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Why won't my meter check this?

Started by Unknown March 25, 2014
I have a DVM made by Extech that has the diode check function. When
checking a regular diode the function works. But when I try an LED it
shows open both ways. And when I try to measure resistance the meter
shows open both ways. Is it possible that the LEDs just need more
current before than the meter provides they will conduct?
Thanks,
Eric

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On Tue, 25 Mar 2014 10:07:19 -0700, etpm@whidbey.com wrote:

>I have a DVM made by Extech that has the diode check function. When >checking a regular diode the function works. But when I try an LED it >shows open both ways. And when I try to measure resistance the meter >shows open both ways. Is it possible that the LEDs just need more >current before than the meter provides they will conduct? >Thanks, >Eric > >--- >This email is free from viruses and malware because avast! Antivirus protection is active. >http://www.avast.com
LED's need more _voltage_ than a diode checker circuit provides. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Tue, 25 Mar 2014, etpm@whidbey.com wrote:

> I have a DVM made by Extech that has the diode check function. When > checking a regular diode the function works. But when I try an LED it > shows open both ways. And when I try to measure resistance the meter > shows open both ways. Is it possible that the LEDs just need more > current before than the meter provides they will conduct? > Thanks, > Eric >
LEDs need more voltage, and the more esoteric the color, the higher the voltage. Back when LEDs that we'd get were of dubious quality, I got into the habit of using a 9v battery with a 1K current limiting resistor to test LEDs. If it doesnt' light one way, reverse it and see. This makes sure the LED lights (back then it was possible for the LED to be a complete reject), make sure it's bright enough (again you might have to go through a few to find one that was right, especially if you had green or orange or yellow, that weren't were pretty bad in the early days), and make sure you got the polarity right (rejects might be rejected because the die was in the LED in reverse). Michael
In article <0md3j9tkgokf5mktcrenbbpaligcpbpv01@4ax.com>, 
etpm@whidbey.com says...
> > I have a DVM made by Extech that has the diode check function. When > checking a regular diode the function works. But when I try an LED it > shows open both ways. And when I try to measure resistance the meter > shows open both ways. Is it possible that the LEDs just need more > current before than the meter provides they will conduct? > Thanks, > Eric >
More like, more voltage. And it depends on what kind fo LED you were testing? Some have greater forward voltage than most DMM's in diode mode. Jamie
"Jim Thompson"
> > >>I have a DVM made by Extech that has the diode check function. When >>checking a regular diode the function works. But when I try an LED it >>shows open both ways. And when I try to measure resistance the meter >>shows open both ways. Is it possible that the LEDs just need more >>current before than the meter provides they will conduct? >>Thanks, >>Eric > > LED's need more _voltage_ than a diode checker circuit provides.
** Both my DMMs, a 29 year old Metex and a 20 year old Fluke series 70, show correct readings with LEDs - except for blue/white ones. Open voltage is 3V on the Metex and 2.5V on the Fluke. The OP must have a low end Extech with only 1.5 volts. .... Phil
On Tue, 25 Mar 2014 19:38:54 -0500, Maynard A. Philbrook Jr. <jamie_ka1lpa@charter.net> wrote:
> In article <0md3j9tkgokf5mktcrenbbpaligcpbpv01@4ax.com>, > etpm@whidbey.com says... >> >> I have a DVM made by Extech that has the diode check function. When >> checking a regular diode the function works. But when I try an LED it >> shows open both ways. And when I try to measure resistance the meter >> shows open both ways. Is it possible that the LEDs just need more >> current before than the meter provides they will conduct? >> Thanks, >> Eric > > More like, more voltage. > > And it depends on what kind fo LED you were testing? Some have > greater forward voltage than most DMM's in diode mode. > > Jamie
Eric, From the Department of Silly Questions: What _is_ the voltage your meter uses to test diodes? That is, and assuming you have more than one meter, if you set the other meter on Volts and connect it to the Diode Test function points, what reading do you get? Jes' curious. Frank McKenney -- The overwhelming message taught in American schools, public and private, is that no group is superior to any other. In America, embracing yourself as you are -- feeling secure about yourself -- is supposed to be the key to a successful life. People who don't live in the present are missing out on happiness and life itself. Whatever kernels of truth may underlie these propositions, the irony is this: America still rewards people who don't buy into them with wealth, prestige, and power. -- Amy Chua, Jed Rubenfeld / The Triple Package -- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney aatt mindspring ddoott com
On Wed, 26 Mar 2014 09:07:00 -0500, Frnak McKenney
<frnak@far.from.the.madding.crowd.com> wrote:

