Cheap way to measure high current DC

Boyntonstu · Sep 2, 2017

Only a few bucks pretty accurate:

[video=youtube]

John61CT · Sep 2, 2017

Uni-T UT203 is a not expensive clamp style ammeter?

Do you think yours is better, more accurate? What model is it?

Trebor English · Sep 2, 2017

AWG # 10 wire is .001 ohms per foot Actually it is supposed to be 0.0009989 but it isn't that close. A foot has a voltage drop of 1 millivolt, .001 volt for each amp flowing.

#10 wire is supposed to be good for 30 or 40 amps. I have used this method with the cheap Harbor Freight meter to measure the 500 amp starting current on a 6 cylinder 466 cubic inch diesel school bus. Actually I was looking for a parasitic draw that made the bus not start on Mondays.

Copper clad aluminum from Amazon is higher resistance.

Boyntonstu · Sep 2, 2017

John61CT said:
Uni-T UT203 is a not expensive clamp style ammeter?

Do you think yours is better, more accurate? What model is it?

My model is a Boyntonstu Mod 1.

Accuracy? Close enough. Do you need to measure currents to the mA level?

Cost? A short piece of copper wire given to me for free a Home Depot.

A multimeter given free at Harbor Freight without a purchase by using a coupon.

I tried, but I could not make any cheaper. :blush:

Boyntonstu · Sep 2, 2017

Trebor English said:
AWG # 10 wire is .001 ohms per foot Actually it is supposed to be 0.0009989 but it isn't that close. A foot has a voltage drop of 1 millivolt, .001 volt for each amp flowing.

#10 wire is supposed to be good for 30 or 40 amps. I have used this method with the cheap Harbor Freight meter to measure the 500 amp starting current on a 6 cylinder 466 cubic inch diesel school bus. Actually I was looking for a parasitic draw that made the bus not start on Mondays.

Copper clad aluminum from Amazon is higher resistance.

I must have been looking over your shoulder when I came up with my method.

BTW You can use about 18" of #12 copper wire as well.

Especially if you calibrate the positions to solder the Volt measurement points.

For short time current flow measurements using short pieces of copper wire, quite a few Amps can flow without heating t up.

John61CT · Sep 2, 2017

I just meant the multimeter.

I've never seen Harbor Freight nor their coupons, so wanted to look at the DMM specs or reviews online, since not that many models measure DC amps accurately.

Boyntonstu · Sep 2, 2017

John61CT said:
I just meant the multimeter.

I've never seen Harbor Freight nor their coupons, so wanted to look at the DMM specs or reviews online, since not that many models measure DC amps accurately.

Sorry, I did not understand your question.

Any multimeter that than can measure millivolts will be OK.

I have gotten about 10 of these meters:

https://www.harborfreight.com/7-function-digital-multimeter-90899.html

Just measure the millivolts and the current will be the same if the wire resistance is 1 milli-Ohm between the leads.

10 millivolts measured means 10 Amps.

>----------+-------------------+---------->

Put the current in from the left and out the right.

Place the multimeter Red lead at the left + point and the Black lead - at the right + point.

Real easy to do.

It does not matter if you bend the wire.

Trebor English · Sep 2, 2017

John61CT said:
not that many models measure DC amps accurately.

This is using the multimeter to measure milivolts. The Harbor Freight cheapest meter has a shunt resistor internaly and can measure 10 amps. An external shunt resistor permits higher current measurements.

An uncalibrated shunt resistor made by measuring a foot of AWG # 10 wire is unlikely to be within 10%. The key advantage is the really really cheap price.

The accuracy over temperature is not to be expected. When measuring 500 amps it gets hot pretty quickly.

Did I mention that it is cheap? A shunt resistor calibrated to 10% will cost $10. Thats a buck per per cent. Is that a unit of measure? Buck per per cent? A SPW (short piece of wire) from junk at zero cost is, did I mention, cheap.

The Harbor Freight meter does not auto-shut off. It does not auto range. AC scales are seriosly limited with nothing low voltage. It is not RMS responding, just a diode and rc filter. On the plus side, it comes with a battery.

If you forget the meter at a job site it is not aggravating. It basically works fine.

Boyntonstu · Sep 2, 2017

Trebor English said:
This is using the multimeter to measure milivolts. The Harbor Freight cheapest meter has a shunt resistor internaly and can measure 10 amps. An external shunt resistor permits higher current measurements.

An uncalibrated shunt resistor made by measuring a foot of AWG # 10 wire is unlikely to be within 10%. The key advantage is the really really cheap price.

The accuracy over temperature is not to be expected. When measuring 500 amps it gets hot pretty quickly.

Did I mention that it is cheap? A shunt resistor calibrated to 10% will cost $10. Thats a buck per per cent. Is that a unit of measure? Buck per per cent? A SPW (short piece of wire) from junk at zero cost is, did I mention, cheap.

The Harbor Freight meter does not auto-shut off. It does not auto range. AC scales are seriosly limited with nothing low voltage. It is not RMS responding, just a diode and rc filter. On the plus side, it comes with a battery.

