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Multimeter: UNI-T 89XD manual ranging

I received a comment or two that said I will not get an accurate amp reading if I use the 20 amp jack and set the dial to anything other than the 20 amp setting, such as 60 or 600mA.

I have used the 20 amp jack and set the dial to 60mA and did not get an OL on the screen. I got mA readings. What is dumber, me or the meter? (Don't answer that!!!)

How true is the admonition that I will not get accurate readings if the 20 amp jack is used with other amp dial settings?

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The 20A socket has its own shunt resistor to common terminal. It measures the current by measuring the voltage over the shunt resistor.

If you select the normal socket for measurement source with the dial, the multimeter may not even be measuring the voltage that is caused by current via 20A socket, or if it is, the measurement will be wrong, since it expects to see voltages in a different range from the normal shunt resistance.

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The 20A current path is not even switched internally (you do that when you move the probe), so you can't expect a reasonable reading. Likely you'd get something like the expected count with a decimal point position that depends on which incorrect position the switch is in.

Of course if you leave the probe in the 20A setting and attempt to measure voltage bad, or very bad, things can happen depending on the source of the voltage, since the 20A current path is pretty much a direct short between the probes. Some meters have an alarm if you set the knob to volts with a probe in the 20A socket.

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    \$\begingroup\$ Interest only: I recently bought a meter that has shutters that block various probe sockets depending on dial setting. An excellent idea - but annoying in that you must remove one plug before transiting ranges where the inserted plug is "illegal". Net result is that it is impossible to measure V while set to I or I while set to V. \$\endgroup\$
    – Russell McMahon
    Commented May 15, 2020 at 23:40
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This depends on what you are trying to measure and how accurate it needs to be. If you're trying to measure 15 amperes and need accuracy to within 10mA, that meter likely won't do it (I didn't go look at its specifications).

The ranges available affect the accuracy because they use different resistors internally. The basic way that current measurement works is by putting a low but known resistance in series with the load, and then measuring the voltage difference across that resistance. The dial is selecting different resistors as well as selecting which port is used (20A uses its own port, bottom left of the unit; all other current measurements use the mA µA port above it). Therefore, measuring current with the probe in the 20A port but the dial on something else will probably result in random values (noise).

Uni-T UT89XD Meter

When you select 20A (and use the correct port), the meter is using a higher-current fuse and resistor, which provides less resolution for low currents. In other words, measuring 50mA with the 20A setting may result in a reading of 0.0 A (it's too little to register) or potentially something like 0.1 A (if it rounds up or something weird).

If you do the opposite, measuring a current that's higher than the dial setting and port, you'll blow the fuse. Meters like this have two fuses for current, a 20A fuse for the 20A setting, and a 600mA fuse for the others.

As @Spehro said, don't measure voltage with a probe in either current port. The 20A port should not be thought of as a "one size fits all" port.

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