Author Topic: Thinking of buying a multimeter.  (Read 23987 times)

0 Members and 1 Guest are viewing this topic.

Offline KJDS

  • Super Contributor
  • ***
  • Posts: 2442
  • Country: gb
    • my website holding page
Re: Thinking of buying a multimeter.
« Reply #25 on: April 18, 2014, 11:03:49 am »
If you want anything* electronic to last, run it at 50% of its rated maximum.

There are exceptions to this, mostly in power devices that get more efficient as you get close to their maximum rated power, but for turning an LED on it's the way to go. Have a look at how resistance changes with temperature, and remember that running a current through a resistor will change its temperature. Find out what the LED volt drop does with time and temperature and you'll be able to run it closer to max, but in general it takes a great deal of understanding to run anything hard without releasing magic smoke and saying it will be fine at 300.5 but not at 300.9 isn't the right approach.

Offline adrianf88Topic starter

  • Contributor
  • Posts: 14
Re: Thinking of buying a multimeter.
« Reply #26 on: April 18, 2014, 11:48:06 am »
Thank you everyone for the patience and for all of the great advice! I settled on the UT139C, and come next paycheck, i will contact Franky and buy one from him since everyone around here seems to recommend him! Thank you again for all the help and for being so nice with a newbie! Have a wonderful day!

P.S. Franky, have you ever shipped to Romania? If so do you know if i have to pay any charges? Import taxes and stuff like that?
 

Offline iloveelectronics

  • Frequent Contributor
  • **
  • Posts: 940
  • Country: hk
Re: Thinking of buying a multimeter.
« Reply #27 on: April 18, 2014, 12:07:17 pm »
P.S. Franky, have you ever shipped to Romania? If so do you know if i have to pay any charges? Import taxes and stuff like that?

Yes, I have shippped a lot of packages of different sizes to Romania and as far as I know my customers there rarely have to pay customs/tax. Obviously I can't guarantee anything though as ultimately I have no control over it.
My email address: franky @ 99centHobbies . com
My eBay store: http://stores.ebay.com/99centhobbies
 

Offline LaurenceW

  • Frequent Contributor
  • **
  • Posts: 258
  • Country: gb
    • It's Time, Jim, but not as we know it
Re: Thinking of buying a multimeter.
« Reply #28 on: April 18, 2014, 05:24:29 pm »
Hold on, Hold on.  Why do you think you need to drive an LED or LED array to such precision as 320.05mA? I suggest that, quite simply, You Don't!. Drive them at 300 mA or 350mA, or anything in between, and I'll bet that you will scarcely notice any difference in the light output, and any impact on LED life expectancy will be so minimal as to be irrelevant.

LED currents just aren't that critical. Neither, then is such a precision current source. If your circuit supply voltage is itself stable to within +/- 5% (or even +0-20% if it's battery operated) I would suggest that that is plenty close enough to be used as a reference. If you still want some sort of voltage reference independent of supply, a small zener or a couple of signal diodes in series will do, but not the cost of a precision reference. In practice, the LEDs just don't warrant it.

Rule of Thumb: unless you are working with instrumentation equipment which actually needs to record voltages to such degrees of accuracy, don't bother designing anything to a precision of more than two significant figures. You are wasting your time, will probably end up wasting money on over-tollerance parts, for no discernable impact on the overall circuit performance.  You design calls for a 1.2K resistor, and you only have 1K to hand? - she'll do - bung it in!
If you don't measure, you don't get.
 

Offline adrianf88Topic starter

  • Contributor
  • Posts: 14
Re: Thinking of buying a multimeter.
« Reply #29 on: April 18, 2014, 06:00:45 pm »
Hey there LaurenceW! Thank you for the imput! I don't need it to 350.05 but i did need it to 350.5 at the time. Or to be precise at 350.4. Why? Because i had 16 LED's with a maximum tolerance of 22mA and i wanted to run them at their peek performance without blowing them out. So i settled for 21.9mA for each, totaling a 350.4 mA draw. At least that was my logic. Could i have powered them at 21 mA? sure! 20? Sure! But i wanted absolute peek performance without killing them. So i decided at 21.9 mA. That lead me to a constant current source of 350.4 mA. And trust me, at even 23 mA per LED, those LED's burned out. As to the design, that's what i found during a quick search on google. Not my design, don't even know if it's any good. Was just a proof of concept, because somebody asked if i needed 0.01% precision on resistors. He believed i meant resistors when i mentioned of powering LEDs. Anyway, let's not drift offtopic. :) Thanks yet again to everyone for the help. I'm now just waiting for the next paycheck to buy this puppy! :D
 

