This example describes the utility over , under voltage automatic protection device , can be in the city less than 170V or higher than 250V electricity is cut off automatically Circuit works Power over the city , under voltage protection circuit automatically from the power circuit , over voltage detection control circuit and under voltage detection control circuit , as shown in Figure 5-178 . Power circuit from the starter button and S2, the power transformer T, rectifier diode VDl, VD2 , and filter capacitor Cl form . Over-voltage detection control circuit consists of resistors Rl-R3, potentiometer RPl, regulator diode VSl, diodes VD3, capacitor C2, transistor Vl relay Kl and composition . Under voltage detection control circuit by the resistors R4-R6, potentiometer RP2, regulator diode VS2, diode VD4, capacitor C3, transistor relays K2 and V2 composition . Sl for the PV of the control switch voltage meter . HLl too , under voltage indicator ; HL2 for the output voltage of the normal light . After pressing the S2 , the utility voltage by T Buck , VDl and VD2 Cl rectifier and filtered , as before, under voltage detection control circuit to provide DC power . When a power voltage is normal , then Vl end , V2 conduction , K2 pull , K2-l and K2-2 in the normally open contact connected , normally closed contact is open, the load of the power is turned , while HL2 point Light . After the release S2 , Kl maintain pull state . If the high voltage electricity at 250V, then VSl breakdown conduction , so that Vl saturated conduction , Kl pull , Kl-l and Kl-2 in the normally closed contact is open, normally open contact connected to the load A power supply circuit and T circuit cut off , Vl and V2 has also closed , Kl and K2 release , HLl light , HL2 off . When a power voltage is low at 170V, the VS2 and V2 closed , K2 release , load and T were cut off the power supply circuit , HLl light . Adjustment RPl of resistance , can change the size of high-voltage threshold . Adjusting the resistance of RP2 , can change the size of low-voltage threshold . Components Selection Rl-R6 are selected 1/4W metal film resistors . RPl and RP2 are selected membrane variable resistor . Cl-C3 are selected voltage value of 16V aluminum electrolytic capacitors . VDl-VD4 were selected 1N4001 or 1N4007 type silicon rectifier diode . VSl and VS2 use 1/4W or l/2W silicon voltage regulator diodes . Vl and V2 use C8050 or S8050, 3DG8050 silicon NPN transistor . T use 3-5W, the second voltage power transformers for the two- gV . Kl and K2 are selected JRX-l3F type gV DC relays . HLl and shame are all made of neon lights H . S1 and S2 of the contact current load should be more than 5A . PV use 0-300V AC voltmeter .