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Circuit Design Help - Resistance Tester

I have a little project I need some assistance on. I have some circuit design experience, but not enough to know what I need to make this work.

I have three long wires that I need to test for resistance. If the wires are corroded, connections are bad, etc. that will be indicated as a larger resistance than if the wire is intact. Usually the cut-off is 3 ohms for a bad wire. The wires are under lots of stress, so they develop breaks and the pins on the ends get pulled out a lot.The pins also corrode, causing problems.

I need to set up a circuit that will light one of two LEDs or lights for each wire. A green signal for a circuit resistance of less than 3 ohms, a red signal for more than three ohms. The wires are installed into a system running at 12 VDC about 0.5 amps.

The idea I have so far is a comparison system. If R1 is less than R2, then turn circuit three on. If R1 is more than R2, then turn circuit 4 on. Repeat for the other two circuits.

Ideally, the system would have an on/off switch and a single power source. I plan to use a battery since I intend the system to be portable. I also plan to have at least one other circuit in the same system, so the battery would power that circuit as well and there would be a switch to toggle between test circuit A and test circuit B.

Any help would be greatly appreciated. I would like to be able to fit the completed system in a small project box, probably 4 inches by 3 inches or thereabout. I plan to get supplies wherever I can get them, so the more simple the components are the better.

I can solder just about anything, though I haven't tried surface mount yet.

Thank you.

Reply to
bhsafe
Loading thread data ...

--
Will you have access to the 12V, will the wires be powered up when
you test them, and how do you plan to connect to them?
Reply to
John Fields

--- The end result of the project will be a stand-alone test device. It will ideally be a small box with an internal power supply. The wires to be tested will be plugged into sockets on the face of the box. This connection method allows the pin-wire connection to be stress tested (wiggled) and also tested for corrosion damage.

---

---

I haven't been able to figure out how to design the resistance comparison circuit. I keep thinking that an if-then statement would work, but that goes to computer logic programming and I don't know how to relate that to simple circuit design. The easiest way would be to have three cheap digital multi-meters mounted in the box and have the user do the check. The LCD readouts increase the size of the system and you almost need three separate power sources, one for each meter. While the non-relevant functions could be removed from the multi-meter, I would prefer to have a simple visual indication of the status of the wire.

Reply to
bhsafe 

--
If you can get ahold of both ends of the wires and there\'s nothing
else connected to them this should work:

View in Courier.

. +6V>----+----------+-------+-------+-----+----------+
.         |          |       |       |     |          |
.       [118]      [1180] +--|---+ [10k] [1800]     [1800]
.         |1%        |1%  |  |   |   |     |          |
.         |          |    |  |  [1M] | [HLMP4740] [HLMP4700]
.  +-->>--+--[1000]--|----+-|+\\  |   |     |K         |K
.  |      |          |      |  >-+---|-----+--|+\\     |
.[WIRE] [1µF]        +------|-/LM393 |        |  >----+
.  |      |          |       |  U1A  +--------|-/U1B
.  +-->>--+       [30.1]     |       |       
.         |          |1%     |     [3K3]      
.         |          |       |       | 
.  GND>---+----------+-------+-------+         

The current through the wire and the 118 ohm resistor will be around
50mA, so I\'d use 4 AA alkaline cells in series for the battery.

I\'ve shown only a single channel, so for 3 with all the wires
plugged in and a capacity of 2450 mAH for the battery it\'ll last for
about 17 hours. 

Interestingly, the circuit is ratiometric so changes in battery
voltage will have very little effect on detecting the resistance of
the wire.
Reply to
John Fields

snipped-for-privacy@gmail.com wrote in news:1193675999.079627.252000@

57g2000hsv.googlegroups.com:

How about incandescent light bulb, or three?

Or a low ohm resistor as:

Power: o------/\\/\\/\\/\\/------x----o Test points o-----> Gnd

Piont x goes into a voltage comparator. A good wire will hold point x low, and bad wire will let it float to your input power, or at least a threshold.

Reply to
Gary Tait

John,

Thank you for your assistance. I am having a little difficulty with some of the components.