Offline LaurenceW

  • Frequent Contributor
  • **
  • Posts: 258
  • Country: gb
    • It's Time, Jim, but not as we know it
Re: Thinking of buying a multimeter.
« Reply #30 on: April 18, 2014, 07:50:09 pm »
Hello again, Adrian,

I think KJDS was "pulling your leg" when he asked about the 0.1% resistors! He was really making the same point - really, you just don't have to by that precise in all this. Sure, the constant current circuit that you have will work, and no doubt beautifully, but it's overkill, that's all.

When manufacturers give ratings to their products, they are usually pretty conservative, and figures are specified over a wide range of temperatures and typical component tolerances. There is a good safety margin built in. An LED rated at 22mA will happliy run at 25, even 30mA for many thousands of hours. If you are experiencing LED failures, I will bet my best shirt on the fact that it is not triggered by minor current overload against some specification figure.

Even different LEDs from the same manufacturing batch will perform a few % differently with respect to one another. Put a few in series and parallel, and you will find minor variation in the voltage across each LED, across each string, and also differences in the current flowing in each string, that make trying to drive an exact current through each LED nigh on impossible. But they'll all still work fine.

Now, such a small current LED, accidently exposed to, say, 1A even for just a few mS may well fry. That will be enough energy and time to burn the delicate LED die out. A simple wiring fault or incorrect component in a driver circuit could easily lead to that.

Here's another thought. Our ears, eyes, and for all I know, probably our noses respond logarithmically to changes in sound/light and smell. This is rather handy, as it enables them to operate over a HUGE RANGE of values. Compared to the very quietest sound you can hear and the point at which the sound is so loud it actually hurts, the ratio of sound pressure is 1:1,000,000,000,000 - that's a million million! And this is one reason why we measure sound levels in decibels, instead (a much more manageable measure). Moreover, ears can bearly detect a 1dB change in sound level, and yet this corresponds to a level change of 25% in the absolute level of the signal.

Now, I don't know what the threshold is for eyes, but I will put another of my shirts on the fact that, if you built a circuit to drive your LED with first 20mA for 1 second, then 22mA for a second, and then repeat this, your eyes would not be able to see the change in level of illumination. And even if they could, it would be so small that, when shown either brightness in isolation, you'd hve no reall idea whether you were looking at 20 to 22mA.

Bottom line is - yes, you can specify and build to this level of precision, but it yields no advantages in the workings of your circuit, whatsoever.  And, at the risk of running out of shirts alltogether, I'll state that no 20mA-rated LED will give up the ghost at 23mA. There, I've said it.

Please don't let me put you off - Keep experimenting and Learning! It's all good.
If you don't measure, you don't get.
 

Offline adrianf88Topic starter

  • Contributor
  • Posts: 14
Re: Thinking of buying a multimeter.
« Reply #31 on: April 18, 2014, 09:23:36 pm »
Ohhhhhh, don't tell me about overkill cuz with that led lamp i shot the proverbial ant with an elephant gun! But it pissed me off that the LED's in the original lamp burned out pretty fast so i went out and bought some better LED's. After that i wanted a longer life for the lamp and so i decided to build a constant current source and boy do they work beautifully now! And don't tell me that there's a single hobbyist on this forum that didn't build something overkill just because he could cuz then i would have to throw some of my shirts in the mix! :D Or maybe just out of spite. Or maybe in a moment of pure madness he wanted something that would survive an nuclear war... Whatever, but i bet everyone around here built something overkill. :D

And yeah, like i said previously, i could have powered them at 20mA or even 19... but i just wanted maximum power. It was my crotchet. Who can blame me? :D

Also i appreciate all the time and effort you put in all your replies but i have to (i'm sorry but i hope you understand) take you up on that bet. *here take my shirt*

The LED's i used were rated for different currents. I don't remember the exact numbers but it was something like this:

10mA - 2lm
15mA - 4lm
20mA - 6lm
max 22mA - 7lm

with 15.000 h of life at 15mA or something like that...

And i tried pushing 23mA thru 1 LED's and it did burn out!

So i just decided to power them at 21,9 mA which was just below their maximum rating...

On the other hand i know that an led rated at 20mA won't die at 23mA but these were rated way below that. Their maximum supported current was 22mA!

Anyway... long story short, i had to build that, i don't know if i actually succeeded knowing now what Wytnucls said about multimeter accuracy, but i built it, it still works, and i just thought i could find something to help me better next time. But after all the replies here, i now understand that even the most accurate meter doesn't quite measure current accurately so i'll just stick to the UT139C. Thank you again for all your time and effort and have a wonderful day!


 
 

Offline Flump

  • Frequent Contributor
  • **
  • Posts: 520
  • Country: gb
Re: Thinking of buying a multimeter.
« Reply #32 on: April 18, 2014, 09:43:39 pm »
I just bought an Extech EX330
http://www.extech.com/instruments/product.asp?prodid=277

It's a cheap meter but good on accuracy

I checked its dcv accuracy using an AD586LN
which is 5.000 V  +/- 2.5 mV
and it read spot on

and I used a 620R, 0.5W, 0.1%, 15PPM Resistor
and again it was spot on.

I used the 1Kz test point on my scope which I know is high and the scope says it is
1043hz and the Ex330 reads 1042.

Auto ranging seems a bit slow sometimes but i don't mind that as i'm never in a hurry

so for a cheap meter i'm very happy
 

Offline PedroDaGr8

  • Super Contributor
  • ***
  • Posts: 1283
  • Country: us
  • A sociable geek chemist
Re: Thinking of buying a multimeter.
« Reply #33 on: April 18, 2014, 10:07:23 pm »
I just bought an Extech EX330
http://www.extech.com/instruments/product.asp?prodid=277

It's a cheap meter but good on accuracy

I checked its dcv accuracy using an AD586LN
which is 5.000 V  +/- 2.5 mV
and it read spot on

and I used a 620R, 0.5W, 0.1%, 15PPM Resistor
and again it was spot on.

I used the 1Kz test point on my scope which I know is high and the scope says it is
1043hz and the Ex330 reads 1042.

Auto ranging seems a bit slow sometimes but i don't mind that as i'm never in a hurry

so for a cheap meter i'm very happy

meter is known to have huge issues with build quality. Plus its just a rebranded CEM.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin
 

Offline Flump

  • Frequent Contributor
  • **
  • Posts: 520
  • Country: gb
Re: Thinking of buying a multimeter.
« Reply #34 on: April 18, 2014, 10:29:34 pm »
what kind of build issues and does it matter if it is a rebranded cem ?
 

Offline PedroDaGr8

  • Super Contributor
  • ***
  • Posts: 1283
  • Country: us
  • A sociable geek chemist
Re: Thinking of buying a multimeter.
« Reply #35 on: April 18, 2014, 10:47:02 pm »
what kind of build issues and does it matter if it is a rebranded cem ?

Where to start. How about this:

https://www.eevblog.com/forum/testgear/teardown-extech-ex570-my-first-but-also-last-extech/ One of the connectors fell off when he opened the meter, as in it basically wasn't soldered to the board. Other soldering issues plus the selector switch was already eating into the board.
http://www.eevblog.com/2010/06/15/eevblog-94-near-death-multimeter-experience/ Issue with Extech RC200
http://www.eevblog.com/2010/07/11/eevblog-99-100-multimeter-shootout-extech-amprobe-bk-precision-ideal-uei-uni-t-part-1of2/ issues with the board and attempted repairs.
https://www.eevblog.com/forum/testgear/extech-ex330-quality-issue/ Ex330 quality issues (ON THE REPLACEMENT! The first one was RMA'd and the replacement was bad too!)
https://www.eevblog.com/forum/testgear/extech-mm570a-arrived-as-replacement-for-ex570-but-is-it-better/ Extech sent him an EX570A to replace a bad older one. It had a BLOWN fuse out of the box and fingerprints inside the LCD. Likely an old demo model they sent him.

I can keep going if you wish. How much is CEM and how much is Extech who knows exactly but it appears both have huge QC issues.

I can find more if you wish.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin
 

Offline dannyf

  • Super Contributor
  • ***
  • Posts: 8221
  • Country: 00
Re: Thinking of buying a multimeter.
« Reply #36 on: April 18, 2014, 10:50:10 pm »
Quote
And i tried pushing 23mA thru 1 LED's and it did burn out!

So i just decided to power them at 21,9 mA which was just below their maximum rating...

I would argue that those LEDs are likely "overrated" - they were really <15ma max leds sold to you, unfortunately, as 22ma leds.

Quote
Their maximum supported current was 22mA!

A 22ma rated led would have a life span far greater than 15,000 hours at 15ma - assuming adequate cooling.

================================
https://dannyelectronics.wordpress.com/
 

Offline Flump

  • Frequent Contributor
  • **
  • Posts: 520
  • Country: gb
Re: Thinking of buying a multimeter.
« Reply #37 on: April 18, 2014, 11:03:30 pm »
maybe you should research more before turning people away from specific multimeters
only 1 of your links is pertinent to the ex330 and even that is not a HUGE build issue.

I just had a look in mine and the soldering is fine.
dave didn't notice any problems with the soldering in his ex330 when he did a review and internal look at it
maybe dave and I are blind, or maybe its not the HUGE issue you try to make out it is ?

so are these HUGE build quality issues for the ex330 just founded on that one thread ?

besides it was £20 new, I was not expecting Fluke quality.
 

Offline dannyf

  • Super Contributor
  • ***
  • Posts: 8221
  • Country: 00
Re: Thinking of buying a multimeter.
« Reply #38 on: April 18, 2014, 11:17:13 pm »
Quote
you try to make out it is ?

It is helpful in a discussion not to impute your opponents' "motive". If you disagree with their points of view, just lay out your arguments and let others be the judge.

PedroDaGr8 simply provided her/his point of view. Even if her/his point of view is factually incorrect (I am not sure about that), that by and of itself doesn't mean s/he had the intent to exaggerate.

Talk about the facts, don't talk about the individuals.
================================
https://dannyelectronics.wordpress.com/
 

Offline Flump

  • Frequent Contributor
  • **
  • Posts: 520
  • Country: gb
Re: Thinking of buying a multimeter.
« Reply #39 on: April 18, 2014, 11:37:21 pm »
I agree Danny and maybe i went a bit OTT
and i apologise to you Pedro, no offence meant
but when i read your post it came across as
you just wanted to put down the meter because i bought one.

 

Offline PedroDaGr8

  • Super Contributor
  • ***
  • Posts: 1283
  • Country: us
  • A sociable geek chemist
Re: Thinking of buying a multimeter.
« Reply #40 on: April 20, 2014, 02:24:02 am »
I agree Danny and maybe i went a bit OTT
and i apologise to you Pedro, no offence meant
but when i read your post it came across as
you just wanted to put down the meter because i bought one.



It wasn't at all an attack on you or your purchase. I'm sorry if you felt it was because I didn't intend it that way AT ALL. I was strictly commenting on the fact that Extech and CEM have huge issues with their quality control department. Does this mean every single meter that leaves their factory is a steaming pile of crap? Not at all. It just means that the end user has a higher chance of receiving a defective unit than they would going with another manufacturer. I'm glad to hear that your meter is good and I hope that it serves you well. I just have seen significantly more people burned by Extech than most other manufacturers (almost all by QC issues which should have been caught). The design quality of Extech meters range in quality from rubbish to quite good. This is something I actually don't have a problem with. They design to various price points. The lower end will be crap the upper end will be good. The designs of the upper end Extechs which are rebadged Brymen not CEM, are top notch. The issue is that the quality control isn't there and that lets down what is a great design.

On the qc note. I do not recommend hp laptops to friends for similar issues. One of their worst designs the dv2000 series had a failure rate of around 25%. This still means three out of every four laptops had no issues at all. Meaning the vast majority had no issues. The problem is the buyer has a one in four chance of getting a pile o crap.

Hopefully, this explains my position much more. Sorry again for you feeling like I was attacking you.
« Last Edit: April 20, 2014, 02:27:16 am by PedroDaGr8 »
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin
 

Offline vladdrake

  • Newbie
  • Posts: 4
  • Country: fr
Re: Thinking of buying a multimeter.
« Reply #41 on: July 25, 2014, 05:46:00 pm »
hello everyone,
i've been reading this forum for 3 days continuously and i find it really helpful on so many points !

So i am considering buying one of these multimeters :
They seem quite great for my purpose (rebuild sub ohm coils  with an average of 0.8 ohm for ecigarette, test battery charge and some other basic house stuff), i'm a simple electronic hobbyist.

