AC current limit

Discussion in 'The Projects Forum' started by geoaad, Oct 16, 2012.

  1. geoaad

    Thread Starter New Member

    Oct 12, 2012
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    Hello ,
    I'm new here and I don't know if this thread was discussed before...
    I have ac power source that should kept at fixed current load ( at least never pass a certain AC current limit ) , I use it to supply multi-equipment that is already at safe side for current source . In addition I want to supply a simple water heater , and this heater might increase the current to over-limit.I need a circuit that auto control the current supplied to the heater (maybe auto dimmer) so the main current never pass the max limit . Thanks for any ideas.
    Source : 220v , Current max : 10 Amps
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    ?? Why? A load will take what it needs.
    Use a circuit breaker or fuses?
     
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  3. wayneh

    Expert

    Sep 9, 2010
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    First step is to measure the current. Use a hall effect (non-invasive, clamp-on) sensor for this.

    The heater can likely be controlled with a triac, more-or-less like a light dimmer. What is the max current the heater can draw? This will help you specify which triac to use.

    Now you need to combine the two systems, tying the amperage measurement to a control signal to the triac. High system current will reduce the heater current. You'll need some details about about both - the sensor and the triac - before you can do that. But in the meanwhile I think you can take for granted that the strategy works, and move forward to finding the right parts.

    Circuits for controlling mains power directly without isolation - which you would be doing for the heater - may be forbidden here, so don't be surprised if/when this thread gets closed, or another approach is suggested.
     
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  4. geoaad

    Thread Starter New Member

    Oct 12, 2012
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    Thanks for your answers,
    first the reason of this circuit is that I have a fixed source of supply protected with a circuit breaker of 10 amps,. I have a main load about 8 amps at normal condition . ( normal load is containing pressure pump and a freezer and those are not always on.. ) now I use sometimes a heater that is about 5 amps load . my circuit have to work as automatic dimmer for the heater so when pressure pump work or freeze the circuit will not break out .
    I think about using a hall effect and isolate the circuit using a optotriac or any opto-isolator , but main difficult was to design the circuit to convert the increased voltage at output of current sensor to decrease the output at load pins ...
    Main supply is 220v , 50 hz.
    thanks for any ideas
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Maybe you just need to cut out the heater altogether whenever one (or both) of the other items needs to cycle on?

    I haven't worked it out but I believe you could rig a relay to accomplish this. The relay would be normally closed so that the heater circuit is complete. If the pump and/or freezer comes on, the relay opens and disables the heater.

    This wouldn't "dim" the heater but might be a simpler solution?
     
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  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    The official electrician's answer would be to install an additional breaker and run new wire to the heater. However you're probably not going to do that, so you might try this thingamajig. Officially, It's sold as a router speed controller, but I believe it is nothing more than a phase angle fire power controller - basically a beefed up household light dimmer. It should allow you to control the amps on your heater.


    [warning - the above is educated conjecture. I have never actually tried this and I'm not 100% sure it would work]
     
    Last edited: Oct 20, 2012
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  7. geoaad

    Thread Starter New Member

    Oct 12, 2012
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    Thanks strantor , what you mentioned is what I'm looking for but I want it to be full automatic (automatic current control),..
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You could use the current detect circuit with a comparator to bypass the switch on the router speed control that strantor referenced. It would select either the "FULL" mode when the other current draw is low or the "VAR" mode when the other current draw is high. You set the variable knob to the low level of current you want for the heater.

    Note that you must use a relay or other isolation to bypass and control the switch from the current detect comparator.
     
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