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macm
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Joined: 26/03/2008
Location: Sweden
Posts: 4

Posted: 10:17pm 25 Mar 2008

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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.Edited by macm 2008-03-27

GWatPE

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Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 11:17pm 25 Mar 2008

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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.

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macm
Newbie

Joined: 26/03/2008
Location: Sweden
Posts: 4

Posted: 12:16am 26 Mar 2008

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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 Edited by macm 2008-03-27

Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 5108

Posted: 01:52am 26 Mar 2008

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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

WineGuy

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Joined: 16/04/2008
Location: United States
Posts: 10

Posted: 01:13am 25 Apr 2008

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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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