But i have some questions before choosing, and i'm turning to you, the connoisseurs to help me. Which of these has best features (i think i don't really need a very accurate multimeter) ?

For the moment my preference goes to the UT139C, it measures temperature, has good resolution and accuracy, it's true RMS, has NCV  but it doesn't have a bar graph !
  • Is the bar graph really useful, in what conditions ?
  • Is the UT139C worth the extra bucks compared to the minipa ?
  • And i would like to know if Franky sells better quality leads than those included with the UT139C ?
I really thank you for your help in my decision.
 

Offline iloveelectronics

  • Frequent Contributor
  • **
  • Posts: 940
  • Country: hk
Re: Thinking of buying a multimeter.
« Reply #42 on: July 25, 2014, 05:50:00 pm »

And i would like to know if Franky sells better quality leads than those included with the UT139C ?[/li][/list]


Yes, I do :) http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=171162377470 or http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=171120395118
My email address: franky @ 99centHobbies . com
My eBay store: http://stores.ebay.com/99centhobbies
 

Offline SeanB

  • Super Contributor
  • ***
  • Posts: 16272
  • Country: za
Re: Thinking of buying a multimeter.
« Reply #43 on: July 25, 2014, 05:59:06 pm »
Buy the Brymen leads..........

Do you still have those Fluke leads available Frankie? They are very nice as well.
 

Offline iloveelectronics

  • Frequent Contributor
  • **
  • Posts: 940
  • Country: hk
Re: Thinking of buying a multimeter.
« Reply #44 on: July 25, 2014, 06:02:13 pm »
Do you still have those Fluke leads available Frankie? They are very nice as well.

I might still have a pair of those Chinese Fluke TL175's, but can't be sure right now.
My email address: franky @ 99centHobbies . com
My eBay store: http://stores.ebay.com/99centhobbies
 

Offline vladdrake

  • Newbie
  • Posts: 4
  • Country: fr
Re: Thinking of buying a multimeter.
« Reply #45 on: July 26, 2014, 09:40:49 am »
Thank you Franky for your prompt reply.
Indeed the Brymen one seems to be really good !
I'm looking for good quality leads with low resistance to calculate resistance under 1 ohm.

Anybody to help me with the bar graph feature :) ?
 

Offline Yago

  • Frequent Contributor
  • **
  • Posts: 651
  • Country: gb
Re: Thinking of buying a multimeter.
« Reply #46 on: July 26, 2014, 10:34:28 am »
The LCD numerical display can not be read when there is a slowly changing level measured.
Think of something similar to the turning signal on a car, the meter just reads numbers racing up then down.

The bargraph simulates an analogue meter, and allows you to get some information from measuring such signal.
 

Online mariush

  • Super Contributor
  • ***
  • Posts: 4983
  • Country: ro
  • .
Re: Thinking of buying a multimeter.
« Reply #47 on: July 26, 2014, 11:33:10 am »
On UT61E, the numbers on the screen update 2 times a second (a bit slower than the average multimeter, which does 2.5 to 4 updates a second).  The bargraph updates about 40 times a second.

The bargraph allows you to see if what you measure is stable or not or if you have something that's active for short periods of time. 

For example, let's say you want to see if the tachometer wire of a fan works. You can connect the fan to about 5v, just enough to get the fan spinning slowly and you  you connect the tach wire to that voltage through a pull up resistor. Now you can put the probes on tach wire and ground and you'll see that voltage on the digits most of the time, because it only updates twice a second, but on the bar graph you may see how the tach wire is pulled down to ground twice for each rotation (well, you'll see the bargraph moving more often than the digits)

 

Offline vladdrake

  • Newbie
  • Posts: 4
  • Country: fr
Re: Thinking of buying a multimeter.
« Reply #48 on: July 26, 2014, 01:54:07 pm »
Ok thanks, so not really useful for me i think ^^
 

Offline torr032

  • Regular Contributor
  • *
  • Posts: 88
  • Country: ba
Re: Thinking of buying a multimeter.
« Reply #49 on: July 26, 2014, 04:04:07 pm »
Bar graph comes in handy on the places where constant voltage should be expected, but for some reason it is not and that is causing the device to malfunction. Because of the slow update, numbers on the screen can't show if it is changing rapidly, so you will need a fast bargraph with enough segments at least like on the ut61e. And I would say numbers updates more like a 3 times per second.
« Last Edit: July 26, 2014, 04:14:06 pm by torr032 »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf