ultra-low voltage continuity tester?

Thread Starter

Mullins

Joined Dec 31, 2021
128
Hi, sometime I need to test the continuity in the circuit board. I mean during rework. Mostly there are one or more MCU on the control board. My Fluke 87V has a 9v battery. Is there any tester who work with low voltage? Maybe under 0.4v so I can exclude the diode during continuity test. Which voltage is safe tu use on an MCU during a continuity test?
Thank you.
 

Reloadron

Joined Jan 15, 2015
7,219
My Fluke 87V has a 9v battery. Is there any tester who work with low voltage? Maybe under 0.4v so I can exclude the diode during continuity test. Which voltage is safe tu use on an MCU during a continuity test?
It depends on the MCU. When in resistance measuring ranges the Fluke 87 outputs a current to measure the I*R drop. That's how the meter gets resistance measurements. On the lowest resistance range the Fluke 87 meter outputs about 200 uA on the lowest 400 ohm range. Read page 20 of the user manual to understand exactly how the meter works. on resistance ranges. Read page 37 for the current on the 400 ohm range. Learn how to use the relative function for low resistance measurements. The 400 ohm range offers 0.1 ohm resolution. The 9.0 volt PP3 type battery has nothing to do with it. If you have further concerns place an external resistance in series with your probe.

Ron
 

MrSalts

Joined Apr 2, 2020
2,767
It depends on the MCU. When in resistance measuring ranges the Fluke 87 outputs a current to measure the I*R drop. That's how the meter gets resistance measurements. On the lowest resistance range the Fluke 87 meter outputs about 200 uA on the lowest 400 ohm range. Read page 20 of the user manual to understand exactly how the meter works. on resistance ranges. Read page 37 for the current on the 400 ohm range. Learn how to use the relative function for low resistance measurements. The 400 ohm range offers 0.1 ohm resolution. The 9.0 volt PP3 type battery has nothing to do with it. If you have further concerns place an external resistance in series with your probe.

Ron
Don't use the diode mode in the Fluke 87, it will keep raiding the voltage until it sees current flow - meaning well over a standard diode. Mine will light a white LED, so 3volts.
 

crutschow

Joined Mar 14, 2008
32,903
You can use another meter to measure the output voltage at the leads when in the resistance mode to make sure it don't exceed about a half-volt.
 

Thread Starter

Mullins

Joined Dec 31, 2021
128
I'm sorry I have user manual book and CD but I never read it :(
I have to chose range or tester do it by his self? I can chose range but the lowest is only 600 Ohm range.
Pushing "Range" button I saw it's going to measure even nS. Can you tell me some video on you tube how to understand it and how to use. I saw it's going OL except with some liquid or other material. Dielectric spray for electronic is about 2.0nS. Pure isopropilici alcohol is about 50, dry paper 0.3nS.

Thank you all
 

MrChips

Joined Oct 2, 2009
29,228
Hi, sometime I need to test the continuity in the circuit board. I mean during rework. Mostly there are one or more MCU on the control board. My Fluke 87V has a 9v battery. Is there any tester who work with low voltage? Maybe under 0.4v so I can exclude the diode during continuity test. Which voltage is safe tu use on an MCU during a continuity test?
Thank you.
If you need to do this often then why not build your own tester to suit your needs?
 

ericgibbs

Joined Jan 29, 2010
18,021
hi M,
I will try to find the original, if not I will draw up a new circuit.
It is a Continuity tester only, not a Resistance value measurement indicator, will that be sufficient for your application?
E
 

Dave Lowther

Joined Sep 8, 2016
187
Edited to correct "coil cell" to "coin cell".
I've been using this for about 7 years. It fits in a small project box (2.25 x 1.25 x 1 inches), including 4mm banana sockets. Powered by a single coin cell.

1677406088840.png
 
Last edited:

Ya’akov

Joined Jan 27, 2019
8,161
I've been using this for about 7 years. It fits in a small project box (2.25 x 1.25 x 1 inches), including 4mm banana sockets. Powered by a single coil cell.

View attachment 288416
Very nice as usual, @Dave Lowther. I just want to point out that the LM3909 (a chip near and dear to my heart) is no longer produced and can be hard or expensive to source. I bought a few when I spotted them on eBay cheap(ish) at around 2 bucks a pop. Currently, they seem to be ≤$6 anywhere I can find them.
 

ericgibbs

Joined Jan 29, 2010
18,021
Hi,
As I understand, the TS's retirement is for a continuity tester that is less than 0.4V across the probe tips, so that it can be used on a logic or MCU PCB and not turn on or damage any internal circuitry of the logic.

E
 

Dave Lowther

Joined Sep 8, 2016
187
As I understand, the TS's retirement is for a continuity tester that is less than 0.4V across the probe tips, so that it can be used on a logic or MCU PCB and not turn on or damage any internal circuitry of the logic.
TS said " Is there any tester who work with low voltage? Maybe under 0.4v so I can exclude the diode during continuity test. Which voltage is safe tu use on an MCU during a continuity test?"
I posted the one I built because it can distinguish between a diode drop and a short. I've used this on everything I have built, including MCU PCBs with components fitted and I've (AFAIK) never damaged anything. At the time I built it I wanted something small, simple and low voltage. I couldn't easily find or design something else to easily meet my requirements. About 45 years ago when I started my first job after uni at British Aerospace, the first task I was given was to build myself a continity tester. IIRC That circuit ran off one AA cell and used a single transistor 'upside down' with a few passive components. I couldn't find it on the internet when I was looking for one to build 7 years ago. That was the time when I decided to get back into building electronic projects after working for > 30 years in embedded s/w.
 

Thread Starter

Mullins

Joined Dec 31, 2021
128
It will be nice if I can "chose" only the continuity track on pcb with resistance under certain resistance. For example it will beep only in case the resistance between two probe is less than 10 ohm. This is the context.
 

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MrChips

Joined Oct 2, 2009
29,228
Here is my idea for a low voltage continuity tester.

Low Voltage Continuity Tester.jpg
Q1 is a germanium transistor.
Q2 can be either germanium or silicon transistor.

I showed a 9V battery for convenience but I would probably use a single or two 1.5V cells. Adjust the resistor values accordingly.

Rx is not part of the circuit. The test probes would replace Rx.
The LED is lit when the resistance shown by Rx falls below about 600Ω.
 

MrSalts

Joined Apr 2, 2020
2,767
Here is my idea for a low voltage continuity tester.

View attachment 288468
Q1 is a germanium transistor.
Q2 can be either germanium or silicon transistor.

I showed a 9V battery for convenience but I would probably use a single or two 1.5V cells. Adjust the resistor values accordingly.

Rx is not part of the circuit. The test probes would replace Rx.
The LED is lit when the resistance shown by Rx falls below about 600Ω.
Have you tried this? Germanium transistors are so leaky and temp dependent that it may never turn off. Ge Transistors are Essentially useless parts and that's why the Gods in charge and the Designers who specified them decided to stop making them.
 
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