Products > Test Equipment
Multimeters With Low Ohms Function
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David Hess:

--- Quote from: EEVblog on January 15, 2022, 09:56:25 pm ---Some old school meters have a "Low ohms" function that has a compliance voltage below 0.6V so it doesn't switch on semiconductor junctions in-circuit. Seems to have fallen out of favour since the 80's and 90's. My first digital meter, a Soar had it.
--- End quote ---

I bought my Tektronix DMM916 in the late 90s and it has 4 "resistance" modes:

Ohms   860 millivolts
Low Ohms   280 millivolts
Continuity Test   3.2 volts   Reads out ohms
Diode Test   3.2 volts   Reads out volts

My Beckman RMS225 from about the same age has:

Ohms   650 millivolts
Continuity and Diode Test   3.2 volts   Reads out volts
EEVblog:

--- Quote from: AVGresponding on January 16, 2022, 04:41:52 pm ---Ok the tests took less time than I thought, so here are the results.



--- End quote ---

Thanks for those.
The V in High voltgae will be reduced by divider of the 10M input impedance of the meter unless you used a High-Z meter input.
ermionesrl:
Hello.
May I add some measurements of my own multimeters?

(EDITED: sorry, I mistakenly wrote some units, this is corrected table)

Meter; V in Cont; A in Cont; V in High R; A in High R; V in Low R; A in Low R
Fluke 177; 2.6941; 980.4u; 1.2439; 120n; 1.2440; 311.5u
Fluke 17B+; 0.5316; 159.2u; 0.5301; 45n; 0.5304; 158.8u
Fluke 15B; 0.4410; 140.7u; 0.4408; 45n; 0.4410; 140.7u

I used the Agilent 34401A in >10Gohm input mode, both for voltage and for current. For current, a 100 ohm 0.01% metal foil resistor shunt has been added. In some time I could eventually repeat the measurements with an electrometer, for better current values. I have a second 34401A and eventually I can measure it with its twin.

AVGresponding: you have used the Fluke 289, but its 10Mohm input voltmeter impedance made your measurement of high ohm V almost surely incorrect, the current source got loaded. You see that even the >10Gohm of 34401A (very) slightly load it.
I think that in most (if not all) cases, the burden voltage is the same between low and high ohm. As you own a 34401A, maybe you can repeat the measurements with it at >10G.
AVGresponding:

--- Quote from: EEVblog on January 17, 2022, 07:12:47 am ---The V in High voltgae will be reduced by divider of the 10M input impedance of the meter unless you used a High-Z meter input.

--- End quote ---


--- Quote from: ermionesrl on January 17, 2022, 12:07:35 pm ---AVGresponding: you have used the Fluke 289, but its 10Mohm input voltmeter impedance made your measurement of high ohm V almost surely incorrect, the current source got loaded. You see that even the >10Gohm of 34401A (very) slightly load it.
I think that in most (if not all) cases, the burden voltage is the same between low and high ohm. As you own a 34401A, maybe you can repeat the measurements with it at >10G.


--- End quote ---

Fair point, I should have thought of that. Just got home from work, and I'm warming up my Fluke 8840A (it's the most recently calibrated high-impedance meter I have).
Neomys Sapiens:

--- Quote from: Kleinstein on January 16, 2022, 09:11:07 am ---One can still test for shorts in circuit, if the open circuit voltage goes higher. A diode junction would still not measure very low ohms, but what you get with some 0.6 V at a resistor and this is usually quite high.

There are 3 points with a limited voltage/power: one is protecting sensitive parts that can not withstand a higher voltage and could be damaged (e.g. BE junction in reverse from more than some 6 V, some LEDs - at least according to the abs. max specs and some were actually sensitive to > 5 V).

The other point is measuring sensors like PTCs / NTCs, where the test current may already cause significant self heating. A small PT1000 tested with 1 mA test current is not ideal. A 10 K NTC with 1 mA test current can be way off. 

The 3 rd point is dry contact testing, so testing the low votlage performance of contacts, where a higher voltage can destroy an oxide layer at the contact and this way hide a contact problem. This dry ohms testing wants a rather low maximum voltage, often realized with a prallel resistor.

A high voltage is nice for diode testing to also measure a blue / white LEDs and maybe a 5 V zener.
On the other side high resistor ranges may need a relatively high voltage to gets low senstivity to hum.

--- End quote ---
Another very good reason for a low test voltage and limited energy would be the presence of squibs/detonators.

That said, both the Tektronix DM501A and DM502A have a HI/LO section switch for their kOhms ranges. For LO, maximum voltage is stated to be 0.2V.
The Siemens B1043 does it differently: when selecting a Resistance range, a diode symbol is shown if the voltage is above 0.5V. If none is shown, it is a 'diode safe' range.
The Metrawatt M2036 just gives a diagram in the manual from which it can be seen that the open-circuit voltage on all ranges except 30MOhms should be <400mV.

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