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SCR controlled bilge pump
"Offbreed" wrote in
oups.com: I cannot relocate the circuit, It's quite easy, actually..... SCR cathode to 12V negative bus SCR gate to 10K ohm 1/4 watt resistor to limit its gate current to a safe level. The other side of the resistor hooks to the higher float switch, whos other lead is hooked to +12V. The resistor limits gate current to 12/10000 = 1.2ma. The anode of the SCR is hooked to one lead of the lower float switch. The other lead of the lower float switch is hooked to the negative lead of the pump motor. The positive lead of the pump motor is hooked through the appropriate inline fuse directly to the house battery + post to prevent some idiot from turning off the bilge pump! NO BILGE PUMP SWITCHES PLEASE! As the water rises, the lower switch closes, powering up the SCR anode, but without a gate + the SCR doesn't fire. As the water flips the upper switch, the +12V applied to the resistor brings the gate current up above the microamp-level firing current and the SCR gates on. Once fired in a DC circuit, the SCR will not open just because the upper switch opens and it loses its gate current. Current runs the pump until the water level in the bilge drops to open the lower switch which shuts down the pump, and ungates the SCR because it has no Anode current, now. The circuit won't cycle, again, until the lower, then the upper switches both close to gate the SCR, again, as the cycle repeats at a much slower rate. Use a powertab 8A SCR and just screw it into the wood somewhere way above the waterline, even out of the bilge if you can to prevent corrosion. Run a gate lead out of the bilge to the 10K resistor soldered directly to the SCR gate lead to support it. As this gate lead will be hooked directly to the 12V plus, make sure the power to the upper switch hooks to the motor side of the bilge pump fuse for safety. Now you have a true hysteresis bilge pump. AT 4A, the SCR barely gets warm In an emergency, just jumper out the SCR and you have a simple bilge pump with the lower switch. |
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