If you forget the meter at a job site it is not aggravating. It basically works fine.

Boyntonstu · Sep 2, 2017

The preview was perfect. However the edit time expired.

This is what it was supposed to be:

"An ammeter shunt allows the measurement of current values too large to be directly measured by a particular ammeter. In this case the shunt, a manganin resistor of accurately known resistance, is placed in parallel with the moving coil galvanometer , thus all of the current to be measured will flow through it."

The drawing that got screwed up is here:

https://www.google.com/search?q=usi...me..69i57j0.7166j0j4&sourceid=chrome&ie=UTF-8

John61CT · Sep 2, 2017

I have a collection of shunts, from 100 up to 1000A, apparently compatible with Bogart and Victron BMs.

It would be a bit unweildy to rig Anderson plugs and use them as semi-portable ammeters, but besides better accuracy in live measurements, being able to log AH usage over time should prove useful.

But I will continue to look for more portable ammeters that are accurate and relatively inexpensive (under $200), so suggestions welcome.

Greenlee CMT-80 may well fit the bill, can come up cheap on eBay.

Boyntonstu · Sep 2, 2017

Compare a 12" piece of #10 wire to your shunts for accuracy.

Zil · Sep 3, 2017

You are using the wire as a shunt resistor. I always had to make a purchase to get the free meter at Harbor Freight. Those meters are rather accurate, but the leads are poor and fail sooner than later. I also have many of them.

Boyntonstu · Sep 3, 2017

Zil said:
You are using the wire as a shunt resistor. I always had to make a purchase to get the free meter at Harbor Freight. Those meters are rather accurate, but the leads are poor and fail sooner than later. I also have many of them.

What current flows when taking a voltage measurement?

A meter at 20,000 ohms/volt passes 12V/20000 0.006 A.

Almost zero.

I do not agree that although technically true, shunting 200 Amps to bypass 0.006 a is really shunting.

OTOH If you were using a shunt to increase the range of a 10 A meter, I would call that shunting.

Zil · Sep 3, 2017

How many amps are flowing through the loop of wire? Are you measuring the voltage drop across the wire, to measure amperes?

Boyntonstu · Sep 3, 2017

Zil said:
How many amps are flowing through the loop of wire? Are you measuring the voltage drop across the wire, to measure amperes?

How many amps are flowing through the loop of wire? All of them.

Are you measuring the voltage drop across the wire, to measure amperes? Yes, indirectly. We measure volts.

1 milli-Volt = 1 Amp if the resistance is 1 milli-ohm which is the resistance of 12" of #10 solid copper wire!

Simple!

Ticklebellly · Sep 4, 2017

Zil said:
Are you measuring the voltage drop across the wire, to measure amperes?

Voltage drop across a known resistance is pretty much how amp meters work. I have manufactured shunts out of Toaster elements in the past using a Wheatstone bridge arrangement to get accurate resistance measurements.

Weight · Sep 4, 2017

I'm confused. Is not the loop of wire carrying all the current, where you are measuring the voltage drop, the same thing as a shunt?

Boyntonstu · Sep 4, 2017

Ticklebellly said:
Voltage drop across a known resistance is pretty much how amp meters work. I have manufactured shunts out of Toaster elements in the past using a Wheatstone bridge arrangement to get accurate resistance measurements.

Multimeters include several functions, Ammeter and Voltmeter.

[font=Roboto, arial, sans-serif]A [/font][font=Roboto, arial, sans-serif]voltmeter[/font][font=Roboto, arial, sans-serif] 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 series with a device to measure its current. At the heart of most analog meters is a galvanometer, an instrument that measures current flow using the movement, or deflection, of a needle.[/font]

[font=Roboto, arial, sans-serif]In the case of a 12" length of #10 copper wire, the wire itself is the device being measured.[/font]

[font=Roboto, arial, sans-serif]The maximum current of most multimeters in the ammeter function is 10 Amps.[/font]

[font=Roboto, arial, sans-serif]The fuse would blow if you tried to [/font][font=Roboto, arial, sans-serif]pass 100 Amps through a multimeter.[/font]

See this link on how to use and calculate a shunt:

http://electricalbaba.com/ammeter-shunt-construction-calculation/

[font=Roboto, arial, sans-serif]A voltmeter does not care how much current is passing through the wire.[/font]

[font=Roboto, arial, sans-serif]A wire is not a shunt paralleling an ammeter![/font]

[font=Roboto, arial, sans-serif]I hope that this explains it.[/font]

Spaceman Spiff · Sep 4, 2017

Boyntonstu said:
[font=Roboto, arial, sans-serif] . . . In the case of a 12" length of #10 copper wire, the wire itself is the device being measured.[/font]
[font=Roboto, arial, sans-serif] . . . A wire is not a shunt paralleling an ammeter![/font]

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

Cheap way to measure high current DC

Help Support Van Living Forum:

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Similar threads

Join the conversation!

Join today and get all the highlights of this community direct to your inbox. It's FREE!