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KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 07:19am 14 Sep 2006

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I would like to monitor current and voltage on my F&P generator system.

What I have is input from the generator going across a 12v battery and the load. The generator drive is very 'peaky' so one moment the battery is charging the next it is discharging.

OK, I can handle the software but what I really need is current and voltage sensors that I can connect to my laptop.

Any ideas on what the sensors should be?

RossW
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Joined: 25/02/2006
Location: Australia
Posts: 495

Posted: 07:42am 14 Sep 2006

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Do you want a long-term or short-term solution?

There are some quite reasonable, fairly inexpensive digital multimeters with RS232 interfaces that could do this for you.

If it were me, I'd probably use something like an ATMEGA series chip, use two of the A/D channels and sample at several KHZ, working out instantaneous power in and out in + and - watt-milliseconds and either logging that in larger time intervals, or spitting it out to your PC every second (or other convenient interval).

Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 5078

Posted: 09:25am 14 Sep 2006

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The PicLog could do what you want.
http://www.thebackshed.com/Windmill/PicLog.asp
There is a new version on the scratchpad, should have it online this weekend.

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ

KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 09:33am 14 Sep 2006

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ATMEGA chips? Are they PICs ?

I was thinking more of the sensor types rather than the processors, something I have found are Hall effect sensors with I2C bus but it does not seem to be easy to find an I2C host to put on my laptop, sure they are available at a few hundred US$ but thats a bit out of budget for this very low priority project.

I have a Hall effect chip right in front of me plus I have cut a yellow toroid from a PC power supply which should be about right for sensing current but it looks like a lot of fiddling around to monitor this from the games port, which only gives 8 bit values (well 7 bit if measuring bi-polar current).

KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 09:36am 14 Sep 2006

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Hey! the PicLog looks like it has a lot of the answers! Perhaps I should stop being an old fuddy-duddy and get with these PICs after all!

Thanks Gizmo

RossW
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Joined: 25/02/2006
Location: Australia
Posts: 495

Posted: 11:06am 14 Sep 2006

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KiwiJohn said ATMEGA chips? Are they PICs ?

Well, technically, no. They're more like "real computers with A/D converters, UARTS, memory, flash, I/O, counters, dividers etc on a chip"

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I was thinking more of the sensor types rather than the processors, something I have found are Hall effect sensors with I2C bus but it does not seem to be easy to find an I2C host to put on my laptop, sure they are available at a few hundred US$ but thats a bit out of budget for this very low priority project.

I like HALL devices, but I just can't get over the variable and unpredictable offset (bias) they seem to have. They never quite seem to deliver what I expect.

If you have small currents, the offsets can be very annoying - you may think you're charging but you're actually DISCHARGING!

The electrical isolation is a bonus, and the very low (ney, zero) losses to the monitored circuit are indeed a bonus, but I think its hard to go past a good shunt.

Gizmo

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Joined: 05/06/2004
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Posts: 5078

Posted: 11:53am 14 Sep 2006

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Yeah I've used hall sensors on some of my projects. For the Charger Controller the erratic readings were acceptable, but I wanted the PicLog to be accurate, and the standard Dick Smith hall sensor wasn't up to the job.
There are accurate hall sensor senders, they incorporate some extra clever bits to give a linear response, but you wont get them from your local DSE.
Another thing to watch is some of those toroid cores are not ferris. If you cut it and the material is metal like ( shinny ) then its no good for a hall sensor. I had one I was going to use for a hall sensor, but found I could magnetise it, permanently! So it would indicate current into a hall sensor, even though no current flowed.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ

RossW
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Joined: 25/02/2006
Location: Australia
Posts: 495

Posted: 08:33pm 14 Sep 2006

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Gizmo said
There are accurate hall sensor senders, they incorporate some extra clever bits to give a linear response, but you wont get them from your local DSE.

Glenn, even those fancy-pants devices that were discussed on this board recently, with their chopper-stabilised amplifier etc, had a non-trivial and variable offset.

Sure, the electronics were good, but the offset was from the sensor itself. It may have only been .3 amps but wow, thats still a fair bit!

Making the output linear is one thing, but simply getting "zero" right is almost impossible. I've never seen a hall device based instrument (even hellish expensive ones) that don't have either manual or auto zeroing circuits - and you can't zero a probe without knowing how much current is passing (preferably none!)

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