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Voltage Suppression

Discussion in 'General Electronics Discussion' started by Alpharsp, Jun 17, 2013.

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  1. Alpharsp


    Jun 17, 2013
    Hello, how is everybody.
    I am currently designing an interface between a RMS (Radio Management System) and some radios.
    One of the things my device do is turning them ON using a relay (this relay ahs voltage suppression) the problem is that the relay required consumes more current than the current my RMS outputs can handle nominally.
    my solution is to use a bus configuration device that uses darligton BJT transistors with a open collector output, the idea is to saturate the darlington base and connect the relay with the collector (as described in the attached file), I am adding a voltage suppressor for possible transient voltage in the relay coil.
    The problem is that my design was rejected, they said that instead of doing that we should use a smaller relay to turn a bigger one ON and OFF, according to them that ay we wont need the voltage suppressor.

    Do you guys know if thats true? I though voltage transients in inductors were related just with de-energizing the relay, not with the use of semiconductors, but i am not an expert so I was wondering if some could help me with this.

    Attached Files:

  2. duke37


    Jan 9, 2011
    The voltage transient will be due to the fact that the current in the inductor (relay) cannot be instantaneously be stopped. A free wheel diode across the inductor is normally used to carry this current until it dies from circuit resistance.

    A transistor does not like its rated voltage being exceeded.
    A Darlington will not be necessary unless you have very little drive cabability.

    A mechanical switch will spark on switch off. The contacts need to be substantial to take the abuse.

    RMS is normally used for route mean square in the electronics context.

    If you are switching DC you may be able to use a transistor (fet) directly without a relay.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    If you're driving a large DC-coil relay from a small relay, you should still have back EMF voltage suppression across the large relay coil. If you don't, the back EMF from the large relay coil will cause arcing in the small relay contacts, and it won't last long. You should also have back EMF suppression on the coil of the small relay. Also, a small relay is more expensive than a transistor. So I think the "accepted" solution is a lose-lose-lose proposition.

    You haven't given enough information in your post for us to be sure though. I would like to know the details of the control signal, and preferably a part number for the large relay.

    BTW I'm not sure why you've specified a 28V bidirectional voltage suppressor across the relay coil. The usual method of suppressing back EMF from an inductive load is just to connect a diode across it. In your case something like a 1N4001 would be suitable, oriented with its cathode connected to the positive supply. Your voltage suppressor will allow the relay to drop out more quickly, but it exposes the transistor to at least 56V. The ULN2003 is only rated for 50V, I think.
  4. Alpharsp


    Jun 17, 2013
    duke: I am using a darlington because its a module with internal ULQ2003A but i agree the darlington configuration is not required.

    OMG how do you know I am using the ULN2003 well it its variant : ULQ2003A
    you are right using a bidirectional module will subject it to 56V 28 from the power supply and 28v due to its bidirectional capability i didnt see that I think using the diode in parallel will the load (inverse polarization) would be the best option.
    I was looking for an amphenol voltage suppressor (just a single zener in parallel with the diode) but couldnt find it so i thought the bidirectional would be a good idea, my mistake.

    Thank you guys for your help
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