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Simple relay or more complex circuit?

Discussion in 'General Electronics Discussion' started by agent_86, Mar 19, 2013.

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  1. agent_86


    Mar 19, 2013
    I'm designing a circuit to control my pool cleaner booster pump and am not sure if I need just a relay or a more complex circuit. Any advice on what I need is much appreciated.

    I have a variable speed main pump which draws different current @220v depending on the speed I run it. It is fully programmable for speed, duration and time on and off via an onboard computer. I need to run my booster pump (either 110v or 220v) during certain times when the main pump is running full speed.

    What I want from the circuit is to monitor the current draw on the line going to the main pump and when it is above a certain level (which I would ideally like to be able to set, but will know what that trigger current is so I could buy something set to that point) turn on the other pump.


    Main pump turns on drawing 300 watts, switch does not come on. Main pump increases speed and draws 600 watts, switch goes on and turn on booster pump. Main pump cuts off, drawing on 50 watts, booster turns off.

  2. Electrobrains


    Jan 2, 2012
    Below is the simplest circuit I can think of.

    It's a current sensing relay, made with a low voltage DC relay as core.
    You set the switching current by adjusting R1, thus splitting the current between the relay coil and R1.

    Be aware of in your case that you have quite high current, so R1 must be a low resistance, high power resistor that can dissipate the power loss (roughly 3V over it). Of course you can build R1 with several parallel resistors.
    Also, the diodes must be able to carry the high current (you can also use low voltage Schottky Diodes)

    The suggested relay has a specified current of 67mA at 3V, or coil resistance 45 Ohm. R1 will probably be approx. 1 Ohm or a bit less.
    I chose a DC relay and rectifiers because of price and availability problems for low voltage AC relays.
    An AC relay would be even easier to use (no diodes needed).

    R2 could be used for fine tuning, but it's better without.
    If the hysteresis of the relay doesn't match your wished switch points, a more complicated circuit must be used.


    EDIT: I don't know the suggested relay well. If it would produce a 100/120Hz noise, you would need to add a low voltage, high capacitance electrolytic capacitor parallel to the relay coil (plus pole towards up, guessing a good value: 2200uF/6.3V).
    Last edited: Mar 20, 2013
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