Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site umd5.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!whuxl!whuxlm!harpo!decvax!genrad!panda!talcott!harvard!seismo!umcp-cs!cvl!umd5!zben From: zben@umd5.UUCP Newsgroups: net.analog Subject: Re: AC current monitor Message-ID: <445@umd5.UUCP> Date: Sun, 24-Mar-85 16:01:17 EST Article-I.D.: umd5.445 Posted: Sun Mar 24 16:01:17 1985 Date-Received: Thu, 28-Mar-85 00:03:28 EST References: <5139@fortune.UUCP> Reply-To: zben@umd5.UUCP (Ben Cranston) Distribution: net Organization: U of Md, CSC, College Park, Md Lines: 49 Summary: Does not sound that difficult... Does not sound that difficult. I suppose I am paranoid, but anything connected to both line 125 and TTL gives me the willies, so I would use a (gasp) relay. You might be able to make do with an optoisolator though, if you distrust moving parts. The key here is that you said it would draw from 100 to 1000 watts. If the minimum power drawn is 100 watts, you will be pulling at least 3/4 of an amp. I would sense the voltage across a SMALL shunt (like 1/100 ohm) with a sensitive relay. If the shunt is that small, even at 1000 watts, about 10 amps, the droop will only be 1/10 of a volt. Strangely enough, I am looking at building a similar circuit for a friend's video system. I need to sense if the VCR is on, and if so supply power to the TV set and stereo system. The object of the game is to make the whole thing go on and off with the VCR remote control box. I will only have to do this if the VCR he gets does not have a switched outlet. They used to have these boxes commercially available, for turning your stereo off when the turntable gets to the end of the record and shuts off. I sensed the current drain of the turntable through a special outlet. I don't think they had them big enough to handle 10 amps though - that would be one heck of a stereo :-) LEDs are current driven devices and don't need that much. How about paralleling the LED in an optoisolator with a shunt that would put the maximum of 50 mills through the LED when the load is drawing its rated maximum of 10 amps, and use the output of the optoisolator as the TTL side? I don't know if 5 mills (at 1 amp) is quite enough. Perhaps something like a zener diode or constant-current diode could be used to fix this. We are looking at something like: -----------------/\/\/\/\/\/\-------------------- AC line | R1 | LOAD | | --\/\/\/----|<--------- R2 opto isolator where R1 is 1/100 ohm and R2 is about 2 ohms. Get the rateings of the particular optoisolator you want to use and arrange the resistors such that when load current is max, optoisolator current is at its rated max. Then see if it works down to 100 watts (one amp). By the way, the circuit as drawn will chatter at 60 cycles. I suggest you remove this with a long time-constant filter at the TTL level... Although you might be able to do something at line levels, why take the risk... -- Ben Cranston ...{seismo!umcp-cs,ihnp4!rlgvax}!cvl!umd5!zben zben@umd2.ARPA