design of a continuity tester

[pc88]

Hi,

I've gotten into pondering the design considerations for a continuity tester, and I found the following schematics on the web:

• **broken link removed** (Tony's second design)
• https://kitsrus.com/pdf/k110.pdf (KitsRUS design)

The first one seems to be driven by the desire to deliver only 200 uA to the test circuit to ensure that the circuit under test isn't inadvertantly powered by the tester and also so that PN junctions do not register as shorts. The second design has a short circuit test current of 6 mA and a voltage threshold of 11 mV (on the low setting). Both are designed to indicate continuity at a resistance of 2-3 ohms or less.

Question 1:
Since a PN junction has a voltage drop of > 100 mV, there really isn't any danger of the second design activating a PN junction, is there?

Question 2:
Is this explanation of the how the first design works correct? The first 741 is simply a non-inverting amplifier with a gain of a 11. The voltage at the non-inverting terminal is just 9V * R/100K where R is the resistance under test. A target threshold of 3 ohms thus translates into a voltage threshold of about 300 uV. At the output of the first 741 this would be about 3 mV. The second 741 seems to be operating as a comparator.

Question 3:
Assuming the above is correct, how is he getting the second 741 to trip at 3 mV? Is he relying on the input offset current of the 741 through the 100K input resistor? And if that's the case, what about the 741's input offset voltage which measures in the mV range?

Thanks!

 

[mneary]

1) Both designs pull the test voltage up to +V. That will activate a PN junction. Either design has a threshold below this level, so their accuracy isn't affected. But the voltage is there.

2) Essentially correct. The unadjusted input offset voltage of the first 741 is nominal 3mV, which makes its "zero" output +/- 50mV. It either requires adding an offset null pot on the 741 or being very lucky.

3) The second 741 should be equally unpredictable. The bias current through 100K helps, but not enough to compensate for up to 3mV of offset voltage.

 

[pc88]

mneary - thanks for the reply!