You could use a shunt but I have found that if the length of wire from
the switch/distribution panel is sufficient and the wire is not totally
overly thick you can get good results just measuring the drop across it.
Typically it is not unusual to have a drop of 500mV from the battery
to the load but this will include the cables to the distribution panel
so the wire you are voltage drop you are working with will be less. On
my boat the wire was so thin the drop was over 1V at the dis panel when
the fresh water pump kicked in. Copper wire is not be as thermally
stable as a proper shunt would be but copper is pretty good anyway
(within a few percent for a 10 degree C temp change). Your electronics
may not be this stable anyway. You will need to calibrate the wire using
a known load or ammeter/volt meter and will still need the differential
amplifier to boost to get your full 2.5V range.

Obviously you would be best using the negative side of fridge circuit or
you will need to bring the voltage down from 12ish volts to 2.5. As a
warning I have found with some data logger circuits that when the
voltages are close to 0 there can be a issues. Might be worth
considering when you test. If your logger power supply is tapped from
somewhere closer to the battery than the fridge you should at least
avoid getting a relative negative on the logger input. Negatives
voltages can be as bad as over voltage I have found.

Steve

Glenn Ashmore wrote:
I should know this but I just can't get my brain around it.

I have a data logger that will accept 4-20 mA or 0-2.5VDC. I want to track
the current going to the refrigeration compressor which I figure will be in
the range of 0 to 25 amps (to be safe for start ups).

What is the best way to accomplish this?

Ohms law brain freeze
 

I should know this but I just can't get my brain around it.

I have a data logger that will accept 4-20 mA or 0-2.5VDC. I want to track
the current going to the refrigeration compressor which I figure will be in
the range of 0 to 25 amps (to be safe for start ups).

What is the best way to accomplish this?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Put a shunt in the negative power lead and measure the voltage
across the shunt. A 30A/50mv shunt should work nicely. An
Inst Amp with a gain of 50 can bring the voltage up to the range
of your data recorder.

Doug
s/v Callista

"Glenn Ashmore" wrote in message
news:y0Ipd.3752$wa1.3225@lakeread04...
I should know this but I just can't get my brain around it.

I have a data logger that will accept 4-20 mA or 0-2.5VDC. I want to
track
the current going to the refrigeration compressor which I figure will be
in
the range of 0 to 25 amps (to be safe for start ups).

What is the best way to accomplish this?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Fri, 26 Nov 2004 10:32:55 -0500, "Glenn Ashmore"
wrote:
I should know this but I just can't get my brain around it.

I have a data logger that will accept 4-20 mA or 0-2.5VDC. I want to track
the current going to the refrigeration compressor which I figure will be in
the range of 0 to 25 amps (to be safe for start ups).

What is the best way to accomplish this?

=================================================

As others have mentioned a current shunt is one way to do this.

However, since you're data logger can also be voltage driven, I think
it would be cheaper and easier to build a voltage divider with a
couple of resistors in series across the compressor circuit.

A voltage divider divides in proportion to the two resistors, i.e., if
you put a 9,000 ohm resistor is series with a 1000 ohm resistor, the
voltage across the 1000 ohm resistor will be 1/10 of the voltage
across both. Keep the total resistance high enough to minimize
current flow.

Assuming a 12 volt compressor and a desired output of 2 volts for your
data logger, you need a resistor ratio of approximately 1/5 in series
with each other. A 1000 ohm and 5000 ohm would work OK assuming your
data logger has a high impeadance input with respect to the resistor
bridge.

"Wayne.B" wrote in message
...
As others have mentioned a current shunt is one way to do this.

However, since you're data logger can also be voltage driven, I think
it would be cheaper and easier to build a voltage divider with a
couple of resistors in series across the compressor circuit.

Indeed: As others have mentioned a current shunt is one way to do this.
You cannot measure the current by measuring the voltage *across* the
compressor. Unles you mean that you want to deduce the current by
subtracting the voltage across the compressor from the voltage across the
battery (that's tw measurements) and divide that though resistance of the
wires. Could be done but you have a resolution problem.

Meindert

Yes, completely true, Meindert. But if the objective is simply to log
on/off cycles, then the voltage divider would be sufficient. It would
even give a crude measure of compressor current (via voltage drop).

Chuck


Meindert Sprang wrote:
"Wayne.B" wrote in message
...

As others have mentioned a current shunt is one way to do this.

However, since you're data logger can also be voltage driven, I think
it would be cheaper and easier to build a voltage divider with a
couple of resistors in series across the compressor circuit.


Indeed: As others have mentioned a current shunt is one way to do this.
You cannot measure the current by measuring the voltage *across* the
compressor. Unles you mean that you want to deduce the current by
subtracting the voltage across the compressor from the voltage across the
battery (that's tw measurements) and divide that though resistance of the
wires. Could be done but you have a resolution problem.

Meindert

I really need to mesure actual amps because the compressor has a "smart"
variable speed control. I found a 25A 50 mv shunt for $10 so I think I will
go that way.

These little 4 chanel Hobo U12 data loggers are neat. What I plan to do is
record ambient, freezer and fridge temps along with amps used for a week on
a friends boats headed for the Bahamas next month. The interior size and
mechanicals of his refrigeration system are the same as mine but
traditionally foam insulated so that will give me a good baseline. This is
a particularly good opportunity because he is a detail fanatic and plans to
record everything he adds to the box.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com