I have designed a circuit for water pump. Please correct me if I am wrong with schematics. The idea is to to switch on/off AC motor pump with 230V and 7.5A, when water is detected.

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I have used opto-isolator MOC3063M and triac BT137.

I have four questions:

  • Can be used 1.5V to signal water detection or should be at least 6V? The current should be not more than 25mA.
  • Is it RC snubber set up correctly including resistors R2, R3?
  • Is it FUSE in right place? Should it places before RC snubber or after that
  • Can be used additional measure for heating? For example heatsink or another alternative.
  • Thanks.

    P.S. Water detector is nothing but plate with two conductors to detect water presence.

    Additional: Yes you are right 1.5V is too small, because of voltage drops of led D1 and the led in MOC3063. I consider to use 6V DC power source.

    • \$\begingroup\$ I dont think you are going to be able to even light both LED's with 1.5v. And depending on what your water detector is, voltage will be dropped across it. If your LED (D1) has a forward voltage of more than 1.5v, nothing will happen at all. If it is 1.2v, it leaves only .3 volts for the water detector and the opto-isolators LED. (Not taking the resistor into account.) You need to re-think your supply voltage. You need enough power to light both LED's and operate your water detector. \$\endgroup\$ Commented Nov 1, 2014 at 5:23
    • \$\begingroup\$ The LED in the optocoupler will drop about 1.3 volts, I think (but check the datasheet), and common red, yellow, and green LEDs are about 2 volts, so the battery on the detector side must be at least 3.3 volts, plus an allowance for the voltage drop of the water detector, and for a current limiting resistor. \$\endgroup\$ Commented Nov 1, 2014 at 6:12

    1 Answer 1

    1. As others have noted in the comments, the voltage is not high enough. Assuming the WD1 output is a relay, you might be able to get rid of the indicator LED and use that circuit, perhaps with an indicator light across the pump motor. Alternately, use a 5V wall wart supply and increase R1 to get the proper current.

    2. The triac drive circuit looks okay, suggest a higher value for C1, maybe 0.1uF with appropriate AC voltage rating.

    3. Fuse placement should be in the 'hot' line- it's not clear if that's what you have done.

    4. Heating will be around 7-10W so you need a fat heatsink for the triac. Note that the 8A BT137 is grossly underrated for a 7.5A motor load and will probably fail 'on' at the first opportunity. Suggest using at least a 25A triac (40A would not be too excessive) and giving it a large heatsink.

    • \$\begingroup\$ Yes you are right 1.5V is too small, because of voltage drops of led D1 and the led in MOC3063. I consider to use 6V DC power source. Also water detector is nothing more than plate with two conductors in order to detect the presence of water. \$\endgroup\$ Commented Nov 1, 2014 at 10:57
    • \$\begingroup\$ Yes FUSE is placed in HOT line \$\endgroup\$ Commented Nov 1, 2014 at 11:20
    • \$\begingroup\$ Can I use BTA24 or BTA26 triac? Max. gate current is 50mA, and hot line current 25A. \$\endgroup\$ Commented Nov 1, 2014 at 13:07
    • \$\begingroup\$ Yes. Much better choices, those are. \$\endgroup\$ Commented Nov 1, 2014 at 13:46
    • \$\begingroup\$ Caution the plates may not work well.. I suggest some circuit that cleanly switches the optocoupler.. If you manage to half-wave you could damage the pump motor. \$\endgroup\$ Commented Nov 1, 2014 at 13:49

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