Toneohm 850A short finder by Polar Instruments - a look under the hood.

[harrimansat]

I will upload the 850A schematics as they may show the difference in the probe input circuits.

Fraser

Can  you please upload schematics?
Thanks!
Regards

[kripton2035]

here it is

[kripton2035]

In terms of a cheap implementation of using a milli ohm meter just for comparison purposes of traces.
This is an excellent implementation, with the addition of a small LED bar graph display and interpreting the ADC values this can be easily be adapter to some use.
https://youtu.be/EYaIrRViKq0

I just made a better one adding a display and a 4-wires probe to it. it now displays milliohms, and seems precise to half a milliohm. perfect when you dont want to wake up people in the next room ... with it I was able to detect a shorted capacitor among a bunch of parralel ones in a motherboard. nice. 2mΩ differences only. not detectable with sound only.

[kripton2035]

some pictures of the device

[harrimansat]

Thanks!

I like it, but for my experience two wires system to measure low impedances dont work very well, you need to make zero often. Are you using a constant curret? I prefer to inject current and use a microvolt meter, 4 wires.

[kripton2035]

It is a shorty device, I used the same schematic. I only added an oled display, and 4 wires probes.
I can't make a zero for the probes as the device detects them as zero.
I will try to add a better ADC than the 10 bits of the arduino, and then may be able to detect the probes resistance.

[kripton2035]

On the subject of low voltage ESR meters... since they need milliohm resolution to be usable for characterizing capacitor ESR they just happen to be good for finding low resistance shorts as well. A basic demonstration on how this works is in the below video featuring another cheap ESR kitset meter the ELV ESR1.

in fact I think they are not that good for short circuits. a DC milliohm meter is better. if you search for a short and there are capacitors between, it show as a short and it is not the short you're searching for.

[Shock]

Meh in circuit component testing (V-I Curve Tracer) for me none of this DCR rubbish.

[kripton2035]

just found another shorted ceramic capacitor in an apple time capsule. 2mΩ difference with the other caps . so easy now !

[Shock]

Does it do anything special in software? What's the max measurement?

[kripton2035]

I can read 0.6Ω but cannot read 0.7Ω, and down to 0.4mΩ (don't know the accuracy ! but it's not a problem I only need some scale for relative measurement)
it is only the reading of the arduino ADC, and the display with some scale. the shorty amplifier gives some 13mV/mΩ at the arduino adc input.
I will definitely try another better adc like a mcp3421 with 16 bits to see if there is a better possibility !


I also added a change in the audio frequency response : I used a decreasing exponential response, 0 Ω = 4khz, 1Ω = 10hz
that way you can better hear the small resistances values.

[harrimansat]

just found another shorted ceramic capacitor in an apple time capsule. 2mΩ difference with the other caps . so easy now !

With your meter? Is an ESR meter? I thougth that it was a DC ohms meter.

How it works?

[kripton2035]

it is a DC milli ohm meter. with an esr meter you search for caps with high esr resistance value, all caps must have low esr, so if you search for short it's not possible.
with a DC milli ohm meter, I can find a shorted cap with others in parallel like on computer motherboards with 2mΩ difference.

now I have added a mcp3421 adc, and I can have a stable measurment down to 0.1mΩ.
I changed the amplification of the shorty op amp from 200 to 27, so that I can measure from 0.1mΩ to 4Ω (5 digits resolution)
I also changed the opamp from mcp6041 to ad8628, I had too much offset voltage with the first one impossible to zero the probes.


original shorty device : http://kripton2035.free.fr/Continuity%20Meters/continuity-short.html

[harrimansat]

thanks,

I know, what I don´t  understand is why you said "2mohms difference with the other caps", one cap is short and is resitive, but the others?

