Low V and I continuty tester for in-circuit probing?

Thread Starter

prometei

Joined Apr 13, 2008
74
hi,
can someone recommend a simple circuit for a low voltage, low current continuity tester for in-circuit probing that will not turn on semiconductor junctions?

thanks!
 

#12

Joined Nov 30, 2010
18,224
I looked at the schematic and conclude that it could work. I also advise that doing very low voltage continuity testing requires some understanding of the limitations of the circuit, for instance the 5 ohm maximum. NCSailor just posted one with a 25 ohm maximum (hackable). I would suggest practicing on some known quantities to get familiar with what to expect.

There are many ways to over complicate this circuit, up to and including a digital readout, but the purpose is to be simple and reliable. I think it's a, "win".
 

Externet

Joined Nov 29, 2005
1,962
The Excelta tester on post #4 uses a blocking oscillator that allows to discern less than an ohm change by emitting a tone that varies with conductivity. No need to turn it off, its cell can last its shelf life.
Sharpened its probes to allow piercing solder and just pressing them-in more or less tells a better connection/lower resistance.

The older model I have is MB-1
 

#12

Joined Nov 30, 2010
18,224
Sharpened its probes to allow piercing solder
One of my favorite methods! I replace an alleged probe tip with a 1/8th inch Allen wrench and sharpen that little bit of tool steel.
I can pierce any kind of crud or conformal coating with that method.
 

RichardO

Joined May 4, 2013
2,270
Here is a version of an in-circuit tester I have used for years:
Beeper_AAC.png

I uses a piezo transducer for both the timing capacitor and the speaker so it may work a bit differently with different transducers than the one modeled.
 

#12

Joined Nov 30, 2010
18,224
Here is a version of an in-circuit tester I have used for years:
Nice use of a current mirror to keep the output voltage low.:) Then a Darlington to completely ruin the current limiting feature? Brilliant.
I was puzzling about a 300pf load characteristic until I realized it is irrelevant to the primary function.:D
Hey, I design them backwards. Sometimes I read them backwards.:p
 

RichardO

Joined May 4, 2013
2,270
Nice use of a current mirror to keep the output voltage low.:) Then a Darlington to completely ruin the current limiting feature? Brilliant.
I was puzzling about a 300pf load characteristic until I realized it is irrelevant to the primary function.:D
Hey, I design them backwards. Sometimes I read them backwards.:p
I think of the operation a little differently. Q1 is run at a current of somewhat less than 1 uA. The Sziklai pair (Q2 and Q3) is used to reduce the current through the base-emitter junction of Q2. This makes the diode drop of Q2 less than the diode drop of Q1. This difference is the voltage seen at the probes.

The output of the Sziklai pair drives the bias input of a Programmable Unijunction Transistor. When the resistance being tested is low enough the Sziklai pair biases the PUT so it can oscillate.

Actually, the load is critical to the function of the circuit. I had to do the model of the transducer to get the simulation of the circuit to oscillate. As I remember, the capacitance spec for the transducer is something like 10,000 pf. That capacitance is the timing cap in the PUT oscillator.

When the probes are open, the only current drawn from the battery is the current through Q1. This is such a small current that the circuit does not need a power switch. I build the circuit on an itty-bitty PCB that mounts right on the 9-volt battery clip.
 

dannyf

Joined Sep 13, 2015
2,197
I need something that will be under 0.1V.
two ways to do it:

1) define what "continuity" means for you and drive it with a current source so that at non-continuity the voltage is 0.1v;
2) take a regular meter and put in serial a sufficiently large resistor so that at non-continuity the voltage on the dut is 0.1v;

the two approaches are conceptually identical.
 
Top