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Multimeters With Low Ohms Function
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floobydust:
With low power ohms, I find it's mainly because you need the ability to differentiate between presence of a semiconductor junction or ohmic resistance - in-circuit. For the compliance-voltage limiter.
Damaged semiconductors frequently test as if they're a leaky diode-junction. With multimeters, there is a resistance (say 500R) on diode-test that looks like a good junction. There is a Vf that looks like a resistor. You have to know these values.

Transistor has three leads, six possible readings (both polarities) and double that if you are using both ohms and diode-test. If repairing gear with many BJT's like audio power amplifiers, missing one damaged transistor can be very costly. They will almost test OK, especially some non-symmetrical VBE and VCB junctions which are usually quite close in Vf but not with damaged parts. Or resistive leakage (carbon) is present. Or one junction is low or high Vf. Have to be really thorough sometimes.
ermionesrl:
AVGresponding:
This is very puzzling then. I have not been able to find the schematic of either meter to check, yet it would be very strange if this was intentional. In this case, you would almost surely see a step change.

Neutrion:
If you used a regular x10 probe with your scope, its input Z should also be 10Mohm so your results shouldn't differ. In general scope readings are less accurate than any meter, in particular if your scope has a 8bit adc.

About variable tone beeping, in Art of Electronics II edition, page 954, there is a circuit useful to discover shorts without desoldering parts, measuring millivolts (caused by an external current source). This circuit uses a meter, but I remember somewhere I saw a more interesting version that emitted a proportional tone instead and had an internal current source.
Wallace Gasiewicz:
Just for attempting to be complete:
My old Fordee PSM 37 is analog and has the low voltage ohms. Manual states it is .094 Volts for Low Ohms.
It has a 20 ohm mid scale reading so it is not that great for anything below 5 ohms.
This unit is essentially a JFET input device, so it has characteristics of both the usual modern multimeters with an analog meter.

Wally
AVGresponding:

--- Quote from: ermionesrl on January 18, 2022, 09:45:04 pm ---AVGresponding:
This is very puzzling then. I have not been able to find the schematic of either meter to check, yet it would be very strange if this was intentional. In this case, you would almost surely see a step change.

Neutrion:
If you used a regular x10 probe with your scope, its input Z should also be 10Mohm so your results shouldn't differ. In general scope readings are less accurate than any meter, in particular if your scope has a 8bit adc.

About variable tone beeping, in Art of Electronics II edition, page 954, there is a circuit useful to discover shorts without desoldering parts, measuring millivolts (caused by an external current source). This circuit uses a meter, but I remember somewhere I saw a more interesting version that emitted a proportional tone instead and had an internal current source.

--- End quote ---

You'd be unlikely to see a step change because of the very low currents involved, 41 nanoamps and 55 nanoamps for the Agilent and Metrix respectively, and the very high impedance (≥10GΩ) means that even with very low input capacitance it takes a while for the indicated voltage to drop.
Kleinstein:
The high Z meters have a high input resitance, but often also some input capacitance that may start in some not so well defined state. So one can still see an initial change in the voltage from the capacitance and just a possible slow setting to a final voltage. For the open circuit voltage the ouput impedance of the meter under test may no longer be that high in resistance even with a low current setting.
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