Milliohm Meter

[foetusmachine]

Had a go at designing a milliohm meter. Started off after watching a youtube video of a guy who copied an Alan Wilcox design.

My background is in architecture (i.e. buildings) so forgive me. Yeah, just forgive me.

Tried to get some feedback on another forum as I was working through it, but that was like talking to myself. Literally.

Anyway, here it is. Will cost way too much, but I went down a rabbit warren and got stuck, so I just kept going until I came out.

Would love to get any thoughts and criticism.

[jonpaul]

Bravo: Yes a bit overkill.

Usually a simple current source with decde ranges and millivoltmeter works.

eg 10/100/1000 MA source>>>R to test>>GRD

Place millivoltmeter (eg x1000 DC diff amp ) across the R.

See many HP and other commercial milliohm meters desig.

See ToneOhm:

https://eevblog.com/forum/testgear/toneohm-850a-short-finder-by-polar-instruments-a-look-under-the-hood/



For circuit short tracing the old Squaker was simple and fine:

https://www.edn.com/short-circuit-tracer-finder-uses-low-power/

Jon

[Gyro]

A couple of recent threads that will save time in waiting for more replies...

https://www.eevblog.com/forum/repair/thermal-camera-vs-milliohm-meter-for-short-finding/  (Ignore the thermal camera bit)

https://www.eevblog.com/forum/projects/design-review-ac-milliohm-meter/  (Ignore the AC bit)

[JohanH]

As noted in the circuit, it's important that the voltmeter is isolated.

I made a milliohmmeter based on the Scullcom and Barbouri designs: https://www.barbouri.com/2016/10/09/milliohm-meter-version-1-5/ (Barbouri is a forum member, too).

I had offset readings before I made an isolated power supply for the voltmeter display that I used. Pics at https://johanh.net/mohmmeter/

The meter works nicely. I cannot say how accurate it is, but agrees for the most with 34401A. But it's portable and starts quickly.

[foetusmachine]

Thanks @JohanH,

My design is very similar in principle to the Scullcom/Barbouri design. There's a lot to be said for piecing a lot of application notes together, which is how I started and I see how Scullcom has implemented some aspects. According to the simulations, i've been able to tune the theoretical accuracy to 0.015%, but realities of construction, temperatures will certainly mess that up.

I'm struggling to find a panel meter of low power. I could use the PM328 LCD panel meter which is only 0.5mA, but requires 7-12vdc and isn't isolated. Conversely, the same panel meter used by Scullcom is slightly less accurate and draws closer to 50mA, but is available in an isolated 5V version and is much cheaper.

One of my aims was to make the meter very compact and low power, so i'll be balancing that with the above panel meter selection. I could make my own, but...