On 13 September 2011 04:34, Isaac Marino Bavaresco wrote: > Em 12/9/2011 11:35, Michael Watterson escreveu: >> Isaac Marino Bavaresco wrote: >>> Em 12/9/2011 10:34, Michael Watterson escreveu: >>> >>>> Forrest Christian wrote: >>>> >>>>> I'm struggling with coming up with a circuit which I actually like fo= r >>>>> measuring AC line voltage and current. =A0 The goal is to be able to = read >>>>> the voltage of AC line within a volt or two within a microchip PIC...= .. >>>>> For obvious reasons I'd like something where in normal operation the = AC >>>>> line is isolated from the PIC side of the circuit. >>>>> >>>>> >>>> Neon and a photo transistor? Should work at about 200uA on =A0mains si= de? >>>> Voltage varies the current thus brightness due to large series resisto= r. >>>> Better suited maybe to 230V AC than 110V AC. >>>> >>>> Hall effect sensor to measure line current. >>>> >>> >>> It seems that my suggestion got lost in the list: >>> >>> "You could make a circuit with a small PIC (8 pins) with A/D converter, >>> connected directly to the AC line, powered by a transformerless power >>> supply and using a resistive divider to measure the line voltage. Then >>> transmit the result of the measurement to the other side of an >>> opto-coupler to a circuit that implement the remaining functionalities >>> of your appliance. This way you can read as many different signals as >>> you wish." >>> >>> >>> Isaac >>> >>> >> I thought that a good idea. But I thought he objected to current draw of >> the LED in the coupler? Can you go less than 1mA? > > > It seems that the OP is worried about power dissipation (heat), not > exactly current consumption. > > The capacitive power supply is mostly reactive, not resistive, so the > power dissipation is small even for moderate currents. > > If he uses a power resistor to drop the line voltage, then a few mA > generate a lot of heat, but with the capacitive power supply the power > dissipation is very small. > > > Isaac > Re the voltage Measurement How about capacitive coupling? Especially if you're not looking at particularly high accuracy. The sensor could be a few 10/s of turns of wire wrapped around the phase wire. The two layers of insulation will provide the required level of isolation, and with "enough" turns you should be able to get to 100pF or so coupling capacitance. Use this as the input cap on a capacitive divider (to reduce frequency variations and the effect of nose spikes) and feed to a very high impedance op amp. If the amp has unity gain, you can feed the voltage back to a guard shield over the wound wire to reduce external stray capacitance effects. This stage may be followed by an appropriate gain/LPF/level shift stage which feeds the ADC. I've used a similar arrangement to detect if a cable is live or not, this just takes it to the next level of actually making a measurement. If you can get 100pF coupling capacitance, the reactance is 26.5Mohm at 60Hz. But the voltage is 340V pp for 120V input. So a voltage divider ratio cap of about 70 is required to reduce it to 5Vpp. This implies an earth leg capacitor of 7nf (6.8nf ?) and reduces the source reactance to about 3.7Megohms. So if you can arrange a bias network and an opamp that will give you ~30Megohm input impedance and a 2.5V offset you should be good to go. RP --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .