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Measuring current up to 20 amps

Technical Forum: PicBasic List 05/2000 to 07/2001: Measuring current up to 20 amps

 
By Anonymous on Thursday, March 1, 2001 - 03:30 am:

I have a deep cycle battery in my boat that is rated at 55 AH. I want to
monitor the input and output current of the battery and thus calculate its
state (how long it has left at a operable voltage). In my earlier days I had
a centre 0 that showed charge or discharge of a current up to 60 amps. It
used to have an internal shunt etc. Quite common and I am sure everyone will
know what I mean.

I figure I can use the same type of shunt and measure the voltage across it
to determine current. However the shunt would obviously develop a voltage
drop else the meter would not work and I need the drop to be of a bare
minimum thus supplying the most voltage possible to the powered devices.

can anyone suggest an alternative method or has anyone done similar.

Regards, Peter.

By Anonymous on Thursday, March 1, 2001 - 05:12 am:

A shunt is the right term and device to use here. If you use a
0.1 ohm shunt, a 50 amp drain will cause a 5v drop across the
resistor (shunt) which you can read with the ADC on a PIC but
you'll have lost 5V to the shunt - probably not acceptable.

You need an even smaller resistance shunt, perhaps a piece
of wire, and an amplifier to get it up to the voltage your
PIC ADC can use. You'll also have to add in a DC offset if
you want to read the other polarity (charging and discharging).
What I've done is to use the heavy wire connecting the battery's
+ terminal to the load. Connect separate wires at either end
of this wire to your opamp input. LM324 is a good one - four
in a package, low-cost and can operate on a single supply. The
PIC will have to be separately powered in this case though as
you don't want to be ground-referencing it back to the battery.
You'll have to calibrate it yourself as it's normally difficult
to accurately measure the resistance of a heavy piece of wire.

Good luck.

-Mike-

By Anonymous on Thursday, March 1, 2001 - 05:55 am:

Peter,

There are Hall Effect devices available to measure both AC and DC current
with very low voltage drop. They also have the advantage of complete
electrical isolation.
Try Honeywell and LEM-HEME. Both these makes are in the RS book in the UK.

Regards

Brian Walsh

By Anonymous on Thursday, March 1, 2001 - 07:46 am:

I agree,
A shunt is lossy, a fire risk if not remote mounted and fused and will cost
just as much asa a hall. the single supply, unipolar is the one to go for,
it gives 2.5V out at 0Amps decreasing for discharge increasing for charge.
They are not damaged by overcurrent so you don't have to worry about the
starter (as long as the wire will fit through the hole).
Robert G8RPI.

By Anonymous on Thursday, March 1, 2001 - 10:34 am:

How about using a section of the main cable as a
shunt (maybe the connection between the battery and
ground) and an op-amp with a common mode capability that
includes ground (LPC66 for example).
Unitrode do a current sensor IC UCC3926 designed to do just this.
Also checkout Burr-Brown INA138 and INA168 which are specifically
designed to measure the voltage across a current shunt in a
positive supply rail.
Cheers
Simon Clifford

By Anonymous on Thursday, March 1, 2001 - 11:55 am:

I built a power supply and I used about 10 inches of a steel clothes hanger
(bending it in a zig zag) and a 3 1/2 LCD voltmeter. It only required 0.2
Volts for full scale. The resistance of the clothes hanger was approx. 0.01
ohms or 4 watts (20amps * 20amps * 0.01ohms). I cal it by using a volt ohm
meter that could read 20 A max. Just solder one lead of the voltmeter
input on the clothes hanger and slide the other end to until the two reading
match. Then solder it in place. Re-check the cal.


Mark

By Anonymous on Thursday, March 1, 2001 - 11:55 am:

Peter,
I havn't done it, but you might look up "sense fets" which are a bunch of
power fets in a package in parallel of which you can get in series with
one only. So you get the very low on resistance of all the fets in parallel
and the small current of only one fet.
Don't have anything more spacific on hand.
Hope that helps
Tom Fowle WA6IVG
P.S. there are D.C."clamp amp" probes too, but kinda costly.
T.F.

