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Kickback protection

Discussion in 'General Electronics Discussion' started by anagon, Jun 14, 2016.

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  1. anagon


    Jun 14, 2016

    Mods, I've posted this already on another website but I haven't got a satisfactory reply yet.

    I've built a step down box to run 110 VAC appliances off incoming 240 VAC (Malaysia). The only components in the box are a cooling fan (240 V) and a great big transformer. It's a single phase 30A output aluminum wire 240V->110V 50 Hz transformer that weighs about 25 kg.

    I think I need inductive kick protection for the transformer so that if the incoming 240 V is interrupted (by the upstream circuit breaker maybe), the inductive kick doesn't fry my 110 V equipment. I don't trust surge protecting power strips (and I can't get 110 V ones here anyway) so I'm looking for some discrete components I can use.

    Does anyone have any experience with this? Would a MOV be suitable? Or a TVS?

    Thanks for helping!
  2. Bluejets


    Oct 5, 2014
    Don't know about the "turn-off" aspect but I would suspect the power on the secondary would just bleed off.
    The "turn-on" I have had experience with in the form of 1:1 isolation transformers used in hospital leakage current monitoring systems in operating rooms. They were 15A 1:1 and a total of 32 in all. At initial (no load) the inrush would trip a 250A sub-main breaker at the main switchboard. I'm not saying that your single unit would do that BUT it will have inrush while the core is magnetising which could cause problems for you.
    The fix was simple. At power-up, a normal old 10ohm 5W resistor was in series with the primary circuit and bypassed after a couple of seconds via a timer.
  3. anagon


    Jun 14, 2016
    Hi Bluejets,

    Turn on has been fine so far. No issues.

    The other day, I worked out the 110 V PC's PSU was fried. When I want to power down, I normally turn off and disconnect the 110 V equipment one by one except the PC which I leave on software shut down. I then flip the breaker to turn off the dedicated 240 V circuit. So I figure at the moment the circuit breaker was cut, the only two components in the circuit were a big transformer and the PC's power supply. I reckon the transformer responded to the abrupt drop in current by causing an abrupt spike in voltage, which fried the PSU.

    Is this a correct understanding?
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