# How do I make an arduino Ohm Meter using 24VDC ?

Hi guys, there are multiple tutorial online on how to do arduino ohmmeter.
But most of it is using Vin = 5V.
My question is how to use 12V or 24V as Vin(external power supply) and then connect it to arduino for resistance calculation without damaging the board?

Do you know what a voltage divider is ?

Tell us why you cannot use 5V.

Yes i do know what a voltage divider is.

I canβt use the 5V because I want to measure the resistance between 2 gnd cables. System Earth and Aux Earth. So over a long distance 5V would be insufficient i guess. What do you suggest I do?

nuub_fella:
Yes i do know what a voltage divider is.

I can't use the 5V because I want to measure the resistance between 2 gnd cables. System Earth and Aux Earth. So over a long distance 5V would be insufficient i guess. What do you suggest I do?

Use your 24V as your exciting voltage. Run the current through a milliohm-scale sense resistor. Use an instrumentation amp to measure the voltage drop across the sense resistor with a full-scale output just less than your Arduino board's AVCC.

What range of resistance do you expect ( .1/1/10/100 etc. ohms ) ?

Blackfin:
Use your 24V as your exciting voltage. Run the current through a milliohm-scale sense resistor. Use an instrumentation amp to measure the voltage drop across the sense resistor with a full-scale output just less than your Arduino board's AVCC.

Urm... this sounds complicated

larryd:
What range of resistance do you expect ( .1/1/10/100 etc. ohms ) ?

10 to 70 ohms.
Basically im working on a project that measures resistance between 2 earth cables. So I want use and external psu to inject 24VDC into the earth cable and then measure the resistance. There will be a dip switch to preset the resistance value. For example, If dip switch 1 is on..then preset is 10 Ohms. If the measured resistance is more than the preset, then an sms will be sent to user notifying that earth resistance is high. However, if measured resistance is lower than dip switch preset, then sms will be sent saying everything is OK.

What type of system are these two earth cables connected to..?

Can you provide a basic layout diagram......?

nuub_fella:
So over a long distance 5V would be insufficient i guess.

Nonsense!

I want to measure the resistance between 2 gnd cables. System Earth and Aux Earth.

What resistance would you expect?

So over a long distance 5V would be insufficient i guess.

TLC Electrical Supplies gives resistances for cables. A 1mm conductor has a resistance of 18 MILLIOHMS per metre. Or 18 ohms per kilometer.

You CAN measure resistance EITHER by applying a current and measuring the voltage; or applying a voltage and measuring the current.

However since a TRUE voltage source can supply infinite current (or a true current source, infinite voltage) that in principle can be risky.

So we generally use a potential divider principle.

Your problem (apart from do you understand ohms law?) is that the resistance may be very small. So you will NOT need lots of volts - because big V / tiny R = massive current.

Use 5V and a potential divider circuit made up of 100 ohm to +5 and then external circuit you are measuring - zero volts.