By Anonymous on Thursday, March 1, 2001 - 05:20 pm:

I once connected a microameter (center 0 ) across the battery cable in
my pickup truck .
One lead on the + terminal and the other on the other end of the cable
and it read just fine.
I used it as an indicator not for accurate current reading. I hooked
it so charge was up and
discharge was down.
Jim

By Anonymous on Thursday, March 1, 2001 - 05:57 pm:

hey that's pretty smart!
Dennis Saputelli

By Anonymous on Friday, March 2, 2001 - 09:17 am:

How did you get a good solder joint to a steel clothes hanger?

Dr. Bob


Message text written by INTERNET:picbasic-net
I built a power supply and I used about 10 inches of a steel clothes
hanger
(bending it in a zig zag) and a 3 1/2 LCD voltmeter. It only required 0.2
Volts for full scale. The resistance of the clothes hanger was approx.
0.01
ohms or 4 watts (20amps * 20amps * 0.01ohms). I cal it by using a volt ohm
meter that could read 20 A max. Just solder one lead of the voltmeter
input on the clothes hanger and slide the other end to until the two
reading
match. Then solder it in place. Re-check the cal.


Mark

<

By Anonymous on Friday, March 2, 2001 - 09:17 am:

Can anyone suggest a specific vendor and part for these hall effect
current sensors?

Dr. Bob


Message text written by INTERNET:picbasic-net
I agree,
A shunt is lossy, a fire risk if not remote mounted and fused and will cost
just as much asa a hall. the single supply, unipolar is the one to go for,
it gives 2.5V out at 0Amps decreasing for discharge increasing for charge.
They are not damaged by overcurrent so you don't have to worry about the
starter (as long as the wire will fit through the hole).
Robert G8RPI.

By Anonymous on Friday, March 2, 2001 - 09:46 am:

LEM-HEME LTS 25-NP Farnell stock No 311 4569 £8.00 www.farnell.com
in the US try Newark.

This is a 0-25Amp sensor single +5V supply outputs 2.5V at 0A increasing for
positive current decreasing for negative. This will interface directly with
a PIC AtoD.
To get 12.5A sensitivity just pass two turns of wire through the Hall
sensor.

Robert G8RPI.


By Anonymous on Friday, March 2, 2001 - 10:09 am:

I don't know how he did it, or if it is still 'legal' but there is a
wonderful flux called "Salmet'. I still have a tube from at least
15 yrs ago. It will solder almost anything to anything, including
aluminum!
It's rather nasty though, be careful and clean thoroughly.

Dennis Saputelli


--
___________________________________________________________________________
www.integratedcontrolsinc.com Integrated Controls, Inc.
tel: 415-647-0480 2851 21st Street
fax: 415-647-3003 San Francisco, CA 94110

By Anonymous on Friday, March 2, 2001 - 11:30 am:

The best way I have found to solder to hard-to-solder materials
like iron and steel and telephone wire with entangled fibers is
to first clean the surface (sandpaper or file) then wrap it
tightly with thin bus wire or ~26 guage stripped solid wire. The
resulting solder joint is very strong and electrically intact....
and very easy to solder!

-Mike-

By Anonymous on Friday, March 2, 2001 - 01:26 pm:

Braze

Thats what we did when I was 12 years of age in metalwork class

Braze??

Multiple global echoes.......................................


Don't try it on PICs!!!!

-Simon-

PS its Friday night dammit-------clarity is thinned...........



PPS coat hangers are a b*g**r tho'


By Anonymous on Friday, March 2, 2001 - 01:46 pm:

I used regular solder, but I sanded the varnish coat off and cleaned it real
well. Also use 50 watt or greater iron.

Mark

By Anonymous on Sunday, March 4, 2001 - 06:06 am:

Hi Mark,
Would you tell us how you soldered to the steel clothes hanger, please?
Thanks!
Paul


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