A typical relay that I fished out for giving some numbers to this answer has a 12 volt DC coil (irrelevant - coil could be AC or DC and designed for any voltage) and the contacts are rated to switch 12A at 240VAC, but only to switch 28V at 12A if it were DC.

    So the de-rating for DC in this case is a factor of 8. Typical, and all dependent on the contact size and separation, ie the physical layout of the relay.

    You haven't cited the power of the 12volt DC lamp you wish to switch, but I concur with Dave's previous answer that switching an amp at 12 volts will be fine for this relay.

    Caveat: we have been talking about the contact and voltage switching ratings here, NOT about whether it is OK to run a coil designed for AC on DC, or vice versa. For that the general answer is NO.
    duke37 likes this.
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