# arduino as ammeter

not sure if this is correct thread location to post, if not please forgive, let me know and i'll repost correctly.

i know that the uno is able to read small current with shunts. my question deals with 200A+, yes - 200Amps plus at ~14.5VDC. i have measured 238A max over 2 minutes and ringing ears. LOL felt so good though!!!
i have 5kW stereo system i want to read voltage and current of 4 seperate amps plus main system so 5 readings total.
i have a 1500A shunt for main system and 4-200A shunts for amps.

just starting to get details worked out on design. will have 20x2 lcd, if not 20x4 lcd and 328p standalone.
with the shunts i have and correct components to build, i should realistically be able to monitor both voltage and amps on separate A0-A4 inputs with good calculations or need 10 inputs, 5 voltage and 5 amps???

The thing with a shunt, it's basically a low-ohmage, high accuracy, resistor.

To know if it will "work" with the Arduino, in that you can a) read decent values from it and b) not fry the Arduino, you need to do a bit of Ohm's law.

You need to know the resistance of the shunt, and you know the expected peak current. Given an example resistance of 0.001? (1m?), we can calculate that for 200A flowing through it you will drop (V=R*I) 0.001 * 200 = 0.2V. That's not much range to play with. If the shunt is 0.01?, however, that would equate to 2V. If it were 0.1? it would be 20V and your Arduino would melt.

Also, if you're thinking of doing high-side readings, you'll need a high-side current shunt amplifier to isolate the higher voltage offsets from the arduino. Do low-side if you can.

If the shunt is 0.01?, however, that would equate to 2V

.. and 400W of heat as well

Typical 150A shunt I hold in my hand 2w ago was made of 3 parallel 3-4mm diameter brass alloy wires (~80mm long), soldered to two 25x25mm 4mm thick copper holders equipped w/ M8 bolts with M8 screws for current and M3 bolts and screws for voltage measurement. It gave 60mV @150A.

I’d feel a lot safer with something like this on one of the cables than trying to connect Arduinos to 200A current shunts:

http://www.ebay.com/itm/231154766766

tnx guys,
small correction, i have a 1200A, not 1500A, shunt. not that it's gonna matter anyhow.
it had stamp of 50mV and i was thinking it was good for input to ic but it reads .4 ohm so i don't believe i will be using it.
the 200A is stamped 75mV with .3 ohm on DMM. fat chance to use that also.

with the amp rings, anyone use any of the like before and with good results? i will do more googling to see what i an find.

small correction, i have a 1200A, not 1500A, shunt. not that it's gonna matter anyhow.
it had stamp of 50mV and i was thinking it was good for input to ic but it reads .4 ohm so i don't believe i will be using it.
the 200A is stamped 75mV with .3 ohm on DMM. fat chance to use that also.

You mean 120A probably.
A 120A shunt for 50mV full range reading has R=0.0004 ohm
A 200A shunt stamped 75mV has R=0.000375 ohm
You cannot measure resistance of those shunts with your DMM that way, I am afraid..

aes92000:
with the amp rings, anyone use any of the like before and with good results? i will do more googling to see what i an find.

It's just a hall-effect sensor attached to the wire. It measures the magnetic field created by your amps. There's not much to go wrong.

OTOH you haven't said what sort of signals will go down the wire. AC? DC? High frequency?

OTOH you haven't said what sort of signals will go down the wire. AC? DC? High frequency?

the signal is to measure DC current from my audio system in truck
i want to have 5 individual readings

• main system reading (4 reserve batteries in parrellel, each @ 170Ah each - system is ~14.4VDC)
• 2 - 2000W amps,
• 1 - 1600W amp
• 1 - 1200W amp

Sounds like you're going to be needing to do high-side readings for that kind of rig. You want to look at the AD8215 and similar chips.

aes92000:
the signal is to measure DC current from my audio system in truck

Audio isn't DC.

small correction, i have a 1200A, not 1500A, shunt. not that it's gonna matter anyhow.
it had stamp of 50mV and i was thinking it was good for input to ic but it reads .4 ohm so i don't believe i will be using it.
the 200A is stamped 75mV with .3 ohm on DMM. fat chance to use that also.

You mean 120A probably.
A 120A shunt for 50mV full range reading has R=0.0004 ohm
A 200A shunt stamped 75mV has R=0.000375 ohm
You cannot measure resistance of those shunts with your DMM that way, I am afraid..

i will take picture of both and upload later so y'all can see.
it does say 1200AMP with 50mV stamps

if that is the case, would i be able to use the shunts i have or still a big risk???

oh, thanx for replies!

the signal is to measure DC current from my audio system in truck

Audio isn't DC.

not trying to be rude but...
no, audio is not DC current but my batteries and power to amps is DC current

aes92000:
tnx guys,
small correction, i have a 1200A, not 1500A, shunt. not that it's gonna matter anyhow.
it had stamp of 50mV and i was thinking it was good for input to ic but it reads .4 ohm so i don't believe i will be using it.
the 200A is stamped 75mV with .3 ohm on DMM. fat chance to use that also.

with the amp rings, anyone use any of the like before and with good results? i will do more googling to see what i an find.

You are being fooled by your multimeter - you cannot measure small resistance
values directly with a multimeter because the resistance of the leads and the selector-
switch in the meter itself will totally dominate the reading.

The way to measure a low value resistance is by using 4 wires. Two wires carry the
current to the resistor, the other two (the meter probes) measure the voltage across
the resistor (multmeter in voltage mode, not resistance mode).

For instance pass 10A through a 0.0005 ohm resistor and see 5mV across it.

here are the shunts i have.

i never thought about the testing procedure, it is almost the same as doing a meg test on grounding system.
thank you for explaining it.

and one more pic

MarkT:
You are being fooled by your multimeter - you cannot measure small resistance
values directly with a multimeter because the resistance of the leads and the selector-
switch in the meter itself will totally dominate the reading.

That depends on the multimeter.

A good multimeter will have a "REL" button on it. You put the probes together and hit "REL" to cancel out the impedance of the leads, etc. Now you can measure the resistor.

aes92000:

Audio isn't DC.

not trying to be rude but...
no, audio is not DC current but my batteries and power to amps is DC current

I suppose it depends on which wire you point to and call "Ground".

(But by that definition, all signals are DC...)

Still, if you have a wire going to a big capacitor from a power supply (with the rest of the system powered from that capacitor) the signal in that wire should be pretty close to DC.

Clamping that wire to measure the current should work. All you need is a clamp that can measure that much current.

Of course, no one has thought to ask the most obvious question...

Why the hell have you got such a stupidly powerful stereo in your truck...?!

What is the problem with measuring the 1200A with the shunt?
50mV to measure is easy - an low noise opamp (you must use +5V and -5V power for the opamp) with amplification set to ~50 will do that. You will get 0-2.5V for 0-1200A, for example. Filter the input voltage a little bit and that is all.
You need a DC-DC converter to get -5V and a single opamp, 2-3 resistors, 2-3 capacitors.

You may isolate the whole stuff by a separate battery (so the only wires will go to the voltage terminals of the shunt - use shielded cables) and send the data to your truck control center via Bluetooth

majenko:
Of course, no one has thought to ask the most obvious question...

Why the hell have you got such a stupidly powerful stereo in your truck...?!

And why do you need a real time output of how many amps it's drawing?