Multimeter design help/ideas

[wizpic]

I'm looking at making a wireless resistance meter like my fluke233, But the trouble with the Fluke 233 meter the range is very short and intermittent dropping in/out.

I've made a wireless meter using an ESP32 using the esp-now protocol that can measure up to +/-100V DC only(that's all I need) and the range is very good.

But I now have the need to measure resistance wireless but need protection in case you accidentally use it in ohms range on a live circuit (upto +/-85V) like I can with the fluke meter without causing damage to it. It's an aid to find shorts in wiring looms where I measure the resistance between ground and the positive side of the cable which should read open circuit but in some places you move the loom and then this is where is could be damaged then sound bleeper and readings show on a remote display(like the fluke meters) if the wires short or any resistance between them if it's not a short but drawing extra current and help trace the issue.

I've looked at the UT61E,fluke27,10 and 79 and some other schematics but can't get my head around how the range switch works and where the signals go or pick out the bits just for ohms part.
Not sure but I've looked at the Wheatstone Bridge circuit ?

But not sure what's the best way or how to go about it ?
I've seen plenty of ESP32/Arduino circuits and that but they use the 3.3V/5V to feed the resistor then measure the voltage across the other one but this would blow if there was any voltage on the line and can only measure positive voltage.

 

[danadakk]

Just as an aside you can always make it talking (thru PC) :

GUI Codeless Processor Design

Micro design has for years relied on ASM and C programming and other languages. Often many tasks are fairly simply but these tools were focused and quite a learning curve, especially ASM and C and C++. There are a number of new GUI based tools that essentially take out the "strong typing"...

www.edaboard.com

Also if using ESP32 for stuff you tried Tuniot ? Quick way of bringing up webserver or client.

https://www.edaboard.com/threads/verizon-remote-hotspot-controller.400349/

Regards, Dana.

 

[wizpic]

Thanks but I don’t always have PC on site plus this could work expensive if you drop it , the remote display I use a Nokia lcd 100x100mm box with magnet for my voltmeter.
My idea is another device to measure ohms so the same display can be used for both

 

[KlausST]

Hi,

You mean a battery powered ohm meter with internal power source?
DC resistance only?
2 wires or 4 wires?
What range, resolution, accuracy?

Klaus

 

[wizpic]

Sorry Yeah battery powered with internal power source (no common ground between meter and testing circuit) Like my fluke meter.
It will be working only on DC voltage circuit and 2 wires.
I know that the circuit should not be live while testing resistance but sometimes you may get a bit of residual voltage on the circuit or go straight into measuring/looking for shorts in the wires and forget to turn off or isolate it like I have done with my fluke meter without effecting it. I did that once on a cheap meter and blew it up.
the range I would say 0.1R to 1M, Resolution I would like to use 16bit ADC accuracy has accurate has the fluke meter or close to it as possible.
With the metal work and surroundings give very short range with my fluke 233 meter but with my EPS32 meter the range is very good and have no issues with measuring voltage with the range compared to my fluke meter.

 

[FvM]

I've looked at the UT61E,fluke27,10 and 79 and some other schematics but can't get my head around how the range switch works and where the signals go or pick out the bits just for ohms part.
Not sure but I've looked at the Wheatstone Bridge circuit ?

Definitely no Wheatstone bridge used here. Fluke 233 specifies 1100V proof overvoltage protection in ohms range, UT61E still 600V. Requires well designed circuit, not easily to achieve in DIY meter. UT61E input protection seems to utilize PTC and limiting diodes.

 

[KlausST]

Hi,

2 wire with internal supply.
--> common GND or not, this will not protect the device from being killed by applying external power. You need extra circuitry to protect your measurement device.

0.1 R on 2 wires ... are you sure?
0.1R using DC method .. are you sure?
0.1R to 1M .. how many ranges? Autoranging? Any idea about measuring current of each range?

"16 bit" has nothing to do with accuracy.
Accuracy is how much the displayed value differs from the true resistor value.
Example:
On a +/-1% accuracy..
* when you measure a perfect 1.000 kOhm resitor, the displayed value is allowed to show: 990.0Ohm ... 1010.0 Ohm
* 1% accuracy on a 0.1 Ohms resistor, means an allowed deviation of +/-0.001 Ohms.
(My opinion: Impossible with DC measurement, impossible with 2 wires)
* means you need at least three ranges on a perfectly working analog circuit including ADC.

Klaus

 

[danadakk]

Handbook on precision measurements might be helpful :

https://download.tek.com/document/LowLevelHandbook_7Ed.pdf

Regards, Dana.

 

[wizpic]

Thanks people I knew it would not be easy,
It’s more to find short circuits more than resistance so may be 500ohms may be ok, I’ll do some reading and testing and may be forget the protection side of things and make sure power is turned off

Thanks Dana some bed time reading looks a good book having a quick look through it