Electronics > Projects, Designs, and Technical Stuff
Is it safe to test for continuity with multimeter
David Hess:
Continuity, diode test, and ohms all work the same way using the same circuits, (1) ohms converter plus voltage measurement, but with variations in test current, voltage compliance, and readout (beeper or not).
On my Beckman RMS225, continuity and ohms mode are the same mode with a compliance of about 3 volts. Ohms mode has a compliance of about 0.6 volts.
My Tektronix DMM916 has separate diode and continuity modes but with the same 3 volt compliance. Normal ohms mode is 0.9 volts and low voltage ohms mode is 0.25 volts.
As I said earlier, more modern multimeters than mine have a higher diode test voltage because they need to be able to test higher voltage LEDs. Older multimeters kept the diode test voltage below the point that which would damage a bipolar transistor base-emitter junction. Older multimeters yet had much higher voltage compliance in ohms mode until it was realized that this might not be such a good idea.
(1) I am sure there are exceptions, but multimeter designers tend to be parsimonious. (2)
(2) Checks off $50 word for today.
ledtester:
So far most of the discussion has focused on the safety of the circuit under test. But what about operator safety?
Here's a design for a continuity tester that employs a TBU (transient blocking unit) to protect the tester in the presence of mains voltages:
https://youtu.be.com/N2M-p-OGvPg
Discussion of the TBU starts around 9:00.
c64:
I have two one-hung-low 9205a multimeters (different brands). I checked how much they output.
First multimeter:
about 2.9V in diode/continuity mode
in resistance mode 0.25V (except in 200MΩ range, it's 2.5V)
Second multimeter:
about 2.8V in diode/continuity mode
in resistance mode 0.5V (except in 200MΩ range, it's 2.6V)
It's hard to say about current limit, current is actually jumping up and down, max I've seen was 1.2mA. Maybe better to check with scope.
fourfathom:
I collected seven of my DVMs and measured the "continuity" and "diode" modes, short-circuit current and open-circuit voltage. The numbers were:
Continuity: 0.17 mA -- 0.66 mA, 0.45 V -- 2.52V
Diode: 0.47 Ma -- 0.86 mA, 1.49V -- 2.5V
With a couple of meters I also measured the currents through a small series resistor, and they changed very little if at all. The meters appear to use a constant-current source for these measurements (or at least not a pure resistive source).
.
bdunham7:
--- Quote from: c64 on July 16, 2020, 02:21:33 am ---I have two one-hung-low 9205a multimeters (different brands). I checked how much they output.
First multimeter:
about 2.9V in diode/continuity mode
in resistance mode 0.25V (except in 200MΩ range, it's 2.5V)
Second multimeter:
about 2.8V in diode/continuity mode
in resistance mode 0.5V (except in 200MΩ range, it's 2.6V)
It's hard to say about current limit, current is actually jumping up and down, max I've seen was 1.2mA. Maybe better to check with scope.
--- End quote ---
Surely you mean the 200R range, not 200M! If so, my $0.00 @ Harbor Freight D830 is the same way, 0.25-sh on all resistance except 200 and diode check, which were 2.5-ish.
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