What is the normal operating current of the appliance? A simple AC current transformer might be appropriate. You can fabricate your own from a toroid core. You wrap multiple turns of insulated wire around the core, going through the middle and then around the outside. This is the secondary. The primary usually consists of omly one or two turns of wire. If using a "single" turn, you just have the wire going through the center of the core. The secondary MUST be connected to a load resistor. You monitor the voltage across the resistor, as it is proportional to the primary current. [NOTE: the AC power line has a hot and a neutral. you may use EITHER one as the primary wire, but ONLY one of the wires goes through the center.] [NOTE: the current transformer provides 100% isolation between the primary and the secondary, which makes the current transformer also a SAFE way to measure current] Let's make up an example so you can see how it all goes together. Max current to be measured: 10 Amps AC. Desired secondary current when primary has 10 Amps: 100 milliamps. Current ratio is 100:1 Use a "single" turn primary and 100 turn secondary. Primary must be capable of handling 10 Amps. Secondary must be capable of handling 100 milliamps. A 1 ohm resistor will develop .010 V per Amp and .1 V at 10 Amps. A 10 ohm resistor will develop .100 V per Amp and 1 V at 10 Amps. A 100 ohm resistor will develop 1.00 V per Amp and 10 V at 10 Amps. Secondary resistor power dissipation at 10 Amps through primary will be P=I^2*R, where I^2 = .01, so .01*1 = .01 Watts for 1 ohm .01*10 = .1 Watt for 10 ohms .01*100= 1 Watt for 100 ohms Personally, I would go for a 10 ohm 1/2 watt resistor and expect an output of 1 volt at 10 Amps primary current. Expect some error with a homemade current transformer. Fr. Tom McGahee ----- Original Message ----- From: Frank Tarsitano To: Sent: Wednesday, August 01, 2001 3:47 PM Subject: [EE] current sensing circuit > I'm looking for a circuit that will detect current > flow. I want to know when a particular appliance is > turned ON. > > If any one can help it would be greatly appreciated. > > > > _______________________________________________________ > Do You Yahoo!? > Get your free @yahoo.ca address at http://mail.yahoo.ca > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics