Harbor Freight freebie meters

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Dan Knapp
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Harbor Freight freebie meters

Post by Dan Knapp » Tue Nov 20, 2018 1:09 pm

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 » Tue Nov 20, 2018 6:30 pm

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. :-)
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Re: Harbor Freight freebie meters

Post by Rex Allers » Tue Nov 20, 2018 10:00 pm

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

http://www.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

http://www.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

http://www.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 » Thu Nov 22, 2018 4:41 am

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.

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Re: Harbor Freight freebie meters

Post by ian_krase » Thu Nov 22, 2018 5:45 am

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 » Thu Nov 22, 2018 9:13 am

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
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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.

Dan Knapp
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Re: Harbor Freight freebie meters

Post by Dan Knapp » Thu Nov 22, 2018 1:53 pm

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 » Thu Nov 22, 2018 7:30 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

ian_krase
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Re: Harbor Freight freebie meters

Post by ian_krase » Sun Nov 25, 2018 5:14 am

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 » Sun Nov 25, 2018 9:31 pm

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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