I found the LM393 unit. The HLMP4740 and HMLP4700 are green and red LEDs. Are there any specific properties that these have? I can't seem to get those parts specifically, they are obsolete according to my suppliers (Mouser and DigiKey). Also, I don't know what the 3K3 unit is. The closest I have been able to come is a power resistor. What is this component?

Thank you,

Ben

Reply to
bhsafe 

--
They\'re red and green LEDs with forward voltages around 2V at
forward currents of 2mA.
Reply to
John Fields

John,

One last question and I think I have this down.

I have three wires to test. Is there going to be a problem if I test all three at once using three circuits like the one you designed, with a common ground and DC rail? Or will I need to add a switch to activate the test circuits one by one? The wire I have to test is part of a three strand cable with a three-prong plug at both ends. Since I have lots of these to test, I wanted to be able to just both ends of the cable into the tester and have the indicators for each circuit light red or green. If that won't work with your circuit, I can add a three position switch, but I would prefer the device to be as simple as possible in operation.

Thank you.

Ben

Reply to
bhsafe 

--
No.
Reply to
John Fields

John,

The wires are about 4 feet long and are 16 or 18 gauge twisted copper with a plastic or rubberized jacket. Think cheap speaker wire. There are about 15 different types of wire used, but they are all common in that they use three insulated strands with the insulation joined to make a single cable.

There are two main types of cable to test. The first is three wires running parallel. The second is two wires running parallel with the third separated. This third wire is about 1/2 the length of the other two and has a large alligator clip soldered to the other end of it.

The cables are body wires for Sport fencing, the first set are for epee the second for foil.

There are several units made that test the wires, but they are all highly expensive and a pain to use. So I am hoping that I can use this setup to test the wires I repair for my club quickly and easily. The tester will also be easier to take along to tournaments and the like.

Thanks,

Ben

Reply to
bhsafe 

--
Indeed!

Maybe you could even undersell and outsell the highly expensive
competitive product and make a fortune!

If you do, I wouldn\'t be at all disappointed if you decided to send
a little of it my way! :-)
Reply to
John Fields

You're not looking for a wire resistance tester, you're looking for a contact resistance measurement, there is a difference. The contact resistance tester will have limitations on open-circuit voltage and current, and there may be transient testing, it can get complicated. You should stick with the factory supplied tester, unless you know better, before you make a fool of yourself.

Reply to
Fred Bloggs

--- OP built it and reported, via email, problems with the LEDs.

After looking the circuit over, it turns out I had U1B's inputs wired backwards, duhhhh...

After rewiring it:

. +6V>----+----------+-------+-------+-----+----------+ . | | | | | | . [118] [1180] +--|---+ [10k] [1800] [1800] . |1% |1% | | | | | | . | | | | [1M] | [HLMP4740] [HLMP4700] . +-->>--+--[1000]--|----+-|+\\ | | |K |K . | | | | >-+---|-----+--|-\\ | .[WIRE] [1µF] +------|-/LM393 | | >----+ . | | | | U1A +--------|+/U1B . +-->>--+ [30.1] | | . | |1% | [3K3] . | | | | . GND>---+----------+-------+-------+

the OP reported that it worked perfectly. :-)

-- JF

Reply to
John Fields

It works perfectly, until the OP accidently feeds the 6V, which you are using for measuring, into some low-voltage logic input, e.g. some expensive and unprotected IC with 2.5V input.

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
Reply to
Frank Buss 

--
Well, Frank, now that you\'ve unknowingly and awkwardly skewered
yourself by not having read the earlier parts in the thread before
casting aspersions, I guess it\'s too late for you to save face, eh?

Clue: The [WIRE] and the connectors on its ends are the devices
under test, LOL. :-)
Reply to
John Fields

I have read the rest of the thread and your schematic is perfect for testing sport fencing wires, but it is very easy to build a safe tester, which can be used without fear to damage electronics. There was an article in de.sci.electronics some time ago and it is possible to measure with as low as 2mV instead your 6V at 118 ohm:

formatting link

The low testing voltage has the advantage that you can use it for detecting pcb track shorts, even on populated pcbs without damaging the parts.

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
Reply to
Frank Buss 

--
Who cares?

The OP asked for a device to test sport fencing wires, and that\'s
what he got.
Reply to
John Fields

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