Sometimes the water pump runs when there is no water in the sump or underground tank or when the treadmill is accidentally left on. When this happens, the motors of these electrical machines run when they don't need to. If you use these for a long time, you might get tired of them. This no-load and overload protector circuit can be used to fix these problems. It can turn off the appliance if it gets too hot. Fig. 1 shows the prototype that the author made. Circuit and how it works Figure 2 is a diagram of the AC motorized circuits no-load and overload protector. It is made up of a NE555 timer (IC1), a decade counter (IC2), a quad comparator (IC3), a quad NOR gate (IC4), a current transformer (CT1), BC547 transistors (T1, T3, T4), SL100 transistors (T2), and a few other parts. The circuit runs on a 12V DC power source. At connector CON1, you can use a 12V, 1A SMPS unit. In most cases, Q4 of IC2 will be in the low state, except when there is no load. If reset pin 15 of IC2 is high, the output of Q0 will be high. When reset pin 15 is in the low state, the counter can count. This means that each output (Q0 through Q3) will change from low to high in order. All four outputs (Q0–Q3) are logically ORed and used to drive T2 and turn on relay RL1.

NOR gate N4 will not let the clock pulse into the counter if Q4 of IC2 goes high. So, T2 will be turned off and RL1 will lose power. This will keep happening until the next time the power supply is turned back on. AC voltage is sent through the coil of the 50Hz, 10A current transformer (CT1) to measure how much load current is going through the motorized electrical device. The load current of an electrical device is proportional to the sampled AC voltage in the CT coil. This 50Hz AC voltage across the coil is turned into a DC voltage by reversing it and filtering it. In two comparators, A1 and A3, of the LM339, the DC sample voltage is compared with the reference voltages that come from two resistive networks (IC3). The voltage from potmeter VR2, which is used to check the no-load condition, should be set to be just a little bit less than the sample DC voltage when the load is normal. This gives pin 2 a high output. If the load current goes down, the reference voltage from VR2 goes above the sample voltage. This makes the output of A1 at pin 2 go low. In normal load conditions, the reference voltage from VR3 is set to be just a bit higher than the sample DC voltage. This makes pin 14 of A3 have a low output. If there is an overload, the sample voltage goes above the reference DC voltage, and the output at

pin 14 of A3 goes high. Gate N2 of IC4 is wired as a NOT (inverter) gate so that an RC network made of R12 and C6 can be used to make an initial reset pulse circuit. This reset circuit is needed to reset counter IC2 when the power is turned on for the first time, so that IC2's output Q0 goes high and turns on T2. This, in turn, turns on relay RL1 so that the load can get AC power. At the cathode of D1, you can get a DC sample voltage from the rectifier. It is used to compare and make gates work logically. Most of the time, the output of A3 at pin 14 is low and the capacitor C7 is empty. Zener diode ZD1 won't work, and SCR1 won't be able to do its job. As the base bias goes through R16, LED3, D7, D8, and R20, T4 turns on. T4 grounds the base of T3, which sets off T3. When T3 is off, it won't pull down T2's base bias voltage. The levels of output from Q0 to Q3 of IC2 control T2. The truth about the NOR gate is shown in the table.