To make Arduino_Multimeter need advice on protections please.

DickCappels

Joined Aug 21, 2008
7,449
How would that hurt anything?
(Edit: I realized that I misinterpreted "R gear". Yeah, that's not the proper way to measure a power supply's output resistance :)

If the current source (or resistor) used to measure resistance could rise to well above the power supply voltage of the controller you would have to worry but I don't see any higher voltage being introduced in the circuit.

You need to be careful about electrostatic discharge (ESD), trying to measure voltages in excess of the controller's power supply.
1623308523851.png
The way I got around this was to use a voltage divider with the resistor at the input of the divider being large enough to limit the current to a few milliamps when connected to the highest anticipated voltage. Even is you don't need the divider, be sure to include some resistance. Do not use the values shown in the schematic because your firmware expects something else.
 
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Thread Starter

LAOADAM

Joined Nov 21, 2018
569
How would that hurt anything?

If the current source (or resistor) used to measure resistance could rise to well above the power supply voltage of the controller you would have to worry but I don't see any higher voltage being introduced in the circuit.

You need to be careful about electrostatic discharge (ESD), trying to measure voltages in excess of the controller's power supply.
View attachment 240872
The way I got around this was to use a voltage divider with the resistor at the input of the divider being large enough to limit the current to a few milliamps when connected to the highest anticipated voltage. Even is you don't need the divider, be sure to include some resistance. Do not use the values shown in the schematic because your firmware expects something else.
Thanks.
I got few of my DMM broken for the same reason of mistest.
I'll try your circuit.
 

Phil-S

Joined Dec 4, 2015
205
Unless you are doing this out of curiosity, it can't really be worth the effort.
By the time you have the Arduino and its ancillaries, a display, some buttons or switches, a case to keep it all safe and so on, it's going to add up cost and timewise to a lot more than a cheap multimeter. Then you need to calibrate it.
Component testing is a different matter and although meters have functions like capacitance, they are rarely good for anything outside a narrow range. Personally, addons like capacitance are at best an indication only.
My now very old Fluke 83 (I think) has been a real, reliable workhorse for 30 to 40 years.
And as the others have said, the only problems have arisen on current measuring, where I didn't get the range right or overloaded it and one of the two chunky fuses has popped.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
569
Did you check the fuses? The most common mis-test is measuring a voltage when it is in a current mode, and this usually blows one of two fuses. I keep spares in stock.

Bob
Thanks.
I'll recheck that. I was knowing the DMM got only one fuse.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
569
Unless you are doing this out of curiosity, it can't really be worth the effort.
By the time you have the Arduino and its ancillaries, a display, some buttons or switches, a case to keep it all safe and so on, it's going to add up cost and timewise to a lot more than a cheap multimeter. Then you need to calibrate it.
Component testing is a different matter and although meters have functions like capacitance, they are rarely good for anything outside a narrow range. Personally, addons like capacitance are at best an indication only.
My now very old Fluke 83 (I think) has been a real, reliable workhorse for 30 to 40 years.
And as the others have said, the only problems have arisen on current measuring, where I didn't get the range right or overloaded it and one of the two chunky fuses has popped.
Thanks.
You are right.
I just like to DIY some thing and trying out some possibility. In fact, it can be of some advantages.
 

DickCappels

Joined Aug 21, 2008
7,449
It looks as though the the circuit measures resistance based on the resistor under test being a voltage divider when combined with the 2.2k resistor. What is probably the best way to protect the controller is to place a resistor in series with the A-to-D converter input. I could not find the maximum input current to a pin for the case of driving the input through a resistor with a voltage higher than Vcc. But from memory this is more than 5 ma.

I propose that limiting current to 3 ma is safe, so the easiest way to protect the input is to put a resistor between the output of the voltage divider and the A-to-D input. The resistor should be greater than ((Maximum expected fault voltage)-Vcc)/ 3 ma.

Do not use a larger resistance than indicated by the formula lest you run into accuracy problems.
 
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