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Relay chatter problem

Discussion in 'General Electronics Discussion' started by thorntonforce, Apr 5, 2012.

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  1. thorntonforce


    Apr 5, 2012
    I have been doing installation work on a vehicle and have encountered a problem. I have 2 AC electrical inputs to the system, a 1kw inverter and a socket for external mains electricity or generator. When the inverter switches on it powers the coil of an ac relay which switches the contacts from normally closed (external socket) to normally open (also the inverter). the common contacts of the relay connect to the input of my consumer unit. Downstream I have multiple sockets and lights etc.

    the system works fine and switches seamlessly between the 2 inputs with no loss of power to my PCs and monitors etc. Unfortunately I cant run the fluorescent arc lighting I have installed from the inverter. There is plenty of capacity to run them but the initial inrush current causes a voltage drop on the inverter which switches off the relay. the result is high frequency relay chatter and the lights never come on.

    If this was a DC voltage drop problem I would consider using a large capacitor but this wont work for AC - will it??

    Is there any other solution??

    Thanks in advance
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You're right, a capacitor won't work for AC.

    You could use DC controlled relays, a bridge rectifier to convert AC to DC plus a capacitor to bridge the time when the inverter's voltage drops due to the inrush current.

    Alternatively you could use AC relays rated for a lower voltage plus a series resistor to limit the current through the relay coil during continuous operation. An auxiliaary relay could short circuit the resistor during the switching-on of the arc lighting. But I think this is pretty more complicated than a DC relay.

    Be safe: high voltages and currents when tinkering with power of this sort. Any failure may be dangerous and even lethal.

  3. jackorocko


    Apr 4, 2010
    Either that or buy a bigger inverter which would cost significantly more then either or haralds suggestions.

    I think the rectifier, relay and extra cap would work pretty well and not increase the price to much.
  4. thorntonforce


    Apr 5, 2012
    Thanks for the responses - Il mull these options over and decide what to do. As an interim measure I have fitted a sticker on the light switch saying do not press unless plugged into mains. Working quite well at the moment.
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