Harbor Freight freebie meters

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Dan Knapp
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Harbor Freight freebie meters

Post by Dan Knapp »

This is not exactly advanced technical discussion, but I didn’t find a more appropriate section. The Harbor Freight red digital multimeters have been mentioned multiple times. I just discovered yesterday that these have only a 1 megohm input impedance. Nowhere in the manual or online specifications do they state an input impedance. I was working on an electron gun controller that only has a 0-200 microamp emission range and was monitoring several voltages with a group of red meters when I noted that the energy reading on the red meter didn’t agree with that on the energy setting and that the controller was indicating significant electron emission with no emitter. I measured the input current on the red meter and calculated only 1 Meg input resistance. When I put a higher quality multimeter with a more typical 10 meg input impedance, the reading agreed with the controller setting. One should be aware that using these meters can load a low current source. They are still very useful for lots of things, particularly in sacrificial application where an arc can zap an expensive meter, and the price is certainly right; but it is helpful to know that they have a significantly lower input impedance than the usual higher quality meter.
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Rich Feldman
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Re: Harbor Freight freebie meters

Post by Rich Feldman »

Yup, 1 megohm input impedance is typical for those cheap meters. Many of that class are also distinguished by having an hFE measuring range, and associated 8-terminal transistor socket.

Their input impedance is even worse in AC volts mode, as I found out a couple months ago, and presented in Neon Sign Transformer Power thread. Makes it hard (or at least unobvious) to increase the AC volts range with external resistors. Not so bad for DC volts, if you respect the voltage rating of your resistors.

That post includes a table of freshly measured input resistances for an assortment of multimeters, on various AC and DC volts ranges. Let's see what it takes for a link to post, instead of link to top of the thread. viewtopic.php?f=11&t=12407&p=80759&hili ... nta#p80759 How does one turn off the highlighting of search words?

On the flip side of the coin, when you want to measure sort of high voltages with minimal loading:
Many old-fashioned analog multimeters have a 1000 V DC range that presents 25 megohms (e.g. the Micronta in linked post) or even 50 megohms.
Look for fine print on the analog scale card, stating ohms-per-volt value (e.g. 50K) or full scale current (e.g. 20 uA). Some of these instruments go for $10 or more at flea markets. :-)
All models are wrong; some models are useful. -- George Box
Rex Allers
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Re: Harbor Freight freebie meters

Post by Rex Allers »

About: "How does one turn off the highlighting of search words?"

I'll take a stab at it. Links in a message usually look like gibberish and don't give much context. Because of this I usually copy the text of the header before the link I post. If I am referencing a reply message in a thread I might also copy the text of the posting date as extra help.

So I think this is what you referred to, put in my preferred format...
Re: Neon sign transformer power: a new look
Mon Sep 10, 2018 2:58 am
viewtopic.php?f=11&t=12407&p=80759#p80759

It takes more work to do that but I think it is more informative.

So you got to your link by doing a search for 'micronta' and getting the link from the results. You asked how to get rid of the highlights. The only way I know of is to edit the message link that you got from the results.

Here is the link you shared. I got it from "copy link location" in my browser and pasting that into a text editor.

Code: Select all

https://fusor.net/board/viewtopic.php?f=11&t=12407&p=80759&hilit=micronta#p80759
From inspection, the hilit tag is what is giving the highlights. My first attempt was to delete all of the link from that to the end:

Code: Select all

https://fusor.net/board/viewtopic.php?f=11&t=12407&p=80759
That got to the thread without highlights but referenced the whole thread, not the desired reply section. For that, the last part after the pound-sign (that I had deleted) was needed. So this is the edited link I arrived at and posted as my example in the top of this message:

Code: Select all

https://fusor.net/board/viewtopic.php?f=11&t=12407&p=80759#p80759
It would be nice if there was an easier way to get this desired link after a search, but if there is I don't know of it.
Rex Allers
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Bob Reite
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Re: Harbor Freight freebie meters

Post by Bob Reite »

Wow! Reminds me of the old days of "1000 ohm per volt" meters (cheap ones in the day) 20,000 ohm per volt meters, and of course if you wanted 10 meg input resistance, you had to dig out your "Vacuum Tube Volt Meter".
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
ian_krase
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Re: Harbor Freight freebie meters

Post by ian_krase »

Ahhh, yes.

