Cheap way to measure high current DC

[John61CT]

Hey Spiff don't be such a tease! Which model?

http://www.p-mastech.com/product/category/80

 

[Spaceman Spiff]

I have a MasTech MS 2108A that I bought a few years ago for ~$18 (don't remember if Amazon, E-bay or ??).  An EE friend was curious about how good it was so we tested it against his calibrated Fluke.  It gets less consistent below 10A, but that can be corrected by wrapping the wire around the clamp a few times.

I never tested how it responds to supply battery voltage.

-- Spiff

 

[Boyntonstu]

What you are describing is in electronics called an ammeter shunt or current measuring shunt, so your wire is a shunt.  This is the method used in amp counting gauges for solar charging systems like Trimetric (measuring voltage across a precision shunt to determine amps).

You have not accounted for the temperature coefficient of resistance of copper = +0.393% per ºC.

I am not aware of any digital multimeters that use a galvanometer to measure amps.  They use a precision shunt (the precision being a function of the cost of the multimeters).

The cheap HF multimeters are notorious for being very sensitive to supply voltage, so make sure you are getting 9.0V from your battery. In my experience the batteries supplied do not hold voltage for long. Voltage reading will drift upwards as supply voltage drops.

The leads that come with the HF multimeters are crap.  Make sure you know the resistance of you leads.

My only bitch with the system you are describing is that you have to disassemble the circuit to insert the shunt.  My $20 Mastech clamp on ammeter is within 1% of a calibrated Fluke for currents over 10 amps.  And I don't have to take my circuit apart for measurements.

 -- Spiff

Spiff,

1> Am I using an ammeter?  No.  It is not an Ammeter shunt.  An ammeter shunt allows the measurement of current values too large to be directly measured by a particular ammeter.

 [font=Arial, Helvetica, sans-serif]Ammeter Operation – Today’s ammeters have an internal resistance to measure the current across the particular signal. However, when the internal resistance is not enough to measure larger currents, an external configuration is needed.

[font=Arial, Helvetica, sans-serif]To measure larger currents, you can place a precision resistor called a shunt in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure larger currents.[/font][/font]

[font=Arial, Helvetica, sans-serif]Using #10 wire, more than 99.99% of the current flows through it.[/font]


2> Make sure you know the resistance of you leads.  IMHO Not important when measuring Volts with almost zero current flowing in the leads.  E=IR  I is almost zero and the resistance does not mater.  We are measuring potential.

3> You have not accounted for the temperature coefficient of resistance of copper = +0.393% per ºC.

I have never needed to measure high Amps for other than for quality testing and I never needed precise measurements.

At 32*F  .9203 Ohms
At  59*F .9792 Ohms
At  68*F .9989 Ohms  almost perfect at this temperature.
At  122*F .1117 Ohms

4> My only bitch with the system you are describing is that you have to disassemble the circuit to insert the non-shunt series wire.   I agree, except for the shunt correction. 

Shunt - Parallel
Non-shunt-Series.

A clamp on meter is the best.  I would prefer to carry a multi-purpose multimeter and a piece of wire instead.
I would always have cable jumpers  on-board.

 

[Weight]

Any ammeter is a voltmeter with a face scale that indicates amps based on voltage drop across a shunt. Your loop of wire is a shunt. Good by.

 

[blars]

I bought a "Bside ACM03" clamp DC ammeter for $25 on ebay after I saw a review. I have not played with it much, it has 40A and 400A full scale ranges on DC amps. Ebay prices seem to have gone up almost $1 since then. A few cents more got me one shipped from the US. For those alergic to ebay, it's about $30 on aliexpress.

Main review complaints were not true RMS, so not good for AC readings from inverters or generators, and it probably doesn't meet the claimed insulation standards, so don't use it at high voltage. (over 240 VAC or 500 DC).

 

[John61CT]

Excellent, thanks.

Other such specific reports would be appreciated.

 

[Boyntonstu]

Any ammeter is a voltmeter with a face scale that indicates amps based on voltage drop across a shunt. Your loop of wire is a shunt. Good by.

when you use an ammeter are the test leads in series with the load or parallel to the load?

You must break a connection to use a series Ammeter.

FYI

[font=Roboto, arial, sans-serif]An [/font][font=Roboto, arial, sans-serif]ammeter[/font][font=Roboto, arial, sans-serif] is a measuring device used to measure the electric current in a [/font][font=Roboto, arial, sans-serif]circuit[/font][font=Roboto, arial, sans-serif]. A voltmeter is connected in parallel with a device to measure its voltage, while an [/font][font=Roboto, arial, sans-serif]ammeter[/font][font=Roboto, arial, sans-serif] is connected in [/font][font=Roboto, arial, sans-serif]series[/font][font=Roboto, arial, sans-serif] with a device to measure its current.[/font]