Low-ohm meter circuit 
Author Message
 Low-ohm meter circuit

To anyone who can help:

A while back -- I would say roughly 10 years ago -- one of the test
technicians on our industrial systems test floor built a small circuit that
would help him find the exact location of a short-circuit on a PC board (We
used to have a lot of problems with inner-layer copper shorts and poor
etching from our board vendor).  Basically, he put the circuit in a small
case, the outputs of this circuit would be connected via banana jacks to the
+ and - input of any digital multimeter set to the volt scale.  The input
would be two EZ hook leads that would be used to find the short-circuit.
The circuit would cause the meter to read close to 5 or so volts with a
highly resistive or open circuit and in the 1mV range when there was a
short-circuit.  Any reading in between 1mV and 5V would tell you how far
away or close to the short-circuit you were.

Somewhere in the circuit there was a constant current source, a sensitive
comparator and I believe some sort of Kelvin bridge, but I am not sure.
Does anyone know of a circuit that would accomplish this task, or has anyone
seen one of these?

Gerardo Rafiki



Mon, 05 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit



Quote:
>Somewhere in the circuit there was a constant current source, a sensitive
>comparator and I believe some sort of Kelvin bridge, but I am not sure.
>Does anyone know of a circuit that would accomplish this task, or has anyone
>seen one of these?

The "Siemens" or diode checker functions of newer Fluke handhelds are
essentially what you describe. Constant current, measure voltage.

I never tried them for PCB work though.

-> From the USA. The only socialist country that refuses to admit it. <-



Mon, 05 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit

Quote:

> To anyone who can help:

> A while back -- I would say roughly 10 years ago -- one of the test
> technicians on our industrial systems test floor built a small circuit that
> would help him find the exact location of a short-circuit on a PC board (We
> used to have a lot of problems with inner-layer copper shorts and poor
> etching from our board vendor).
[snip]
> Somewhere in the circuit there was a constant current source, a sensitive
> comparator and I believe some sort of Kelvin bridge, but I am not sure.
> Does anyone know of a circuit that would accomplish this task, or has anyone
> seen one of these?
> Gerardo Rafiki


I've seen a similar project in one of the electronics magazines in the
past few years.  The circuit used a PNP transistor to make a constant
current source.  Then there was a dual opamp which was an LM1458, to
amplify the DC voltage by 100 times.  And there was the other half of
the opamp used as a comparator with a pot used as a voltage divider to
find the resistance.  It used just a regular LED to tell when the
comparator toggled.

I built this circuit with the opamp that they called for, an LM1458.
This opamp will not work with either of the inputs at the negative rail,
which is what the circuit has them at.  So it failed to work, period,
not because of anything that I did wrong, but because the author or the

opamp that worked with the inputs at the negative rail, and that fixed
the problem.  It worked great, but by that time, I had spent so much
time scratching my head to figure out what was wrong, that I was going
bald!  So, contrary to the article, do NOT use the LM1458 as shown, use
a LM358.

And NEVER trust a magazine article to give you adequate and/or correct
information to complete a project!

--
Please remove NO and SPAM from my email addr to reply.



Mon, 05 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit

altavoz
You would have to know exactly the resistance of that winding,
undulating trace . You can get a very rough guesstimate .
  When measuring traces , you must use high enough current
to get a valid reading . Millamps are not high enough .
 I would guess a simple milliohm meter that has adjustble
current steps . But a milliohm meter needs to have it's
leads reversed and the readings averaged to get accurate results.
 An accurate current source (CS) is not required . Use the ICL7106
( Hosfelt elect' 1-800-524-6464 , $8 Digital Panel Meter ).
 Run a 1 amp current thru a 0.1 ohm R , and put this R across
the "ref" inputs of the 7106 . The minus input ( DPM) is hooked
to the minus REF and the pos input goes to your PCB resistor .
  The complete circuit is minus CS to PCB , pos CS
to pos REF , thru the 0.1 R to minus REF ( minus input also
hooked here ) to the + test lead ( minus test lead is minus CS
 If the CS goes up or dn , the voltage across all the resistors
goes up/dn , so CS has no effect on accuracy .

