Harbor Freight freebie meters

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Richard Hull
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Re: Harbor Freight freebie meters

Post by Richard Hull » Sun Nov 25, 2018 11:55 pm

I really admit the 99.9% of the digital meters from Harbor Freight (free) to Fluke and Beckman ($150+) with a 10 amp range are a joke. The leads even in the finest will become part of the load at 10 amps and drop the current horribly. I think the Harbor freight meter might have a nice flexible 22 gauge lead set. I have actually cut up over 10 Harbor Freight meter lead sets to add pins to them for plugging into proto-boards; so I am acutely aware of all the issues. Thus, even on the 200ma range you can get a false impression of the real current at the full 200ma.

Current is current and voltage is a totally different animal. Accurate current measurement is solely dependent on the resistance of the wire leads to the meter. Accurate voltage measurement has nothing to due with the leads at all but the input impedance of the internals of the meter, itself. A good manufacturer will typically force you to use a separate plug in lead for the higher current reading and not just switch the positive lead to the special 10 amp banana jack. In fact, both leads to measure high currents will need to be hand made up by the user for the 10 amp range. As most people are dolts and uninformed stuff, this never happens making the 10amp range kind of a total joke played on the purchaser.

As noted by Bob, in the old days, we typically used VOMs and VTVMs. VOMs were, at best, 20,000 ohms per volt and fell on their face when working on high impedance vacuum tube gear voltages in sensitive circuitry. The VTVMs had a fixed 10 megohm input impedance which quickly became the industry standard including on many oscilloscopes with 1X-10X probes and remains so today. (Most scopes sport a 1meg impedance with fixed capacitive loadings). The HF meter is made for the average handy-man working on his auto or in the home wiring arena. In the hands of an adroit electronics person, the HF meter is just fine, as the limitations are understood. Good electronics folks always have a separate, single trick pony, meter for high currents with leads to match. They also possess a separate, single trick pony, meter for high voltage measurement.

There are many circuits today that use FET related circuitry where 10 megohms is a virtual dead short load on them. Blessedly, their is little old fashion trouble shooting on such circuits anymore. ( you throw the entire item away rather than service it.) Likewise, many ICs that work internally in the pico and femptoamp range convert any output pin to a lower impedance "real world" capable signal.

There remain a number of folks who acknowledge the need for understanding such matters in measurement situations and how instruments can lie to the user based on inexperience. (This diseased curse applies to radiation measurement and neutron measurement especially due to lack of understanding by the newbie)

I investigated the issue, and the down grading of the voltage of the HF metering max range (1000v to 200 volts). It is due to a change in circuit board design on the newer meters where arcing is possible on the circuit board internal to the meter at 1kv. The leads supplied will take more than 1000v OK. The guts are the limiting factor. Narrower, closer PC board conductors increase the field strength between conductors on the board that can lead to arcing.
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Rich Feldman
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Re: Harbor Freight freebie meters

Post by Rich Feldman » Tue Nov 27, 2018 8:12 am

Respectfully beg to differ, sir, on one small detail.

>> Accurate current measurement is solely dependent on the resistance of the wire leads to the meter. Accurate voltage measurement has nothing to due with the leads at all but the input impedance of the internals of the meter, itself.

How can lead resistance affect the accuracy of current measurement? The current in circuit under test is identical to the current in the lead wires and the current in the meter's guts. Instrument can accurately report the circuit current with meter present, just as in voltage mode it reports the circuit voltage with meter present.

Sure, ammeters and their lead wires have a finite voltage drop. Metering changes the behavior of circuit under test, sometimes negligibly and sometimes intolerably. Same can be said for voltmeters and their lead wires, with finite current draw.

A generally negligible fraction of the measured variable can actually be lost in the path between circuit connections and meter guts.
In voltage mode, that would be voltage drop in the wires due to the tiny meter current.
In current mode, the analogous error is from current leakage between the lead wires due to the tiny meter voltage. :-)


I do agree that typical lead resistances are very very small compared to typical voltmeter resistance, and not so small compared to typical ammeter resistance. Many handheld Fluke meters have max voltage drop of 100 mV in 10 A range, and 400 mV in 400 mA range, not counting the lead wires. Not exactly negligible when measuring, say, flashlight circuits.

And agree that even fancy meters with 10 amp ranges often don't come with 10 amp test leads. As reported last summer in "277 volt power at home" thread, ...One run using 10 amp range on Fluke meter, until the probe leads began to smoke. Second run using measured voltage across the heat-sinked sense resistor. ... ( I knew that the former would be abusive to the lead wires, but was impatient. And the alligator-clip-terminated wire might have been heated to the smoking point by proximity to 2600 watts of bare halogen lamps.)
Richard Feldman

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Richard Hull
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Re: Harbor Freight freebie meters

Post by Richard Hull » Tue Nov 27, 2018 9:45 am

Current measure on 10 amp HF meter.********************************

Wire leads on harbor freight meters is 24 gauge (measured diameter) .025 ohms per foot.
The HF meter leads are 2 foot long each. .025ohm X4 = .1 ohm

A test circuit of known values of 10 volts from a suitably "stiff supply" into a 1 ohm load =10 amps. This is a given by ohms law.

The 10milli-ohm shunt in the meter (heavy gauge solid copper wire) will read 100mv to the internal meter when 10 amps run through it. The decimal point setting (scaling) will show 10.0 amp.

You now have a total load circuit resistance external to the meter of 1.1 ohms powered up by 10 volts. this means the shunt in the meter will see only, 10 / 1.1 or 90.9 milli-volts and the meter will read 9.09 amps. A failure of almost an amp error in the circuit we know to be 10 amps, (without the meter).

The wire leads will suck up .5 watt of power each without burning up. They will, as seen above, screw up the measurement accuracy.

Wire leads of the meter were the sole cause of error when reading the meter's max current range. The guts of the meter are good.

Voltage measure on a HF meter.*******************************

However the same wire leads will have no effect on voltage readings of moderate to low impedance voltage sources.
The meter is known to have a 1megohm input impedance. If we take the 10 volts noted above and put a 1 megohm resistor in series with it we have made a high impedance full 10 volt source supply. For there is and will always be 10 volts out of this circuit's open leads. However on the 20 volt range, the HF meter will measure only 5 volts. We could take #36 gauge hair fine leads of 100 foot length, each, and connect it to the meter and the 10 volt supply.....It would still read 5 volts.

For those less informed. the voltage halved due to impedance issues not with the wire leads but with issues dwelling in two places. The internal guts of the meter, and the poorly used brain of the person trying such a measurement not having any idea about his voltage source. Even a 10 megohm VTVM meter would read 9.09 volts.

Here the wire leads to the meter were of no importance whatsoever.

As in all measurement work, be it electrical, electronic or radiation. Know all about you instrument, you must become one with it. Know all about what your measured source is and how both items, ill-applied can conspire to fool you in many situations. Be especially aware that at the extreme high and low end of all measurements there are demons lurking to pull the rug from beneath you.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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