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Electrician looking for advice on resistance measurer

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sambam

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Hi all

Sorry to be such a complete beginner, I am from an electrical installation background, not electronics. I understand some fundamentals, but would rank as a complete beginner next to you guys.

I wanted to know if there is a kit, or an easy method, of creating a small resistance measuring device with an LCD output (or some other form of easy reading)? I would be looking at typical circuit measurements so from 0.01Ω upto say (a rather arbitrary sorry) 50Ω.

Basically it needs to be small and cheap (college budgets :rolleyes:) so I could either reuse it or the kit is cheap enough to buy each year for the students. I want them to create an electronics project, but I would rather it was a pertinent one rather than a silly one, you can imagine that resistance is rather important to future electricians and the safety of everyone else!

Thanks in advance

Neil
 

tcmtech

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You are looking for a milliohm meter. Its the opposite of a mega ohm meter.

Typical digital multi meters are not so accurate at low ohms or high ohms and usually have 200 ohms as their lowest Resistance range and 20 megohms for their top range.

Megohm meters go up from the top end of the standard mulltimeters and can go well into the thousands of megohms ranges.

What you want is the far less common Milli ohm meter that does the lowest end resistance measurements accurately. I dont recall where I saw a schematic for one but it was based on a simple fixed current source and a simple analog to digital converter IC like the ICL7106CPL digital meter IC.

You can get these IC's an eBay as a complete LED or LCD meter fully assembled for around $10 including shipping. With a little circuit hacking on the voltage divider section and some additional parts a basic but accurate Milli ohm meter could be made for around $30 or so.
 
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Nigel Goodwin

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Basically it needs to be small and cheap (college budgets :rolleyes:) so I could either reuse it or the kit is cheap enough to buy each year for the students. I want them to create an electronics project, but I would rather it was a pertinent one rather than a silly one, you can imagine that resistance is rather important to future electricians and the safety of everyone else!
Unfortunately such devices are neither small, light, or cheap - in order to check low resistances for electrical safety you need to send a high current down it. This is how PAT testers work (for the earth continuity test), a low current wouldn't be a sufficient test of safety.
 

tcmtech

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The simple circuit I saw used something like 1 amp for the base reference current. So I think it would be a battery hog if it was portable. A standard multi meter uses around 2 - 3 milliamps or so.

I did get to see a micro ohm meter in use once. It used a 50 amp current and had to be plugged into work. But it could accurately read down to around 1/100,000 of an ohm when calibrated right.
 

sambam

New Member
Unfortunately such devices are neither small, light, or cheap - in order to check low resistances for electrical safety you need to send a high current down it. This is how PAT testers work (for the earth continuity test), a low current wouldn't be a sufficient test of safety.
I am really looking at something that will measure R1+R2 (resistance of Line and CPC). Real world testers, such as the Megger range (like this Megger MIT320-EN Insulation and Continuity Tester 500V Megger MIT-320 MIT 320 EN MIT320EN) are considered more than acceptable for doing so on standard circuits.

The Wiring Regulations states an accuracy of 5% for digital readouts, between 4v and 42v with a short circuit current of not less than 200mA. The figures quoted by the Regs and manufacturers are always erring on the side of caution.
 

Willbe

New Member
I wanted to know if there is a kit, or an easy method, of creating a small resistance measuring device with an LCD output (or some other form of easy reading)? I would be looking at typical circuit measurements so from 0.01Ω upto say (a rather arbitrary sorry) 50Ω.
I use a 100 ohm 100w rheostat. With 120v feeding it you've got a ~1.2A current source. Reading 20mV across a load with 1.2A gives you 17 milliohms.

Or, use a hair dryer (10A).

With a car battery and an opamp with a voltage gain of 10x or 100x you can do even more.

Also, one of these Weller soldering guns probably puts out 150A at a very low AC voltage.

You need to use a four terminal Kelvin method; the leads pushing the current into the load must not touch the leads you use to measure voltage.
 
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tcmtech

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With a fixed current output source like a LM317 or one of its lower drop out cousins working as an accurate 1 amp constant current source making measurements down to .0001 ohms with a simple digital meter IC would not be hard at all.