-------------------------------------------------------------

Gerardo Rafiki
A while back -- I would say roughly 10 years ago -- one of the test
technicians on our industrial systems test floor built a small circuit
that
would help him find the exact location of a short-circuit on a PC board
(We
used to have a lot of problems with inner-layer copper shorts and poor
etching from our board vendor).  Basically, he put the circuit in a
small
case, the outputs of this circuit would be connected via banana jacks to
the
+ and - input of any digital multimeter set to the volt scale.  The
input
would be two EZ hook leads that would be used to find the short-circuit.
The circuit would cause the meter to read close to 5 or so volts with a
highly resistive or open circuit and in the 1mV range when there was a
short-circuit.  Any reading in between 1mV and 5V would tell you how far
away or close to the short-circuit you were.

Somewhere in the circuit there was a constant current source, a
sensitive
comparator and I believe some sort of Kelvin bridge, but I am not sure.
Does anyone know of a circuit that would accomplish this task, or has
anyone
seen one of these?

Gerardo Rafiki



Mon, 05 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit

Quote:


> > To anyone who can help:
> > A while back -- I would say roughly 10 years ago -- one of the test
> > technicians on our industrial systems test floor built a small circuit that
> > would help him find the exact location of a short-circuit on a PC board (We
> > used to have a lot of problems with inner-layer copper shorts and poor
> > etching from our board vendor).
> [snip]

[...]

Quote:
> And NEVER trust a magazine article to give you adequate and/or correct
> information to complete a project!

There is a nice short circuit locater in:

  "Beeper Finds Circuit Shorts"

  JIM WOOD,  Inovonics  Inc., 1305 Fair Ave.,  Santa  Cruz,  CA 95060;

  Voted Best of Issue, Electronic Design, April 3, 1995

The complete article is in 18A.PDF.

I downloaded it - but I can't find it on the EDN site anymore. None of
EDN's search engines will produce a hit on anything to do with this
article.

If anyone takes a look and finds it, please post how you did it!

Best Regards,

Mike
CEO, Analog & Digital Design
Automated Production Test
  http://www.csolve.net/~add/home.htm

Hosting Jonathan Ramsey's Pascal TCP/IP for DOS:
  http://www.csolve.net/~add/zips/tcp.htm



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit

[...]

Quote:
> There is a nice short circuit locater in:

>   "Beeper Finds Circuit Shorts"

>   JIM WOOD,  Inovonics  Inc., 1305 Fair Ave.,  Santa  Cruz,  CA 95060;

>   Voted Best of Issue, Electronic Design, April 3, 1995

> The complete article is in 18A.PDF.

> I downloaded it - but I can't find it on the EDN site anymore. None of
> EDN's search engines will produce a hit on anything to do with this
> article.

> If anyone takes a look and finds it, please post how you did it!

[...]

OK, I will. First, you got to look in the right place - it was not EDN,
it's Electronic Design OnLine. (Tho I did get a lot of good stuff
browsing through the back issues of EDN!)

Here's the URL's:

  Electronic Design OnLine
  http://www.penton.com/ed/resource/index2.htm

  Design Ideas (some articles relate to recent S.E.D threads)
  http://www.penton.com/ed/resource/circuit/

  Short Circuit Locator
  http://www.penton.com/ed/resource/circuit/18a.pdf

Best Regards,

Mike
CEO, Analog & Digital Design
Automated Production Test
  http://www.csolve.net/~add/home.htm

Hosting Jonathan Ramsey's Pascal TCP/IP for DOS:
  http://www.csolve.net/~add/zips/tcp.htm



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit


 :>
 :> To anyone who can help:
 :>
 :> A while back -- I would say roughly 10 years ago -- one of the test
 :> technicians on our industrial systems test floor built a small circuit that
 :> would help him find the exact location of a short-circuit on a PC board (We
 :> used to have a lot of problems with inner-layer copper shorts and poor
 :> etching from our board vendor).
 :[snip]
 :> Somewhere in the circuit there was a constant current source, a sensitive
 :> comparator and I believe some sort of Kelvin bridge, but I am not sure.
 :> Does anyone know of a circuit that would accomplish this task, or has anyone
 :> seen one of these?
 :
 :> Gerardo Rafiki

 :
 :I've seen a similar project in one of the electronics magazines in the
 :past few years.  The circuit used a PNP transistor to make a constant
 :current source.  Then there was a dual opamp which was an LM1458, to
 :amplify the DC voltage by 100 times.  And there was the other half of
 :the opamp used as a comparator with a pot used as a voltage divider to
 :find the resistance.  It used just a regular LED to tell when the
 :comparator toggled.
 :