Then of course there are the Keithley vacuum tube frontend electrometers, with input impedance measurable only in scientific notation.
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Richard Hull
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Re: Harbor Freight freebie meters

Post by Richard Hull »

The stock Keithley electrometers can have input impedances as high as a teraohm (1000 gigohms). Some old engineers I hung with had a term...
" A qualified precision open" for the occasional one teraohm resistor we would come across. Unfortunately the highest voltage you can read on most Keithley electrometers is typically 10 volts.

A high-end vibrating reed electrometer can approach the petaohm input range and can differentiate in special measurements, a difference of +/-100 electrons of charge.

The Harbor Freight meters are what they are. I put out the post here using an approx. 99 megohm "adjusted resistor" in series with the Harbor Freight 1 meg input resistance to create a 100 megohm input impedance 2000 volt meter with the meter on the 20 volt range.

I needed the 100 meg impedance to calibrate GM supplies, PMT, supplies and bias supplies that typically have ultra low current output capabilities. It is used a lot by me in my work here at the lab. Free is always free and in the case of the free H.F. meters you get far more value than you pay for.

The new batch of H.F. free meters, probably to satisfy ridiculous safety nerd standards, no longer have a 1KV DC range or a 750volt AC range. All ranges end at 200-250 volts AC or DC. They are still 1 meg input impedance.

The meter I made up with 100meg input, if placed on the 2 volt range, will remain a 100meg input impedance and measure 0-200 volts. Likewise on the .2 volt range it will still be a 100meg meter to 0-20 volts. Needless to say if you want hyper accuracy you gotta' hand cob yer 99 meg resistor with .1%, low temp co resistors. Something no one here typically has access to or money to pay for them if they did have access.

The little 2kv meter I made up could be 50 volts off at 1500 volts and the devices I work on would not care. However, in assembling my resistor I used a Keithley 50ma capable, (low output impedance), 0-3500 volt, precision, decade switched, regulated supply to hum my resistor trim to dead on at room temp with 2% flame proof resistors.

Fluke and Keithley both made such precision, beefy supplies in the 40s to the 60s and used transmitting power output tubes to regulate and supply current to the output. I have two of them that I picked up at hamfests over the years and they run about $30 if working. You'll need a strong back or a cart at the hamfest to drag one of these baby's back to your car.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Dan Knapp
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Re: Harbor Freight freebie meters

Post by Dan Knapp »

It looks like I should have been aware of the 1 Meg input resistance if I had remembered some previous posts on these meters, but at my age I’m doing well to remember my name. At any rate, my post precipitated some interesting discussion.
I have an unrelated question that relates to my present task of trying to fix a Kimball Physics electron gun controller without the benefit of a circuit diagram (EGPS-7FX5 in case someone out there has the schematics). I think I recall Richard stating that when he fixes NIM modules, the first thing he does is replace all of the tantalum electrolytic capacitors. Did I recall this correctly (the controller is about thirty years old and full of these)?
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Richard Hull
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Re: Harbor Freight freebie meters

Post by Richard Hull »

The bad tantalums in all older NIM gear are early tantalums. Early tantalums are prone to drying out and failing. The typical failures in NIM gear are those located near the 1/8 watt 10 ohm, "fuse resistors" in the NIM module's power supply lines. Often, the resistors are burned up and open. Replace them and all tantalums near the cluster of 10 ohm tiny resistors.

Richard Hull

P.S. As I write this, Thanksgiving afternoon, the smell of roasting turkey and fixin's waft through the house as the old gal does her thing in the kitchen. The two cats are under her feet as they know the "good stuff" is on the way. RH
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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
ian_krase
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Re: Harbor Freight freebie meters

Post by ian_krase »

I'm not sure if the meter ratings should be Considered ridiculous safety nerd stuff. I'm not willing to use HF meters, or most other cheapie meters, on moderate-voltage high-current stuff like wall current.
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Bob Reite
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Re: Harbor Freight freebie meters

Post by Bob Reite »

I'm guessing it's because the test lead wire is so flimsy on the current batch of meters is that they had to derate them to 250 volts.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Richard Hull
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Re: Harbor Freight freebie meters

Post by Richard Hull »

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 do 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 10 amp 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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Rich Feldman
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Re: Harbor Freight freebie meters

Post by Rich Feldman »

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.)
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Richard Hull
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Re: Harbor Freight freebie meters

Post by Richard Hull »

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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