I have often thought about building one for many years. It was not until this last year or so I started finding the LED and LCD based digital meters for under $10 that its made it seem like a cost justifiable project.

With a simple calibration resistance its not hard to get reasonable and repeatable measurements. For typical electrical wiring I doubt Resistance measure mets under .001 ohms or over 200 ohms would be needed.

At one amp .01 ohms would still give you a 10 millivolt reading and thats well within the 0.0 to 200.0 Mv range of a ICL7106CPL digital meter IC.
I have used them and modified them before with great success. They also have a gain control circuit that will give them a full range reading at 50 mv direct input for use with common 50 Mv current shunts.

If you had full scale readings at 50 mv using a one amp current source you would have an available resolution down to .000025 ohms respectively with that IC. Given its a 3.5 digit output there is still a .0001 reading resolution limit though. However with a 10x mode it would still make .00001 readings theoretically possible.

Even with a 5% deviation accuracy measuring that low is more than enough for anyone under typical use applications. ;)

Just something to think about. And like I said earlier it could be built for around $30. I had lab fees far greater than that in my college lab classes and typically got nothing to take home with me when the semester was over. :mad:
 

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Mr RB

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I've got some digital low-ESR meters that was a kit from Silicon Chip magazine, I built them in about 2000 when I owned a few TV repair shops or it might have been earlier. They measure low-ohms in either caps or in a pure resistance.

Here's an example;
K-7214 ESR meter kit page

Altronics in australia might still sell the kits. Since I haven't done TV repairs in a lot of years mine only get used sometimes now for low ohms measuring of resistors and wires.

Yep, Altronics still sell the kit;
http://www.altronics.com.au/index.asp?area=item&id=K2574
about $70 USD.

One thing I like about it is it measures ohms and ESR with a voltage less than 0.3v, so you can measure low resistors and caps ESR in circuit, because it will totally ignore any diode or semiconductor junctions.
 
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MikeMl

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As an electrican, you probably own a decent Digital Multimeter which has a milliVolt scale. Why not just use a 1A precision current source (LM317 and 1.25 Ohm resistor) powered by a small 12V Sealed Lead Acid re-chargable battery to force a current though the circuit under test, and then just read the voltage drop in mV. R=E/I, since I=1A, then the arithmetic is trivial...

I use this method in finding corroded/loose connections in aircraft DC power distribution systems...
 
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sambam

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With a fixed current output source like a LM317 or one of its lower drop out cousins working as an accurate 1 amp constant current source making measurements down to .0001 ohms with a simple digital meter IC would not be hard at all....<snip>

Even with a 5% deviation accuracy measuring that low is more than enough for anyone under typical use applications. ;)

Just something to think about. And like I said earlier it could be built for around $30. I had lab fees far greater than that in my college lab classes and typically got nothing to take home with me when the semester was over. :mad:

That looks like an interesting option. Would that be a project easy enough for 16/17 old students to produce? I envisage a small plastic box with a cut out for the digital readout and 2 x 4mm outputs for the test leads and a button that needs to be pressed to get the readout.

It's very much a case of simplicity being key here, the majority of our students will make good electricians but are not always strong with the theory aspects.

When I say that this is a test for electrical safety, it is a test that would measure the fault path, not the actual integrity of the electrical circuit. We have insulation resistance testers for this (commonly set at 500v in the UK) so the check is really just a continuity test with a reading taken to ensure it is low enough to disconnect the protective device under fault conditions.
 

tcmtech

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I dont see why not. With a little supervision I know may 7 year old niece could make one. The meter is already premade. A LM317 set up as a constant current source is only the three pin IC and a resistor. After that its just a sensing resistor for the meter to reference from.
The meter I referred to takes 5 volts so a either a second LM7805 regulator or a pair of diodes could be used to get the drive power for the meter if it was 6 volt powered. I was thinking if it used 4 C or D alkaline batteries it could be portable and self powered too!
Around 10 - 15 total components including the box.
 
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