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Forum Index : Electronics : Piclog and current via shunt resistor
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| macm Newbie  Joined: 26/03/2008 Location: SwedenPosts: 4 |
Hi All from the far north (and a big thanks to Gizmo). I have built a Piclog ver 2 for my Lenz2 turbine and I can get everything to work except the current measurement. I have a 100 amp shunt class 1 (don't know what class 1. means), with a resistants of 4 ohm. Everything is built as described in the excellent manual. But I cant get a reading on the current. Have I done something wrong or do I need to change something to match the shunt? Thanks for a very important DIY-site. Regards Mikkel @ Gotland - Sweden. |
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi Mikkel, are you sure the resistance is 4 ohms. I would think a 100A shunt would have a 4 milliohm resistance, otherwise at the peak 100A, the device would need to dissipate 100 x 100 x 4 Watts. If the shunt is 4 milliohms, then the voltage developed will be 4mV/A. Check the sensitivity of the shunt required by the picaxe. I think the piclog cct used a 100 milliohm resistor as a shunt. You may need to amplify the signal from your shunt to a level needed by the picaxe chip, or replace the shunt with the necessary value resistor. cheers, Gordon. become more energy aware |
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| macm Newbie Joined: 26/03/2008 Location: SwedenPosts: 4 |
 ooooh yes - its 4 milliohms, the shunt is rated 50mv at 100 amp. Im not that strong at the ohm thing.
And - 100 milliohm - that explains alot (the lack of a reading) Is there an example of such a shunt on the net? Thanks! //Mikkel |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah that will be it. The picaxe ADC, while very linear, only has a resolution of 1024 steps over 5 volts input. So that means the picaxe reads in 5 millivolt increments, give or take a few millivolts. This does limit our current measuring ability some what. Either read big values or little values, not both. So you'll need to use a higher resistance shunt, try using a length of wire to start off with. I've never been happy with the way the PicLog measures current, very messy, but cheap. We are looking into using a current transducer ( PhillM and I ), but time is a little scarce for me at the moment. The other option is for someone to come up with a linear high side current measurement circuit using common Op-amps ( differential amp ), beyond me but maybe someone has the skills. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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WineGuy Newbie Joined: 16/04/2008 Location: United StatesPosts: 10 |
This won't work for a DC generator, but if you are measuring AC (assuming a 3 phase gen) you could use a small CT (current transformer) with a resistor for voltage measurement? Place this before your rectifier. I would thing a winding out of a small motor would work. You would need to count the windings to determine the resisor size. just a thought. Sure wish I was smart instead of so darn cute! |
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