[kripton2035]

you have a group of 20 tantalum smd caps in parallel at the output of a power rail. almost in every actual computer.
one of the cap is dead short and prevents the power rail from working.
you have to find it without removing all the caps one by one until you find the bad one.
the faulty caps I have found are around 40mΩ. if you measure any of the cap with an ohmmeter you get 0.01Ω of resolution at best, and given that 1mm of copper is less than 1mΩ you can't find the bad capacitor.
with my device, I get 2mΩ more reading on the cap next to the faulty one, and around 10mΩ more on the caps that are some cm away.
so I only have to measure each group of caps, find the smallest ohm value, then find the smallest value inside that group and it's done.

[harrimansat]

Thanks for your explanation!

The last board that I fix with shorted capacitors, had ten tatanilum in parallel next to CPU, apple mac pro, one of them very shorted, with 2 Amp it did not heat. With two probe needels connected to a differential amplifier, I find out that I can measure which capacitor is conducting meassuring the drop voltage in smd cap lead  lenght, aprox 1mm., injecting 2A DC in DC rail

In one photo you can see the drop voltage, moving de needle up and down through capacitor lead.

300uV at 2A, 0.15 milliohms at capacitor lead???


I used a tektronix 7000 series diferential amplifier, 7A22

[kripton2035]

https://www.youtube.com/watch?v=4F6QNSpWTos


small video I just made with my device in action. more to come.

[kripton2035]

another video showing the resistance of a soldier roller

https://youtu.be/8mYyr9uKpFU

[harrimansat]

Thanks, I want to make one!, do you have schematics and firmware?

The only thing that I don´t like is that you need to hold tight against the circuit to measure well low resistors.

[kripton2035]

I just started a new thread for it here : https://www.eevblog.com/forum/testgear/finding-short-on-motherboards-with-a-shorty-(with-display)/new/#new

with a new video showing how to find shorted capacitor

[kripton2035]

Thanks, I want to make one!, do you have schematics and firmware?
The only thing that I don´t like is that you need to hold tight against the circuit to measure well low resistors.

will share this when they are cleaned.
yes if you want stable reading, you have to be quite strong in holding the probes, you could use some kelvin clips and be lighter with it.

[BobBeBob]

WOW, this is wonderful. thank you for all that you have done to share the knowledge.
May I ask you the resister in the current probe is connected to the shield wire? and there are only 2 wires in there, correct? how far from the tip of the current probe would you say is the Inductor placed?
so below is my story with the toneohm.
I purchased a toneohm 950 about 8 months ago. Polar here in the US wants over $700 for probes of all 3 setups.
that's about how much I spend on the 950. I was not going to spend that much more on Probes only.
I had seen on ebay, pics of items (ebay-claric239) was selling, a toneohm 950 in the background, which was not for sale. so I massaged him to get info on its probes. This guy went all out, sent pictures and measured them for me. if you are looking for electronics check him out on ebay.

I think 850 and 950 current probes are the same but the 850 is the current probe by itself and the 950 has the drive source with it on the same jack.
ebay-claric239, did say it is an Inductor measured 220 uH and 10ohms resistance at the DIN pins. and your pictures explains it.
   
I started making my probes last week. Needle probes was really easy and it works really good.
the current tracer is work in progress. I have gone through a couple so far. on my last one I made an inductor in house with really tiny wire .004 inch, I didn't know there is a resister in lines. so, I wound enough wire to give me 10 ohm, about 12 feet. it works and is very directional. but I can use a smaller one, lol.  the next version will be smaller. got to order an inductor and a resister.   
the 950 also has the drive source voltage variable up to 550mvdc. it has the needle probes, current probes and the Planer probes. 
the needle probes: voltage, current and resistance can be traced.
with the non contact current tracer: well it says it.
I don't think the Planer function on the toneohm 950 is going to of much use to me, for now.

thank you for the info and pics.

[fcb]

Got a mint 950 here with all the probes, etc.. shout/PM if you need any technical info.

[BobBeBob]

thank you very much. I wonder how the trace and the planer probes are wired at the DIN and inside the probe. Attached is what I been able to figure out on the machine side so far.

[fcb]

Conectors are drawn from the backside (solderside) of the cable mounted connector.
Hope this helps.