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Forum Index : Microcontroller and PC projects : modular power logger
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| MOBI Guru  Joined: 02/12/2012 Location: AustraliaPosts: 819 |
[quote=greybeard]If you look at the led vs collector/emitter current curves for a opto there are linear regions at a constant temperature. [/quote] You know, in all the years I've used opto isolators, I've never considered using them as anything but a switching device. Used them in Dept of Defence secure sites as an isolator between the PMG lines and Crypto equipment. Never considered them linear enough over a big enough range to be of use in that department. d.m. David M. |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
[quote] Never considered them linear enough over a big enough range to be of use in that department. [/quote] That is true for a broad scale window of range, but they are quite accurate for a narrow window with the circuit adjusted to suit the desired range. The problem with thermal drift becomes more of a problem with current draw rather than ambient temp, so a voltage divider at a millvolt level should pose little thermal drift. Pete. Sometimes it just works |
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| greybeard Senior Member  Joined: 04/01/2010 Location: AustraliaPosts: 161 |
ok I built the following cct 
Without spending to much time optimising values I basically tried for a reasonable o/p range ( 1-4V ) for relevant input range ( ~6-15V ) 
I used two different opto's to get an idea of repeatability. Input voltage left constant and opto swapped out to reduce variation. As expected there was variation with temperature. The temperature variation was very roughly done. Basically a flat bladed screwdriver was cooled down in the freezer and then applied to the optocoupler and the variation in o/p voltage from ambient was noted. For heat a specially warmed finger was used. Not very scientific and pretty hard to repeat I know, however it gives an indication of temperature variation. Definitely a cheap option if someone uses a calibration curve,compensates for temperature and adjusts for a specific measurement range. I'll stick with using a picaxe at each measurement point and optocouple the serial coms. ymmv |
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| MOBI Guru Joined: 02/12/2012 Location: AustraliaPosts: 819 |
Isolated measurement of DC amps...I wonder, has anyone tried a hall effect device strategically placed on a current shunt? Is it even feasible? I've got one somewhere but yet to locate it. I'll give it a go when I do. david m David M. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Mobi, Interesting thought, i have tried similar but not a shunt. When one adds the cost of a shunt and hall device, opamp and circuitry, then the $15.00 Allegro sensor is not a bad option, pre mounted on a board that one just needs to connect to. But like yourself, i always find it an interesting experiment to find out if it will work. Pete. Psst.... try the 3rd draw down on the right. (its always the 3rd draw down  )Sometimes it just works |
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graynomad Senior Member Joined: 21/07/2010 Location: AustraliaPosts: 122 |
There are special optos designed specifically for isolating analogue, for example the IL300. See here for an example circuit http://ruggedcircuits.com/html/circuit__24.html That said I think all readings, either digital or analogue, should be taken as close to the source as possible and digital-only data passed back to the master. With CPUs at < $2 these days I'd have intelligent nodes and an RS-485 network. RS-485 has a CMRR of 7 volts, that has to be enough for just about any application and there are no large currents flying around. _____ Rob Rob Gray, AKA the Graynomad, www.robgray.com |
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| MOBI Guru Joined: 02/12/2012 Location: AustraliaPosts: 819 |
After looking at the Allegro device Peter mentioned, I think I will be going that way. It is all nicely factory trimmed for consistency. My camper battery monitor will be only a few centimetres from the battery, so I think I can get away with sending analogue to the monitor. I can understand why someone would base their system around the maximite - they are a versatile system. However for me, I have the i2c devices and what I need will fit in a smallish box so that's the way I go. I will have to have a paly with MM sometime though. Sorry Pete, not there either david m ps. I love the quote button Glenn. David M. |
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| graynomad Senior Member Joined: 21/07/2010 Location: AustraliaPosts: 122 |
I guess you can forego the RS-485 network then 
_____ Rob Rob Gray, AKA the Graynomad, www.robgray.com |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Silly bugger, not the 3rd drawers in the bedroom (dont want to know what you found there  ) or the ones in the kitchen (everything ends up in the kitchen 3rd drawer ) just the drawers on the electronic bench i meant.
Pete Sometimes it just works |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6098 |
For digital isolation there are IC's for I2C such as SI8400/SI8405 and I did find a few for isolated RS232. The ones that claim to be buss powered might need the full +/-12V rather than TTL levels. All very simple to implement. Jim VK7JH MMedit MMBasic Help |
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