>On Tue, 25 Mar 2014 19:38:54 -0500, Maynard A. Philbrook Jr. <jamie_ka1lpa@charter.net> wrote: >> In article <0md3j9tkgokf5mktcrenbbpaligcpbpv01@4ax.com>, >> etpm@whidbey.com says... >>> >>> I have a DVM made by Extech that has the diode check function. When >>> checking a regular diode the function works. But when I try an LED it >>> shows open both ways. And when I try to measure resistance the meter >>> shows open both ways. Is it possible that the LEDs just need more >>> current before than the meter provides they will conduct? >>> Thanks, >>> Eric >> >> More like, more voltage. >> >> And it depends on what kind fo LED you were testing? Some have >> greater forward voltage than most DMM's in diode mode. >> >> Jamie > >Eric, > >From the Department of Silly Questions: What _is_ the voltage your meter >uses to test diodes? > >That is, and assuming you have more than one meter, if you set the other >meter on Volts and connect it to the Diode Test function points, what >reading do you get? > >Jes' curious. > > >Frank McKenney >-- > The overwhelming message taught in American schools, public and > private, is that no group is superior to any other. In America, > embracing yourself as you are -- feeling secure about yourself -- is > supposed to be the key to a successful life. People who don't live in > the present are missing out on happiness and life itself. Whatever > kernels of truth may underlie these propositions, the irony is this: > America still rewards people who don't buy into them with wealth, > prestige, and power. > > -- Amy Chua, Jed Rubenfeld / The Triple Package
Thanks everybody who posted replies to my question. I replied yesterday but today I don't see that reply and my outbox says it went out OK. I don't know what the voltage diode test function uses. I'll check it. It seems that the voltage used for resistance must also be low because testing LEDs for resistance shows open both directions. Eric --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com
In article <c4u5j9peodeiqcon2jqqulv546g87n78i7@4ax.com>, 
etpm@whidbey.com says...
> Thanks everybody who posted replies to my question. I replied > yesterday but today I don't see that reply and my outbox says it went > out OK. I don't know what the voltage diode test function uses. I'll > check it. It seems that the voltage used for resistance must also be > low because testing LEDs for resistance shows open both directions. > Eric > > >
DMM's use very low voltage so not to damage any circuits while using it on aboard, also helps to prevent circuits from being biased and counter acting your readings in circuit. P.S. Diode test in low voltage circuits should be done with caution and don't forget to turn it off when you resume normal operations. Jamie
<etpm@whidbey.com>
> > I don't know what the voltage diode test function uses.
** It uses a fixed current ( like 1mA) - then it reads the voltage.
> I'll check it.
** With what ?
> It seems that the voltage used for resistance must also be > low
** Duh ?? That is WHY there is a separate diode test function on many DMMs. FFS look up the specifications for your meter. We cannot because YOU have not supplied the model number. .... Phil
On Thu, 27 Mar 2014 14:12:20 +1100, "Phil Allison"
<phil_a@tpg.com.au> wrote:

> ><etpm@whidbey.com> >> >> I don't know what the voltage diode test function uses. > > >** It uses a fixed current ( like 1mA) - then it reads the voltage. > >> I'll check it. > >** With what ?
--- +------ + +------+ | -->||---[R]---|mA | | | | | | COM|---------|COM | +-------+ +------+ DIODE 2mA TEST John Fields