I fancy doing a bit of ascii art, so here's one I built at home;

                 AMP-BOX FOR LOW-OHM MEASUREMENT (1 ohm max)

      (use pot to set current to 1 Amp through unknown resistor,
       then measure voltage across res, read volts as ohms)

         __________________________________________         _____
         |         |                               |--------|   |
         |          |- pushbutton                 res       |+  |
         |         |  (good quality)             under      |VOM|
         |        res                             test      |-  |
         |         33             _________________|--------|___|
         |         |              |          |
         +         |____          |          |
     2 D cells     |    |         |         gnd
     alkaline      V   pot        c
         -         |   200<-----b   2N3055
         |  3 recs V   w/w        e  (collector grounded to case)
         |  1N4002 |----          |
         |         V             res
         |         |            0.5 2W
         |_________|______________|

And a merry Christmas to one and all!

Walter

Disclaimer: My employer is not responsible for this stuff.



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit



Quote:
> To anyone who can help:

> A while back -- I would say roughly 10 years ago -- one of the test
> technicians on our industrial systems test floor built a small circuit
that
> would help him find the exact location of a short-circuit on a PC board
...    <snip>   ...
> Does anyone know of a circuit that would accomplish this task, or has
anyone
> seen one of these?

Yes I saw something similar.
At about the same time I saw a different instrument in operation for the
same task, but as far as the circuit configuration is concerned I am only
guessing:
So, the instrument injects a regulated (or just limited) 1 kHz current
(square wave, I guess in the order of 500 mA.)
On the tip of the probe is a magnetic head (I guess with a very wide gap)
Now, as the probe is moved along the track, the 1 kHz can be heard, but it
weakens if the probe is not close to, or not parallel with the track where
the current flows. This way the connection can be tracked, and the S/C
located, even if it is below an IC.

An other idea, closer to the one you saw:
The injected current should be AC (square wave), so that the DC offset of
the probe amplifier does not mess up the measured results. A synchronous
rectifier or a phase detector could help identify the direction of the
current flow.

I hope this helps

Bob



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit


Quote:

> A while back -- I would say roughly 10 years ago -- one of the test
> technicians on our industrial systems test floor built a small circuit that
> would help him find the exact location of a short-circuit on a PC board (We
> used to have a lot of problems with inner-layer copper shorts and poor
> etching from our board vendor).  Basically, he put the circuit in a small
> case, the outputs of this circuit would be connected via banana jacks to the
> + and - input of any digital multimeter set to the volt scale.  The input
> would be two EZ hook leads that would be used to find the short-circuit.
> The circuit would cause the meter to read close to 5 or so volts with a
> highly resistive or open circuit and in the 1mV range when there was a
> short-circuit.  Any reading in between 1mV and 5V would tell you how far
> away or close to the short-circuit you were.

> Somewhere in the circuit there was a constant current source, ...

 I designed a super-simple circuit to perform the short-circuit tracing
 function a few years back, and it proved so popular several others have
 copied it.  It's basically a 0.35A current source, with a 0.35V clamp
 to prevent damage to any semiconductors, ICs, etc in the circuit.  It
 does require a small external floating 5V supply.

      +5 ---- 10 ohms ---- E   C --------+-o OUT
              5W  ww         B    1N5822 |
                      TIP125 |  ,---|<---'
      5V return -------------+--+----------o RTN

 An ordinary 3.5-digit multimeter can measure 0.1mV on its 200mV scale,
 and is easily able to see the increasing voltage along a trace as you
 move the probe, since 0.1mV/0.35A = 0.00029 ohms.  It's easier to use
 if the test probe has a very sharp point.

 Those who are in a hurry can eliminate the TIP125 transistor, and just
 use the 10-ohm wire-wound resistor and the schottky diode.

--

Rowland Institute for Science
Cambridge, MA 02142



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit


Fri, 19 Jun 1992 00:00:00 GMT
 Low-ohm meter circuit

I'm looking for a schematic for a switch to activate a tape recorder as
soon as the reciever is picked up and stop the tape as soon as the
reciever is returned. The switching voltage should be around 12 volts.
attached is what I have tried with no luck. Can someone help me??

  switch.bmp
< 1K Download


Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit



|I'm looking for a schematic for a switch to activate a tape recorder as
|soon as the reciever is picked up and stop the tape as soon as the
|reciever is returned. The switching voltage should be around 12 volts.
|attached is what I have tried with no luck. Can someone help me??

You're going to need some power, try a 9V battery.  You should be
sensing when the line voltage is less than about 15V, thus your
polarity of line voltage and "record" is reversed.  Also, the phone
line is polarized...you can either reverse your leads if it doesn't
work or use a diode bridge to sense the MAGNITUDE of the line voltage.

                                        ...Jim Thompson

|  James E.Thompson, P.E.                           |        mens         |
|  Analog Innovations, Inc.                         |         et          |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |        manus        |
|  Phoenix, Arizona           Voice: (602)460-2350  |      Brass Rat      |

 HEY, LOOK!  IT'S NOT MY FAULT; IT'S SOME GUY NAMED "GENERAL PROTECTION".
     (Ratbert, the Consultant, speaks, in Dilbert, by Scott Adams)

     For proper E-mail replys remove "numeric" from E-mail address.



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit


Fri, 19 Jun 1992 00:00:00 GMT
 Low-ohm meter circuit

Quote:

> altavoz
[snip]
>   When measuring traces , you must use high enough current
> to get a valid reading . Millamps are not high enough .

That's not true at all.  The gain of a simple opamp will multiply the
voltage drop across a low resistance enough to make a few tens of
milliamps give a perfectly adequate reading.  The current can be AC, at
some frequency that is easily amplified and filtered and then rectified
with an opamp'd rectifier.  Using AC makes the signal insensitive to any
DC that might be flowing in the circuit.

[snip]

Quote:
> Gerardo Rafiki


--
Please remove NO and SPAM from my email addr to reply.



Tue, 06 Jun 2000 03:00:00 GMT
 Low-ohm meter circuit

Quote:



>  :>
>  :> To anyone who can help:
>  :>
>  :> A while back -- I would say roughly 10 years ago -- one of the test
>  :> technicians on our industrial systems test floor built a small circuit that
>  :> would help him find the exact location of a short-circuit on a PC board (We
>  :> used to have a lot of problems with inner-layer copper shorts and poor
>  :> etching from our board vendor).
>  :[snip]
>  :> Somewhere in the circuit there was a constant current source, a sensitive
>  :> comparator and I believe some sort of Kelvin bridge, but I am not sure.
>  :> Does anyone know of a circuit that would accomplish this task, or has anyone
>  :> seen one of these?
>  :
>  :> Gerardo Rafiki

>  :

[snip]

- Show quoted text -

Quote:
> I fancy doing a bit of ascii art, so here's one I built at home;
>                  AMP-BOX FOR LOW-OHM MEASUREMENT (1 ohm max)
>       (use pot to set current to 1 Amp through unknown resistor,
>        then measure voltage across res, read volts as ohms)

>          __________________________________________         _____
>          |         |                               |--------|   |
>          |          |- pushbutton                 res       |+  |
>          |         |  (good quality)             under      |VOM|
>          |        res                             test      |-  |
>          |         33             _________________|--------|___|
>          |         |              |          |
>          +         |____          |          |
>      2 D cells     |    |         |         gnd
>      alkaline      V   pot        c
>          -         |   200<-----b   2N3055
>          |  3 recs V   w/w        e  (collector grounded to case)
>          |  1N4002 |----          |
>          |         V             res
>          |         |            0.5 2W
>          |_________|______________|

Geez, Louise, you used a 115W power transistor, and a pair of D cells
that will get drained in no time flat!  Holy Smokes!

I can't see using this circuit when it might put enough current thru the
PC board to do some serious damage to the board and/or components!

The way it should be done is to use a voltage that is **NEVER** above
0.6 volts, so that any PN junction, such as a diode or transistor
junction, will ever be forward biased enough to conduct.  So that way,
the transistors have no effect on the readings.  Yeh, right!

So use a small current, and low voltage, and amplify the result with an
opamp.  Nothing should have to be more than a few tens of milliamps.

Quote:
> And a merry Christmas to one and all!

& Happy Holidays.

Quote:
> Walter

--
Please remove NO and SPAM from my email addr to reply.



Tue, 06 Jun 2000 03:00:00 GMT
 
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