Why won't my meter check this?

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

 

[Jim Thompson]

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 | |
| Voice480)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.

 

[Michael Black]

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

 

[Maynard A. Philbrook Jr.]

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

 

[Phil Allison]

"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

 

[Frnak McKenney]

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

 

[Maynard A. Philbrook Jr.]

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

 

[Phil Allison]

<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

 

[John Fields]

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

 

[Phil Allison]

"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???


..... Phil

 

[John Larkin]

On Thu, 27 Mar 2014 18:16:15 +1100, "Phil Allison" <phil_a@tpg.com.au> wrote:

"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???


.... Phil

Put the meter on the diode test range.

Connect it to an electrolytic capacitor for an hour maybe. 100 uF ballpark.

Disconnect the cap, switch the meter to voltage, and measure the cap voltage.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation

 

[Phil Hobbs]

On 3/27/2014 11:25 AM, John Larkin wrote:

On Thu, 27 Mar 2014 18:16:15 +1100, "Phil Allison" <phil_a@tpg.com.au> wrote:


"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???


.... Phil



Put the meter on the diode test range.

Connect it to an electrolytic capacitor for an hour maybe. 100 uF ballpark.

Disconnect the cap, switch the meter to voltage, and measure the cap voltage.

It's quite reasonable to measure the open-circuit voltage coming from
the diode test circuit, to get an upper limit for the voltage
compliance. If it's 1.5V open circuit, you aren't going to be measuring
many LEDs.

My 25-year-old Fluke 87 is 3.5V open circuit, but it won't measure
diodes past about 2.2V.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Allison]

"John Larkin"

Put the meter on the diode test range.

Connect it to an electrolytic capacitor for an hour maybe. 100 uF
ballpark.

Disconnect the cap, switch the meter to voltage, and measure the cap
voltage.

** FYI:

a 4.7uF film cap does the job very nicely in a couple of seconds.



..... Phil

 

[John Fields]

On Thu, 27 Mar 2014 18:16:15 +1100, "Phil Allison"
<phil_a@tpg.com.au> wrote:

"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???


.... Phil

---
Capito, but what I read was that he was going to test whether the
current was fixed or not and - since he said he was going to check
it - he must have had some means to do so, like another meter.

In any case, the earlier sketch was meant to convey that the test
could be done by varying R and watching the output of the
milliammeter.

Just for fun, and because of Phil Hobbs' related post in this
thread, I decided to find out what's up with the diode test
circuitry in a fairly high-end meter, the Fluke 4060A

Here's the setup:
LOAD
Vin VOLTAGE
\ +-------+
+--------------->|+ |
| LOAD | 20VDC |
+-----[R]-----+->|- |
| Iload--> | +-------+
| | WAVETEK
| | 27XT
| |
CURRENT | SOURCE | LOAD
SOURCE | VOLTAGE | CURRENT
+-------+ | +-------+ | +-------+
|DIODE +|<-+->|+ | +->|+ |
| TEST | | 20VDC | | 2mADC |
| -|<-+->|- | +->|- |
+-------+ | +-------+ | +-------+
FLUKE | FLUKE | CSI
4060A | 4050 | DMR250
+-------------+

And here's the data

Vin Vload Rload Iload
-------|-------|-------|-------
0.094 0.000 0 0.940
0.282 0.188 200 0.941
0.470 0.376 400 0.941
0.658 0.564 600 0.941
0.842 0.749 800 0.941
1.026 0.934 1000 0.940
1.948 1.854 2000 0.940
2.874 2.78 3000 0.938
3.781 3.69 4000 0.934
4.620 4.53 5000 0.917
5.263 5.17 6000 0.874
5.561 5.58 7000 0.808
5.739 5.86 8000 0.744
6.157 6.09 9000 0.686
6.344 6.28 10000 0.632

 

[Phil Allison]

"John Fields"
"Phil Allison"

"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???



---
Capito, but what I read was that he was going to test whether the
current was fixed or not

** But that is a very obvious *misreading*.

The OP's post says no such thing.

YOU did not FOLLOW the thread.



..... Phil

 

[John Fields]

On Sat, 29 Mar 2014 01:07:21 +1100, "Phil Allison"
<phil_a@tpg.com.au> wrote:

"John Fields"
"Phil Allison"
"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???



---
Capito, but what I read was that he was going to test whether the
current was fixed or not


** But that is a very obvious *misreading*.

The OP's post says no such thing.

YOU did not FOLLOW the thread.



.... Phil

---
The part of the thread that I was interested in responding to
started with the op saying:

"I don't know what the voltage diode test function uses."

Your response was:

"It uses a fixed current ( like 1mA) - then it reads the voltage."

So, someone casually reading the post would conclude that you had
answered his question, and that the question he asked had to do with
current.

After all, that's what your response assumed, yes?

Now, with the gist of the thread leaning toward current, and the OP
stating: "I'll check it." the assumption is that he has some means
of checking whatever's being talked about, which is, as far as I'm
concerned, the current being pushed through the diode.

Consequently, I posted a circuit which would allow him to vary the
current through the diode while watching the drop across it on the
meter supplying the current.

What's wrong with that?

 

[John Fields]

On Fri, 28 Mar 2014 09:57:53 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 29 Mar 2014 01:07:21 +1100, "Phil Allison"
phil_a@tpg.com.au> wrote:


"John Fields"
"Phil Allison"
"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???



---
Capito, but what I read was that he was going to test whether the
current was fixed or not


** But that is a very obvious *misreading*.

The OP's post says no such thing.

YOU did not FOLLOW the thread.



.... Phil

---
The part of the thread that I was interested in responding to
started with the op saying:

"I don't know what the voltage diode test function uses."

Your response was:

"It uses a fixed current ( like 1mA) - then it reads the voltage."

So, someone casually reading the post would conclude that you had
answered his question, and that the question he asked had to do with
current.

After all, that's what your response assumed, yes?

Now, with the gist of the thread leaning toward current, and the OP
stating: "I'll check it." the assumption is that he has some means
of checking whatever's being talked about, which is, as far as I'm
concerned, the current being pushed through the diode.

Consequently, I posted a circuit which would allow him to vary the
current through the diode while watching the drop across it on the
meter supplying the current.

What's wrong with that?

---
Oops...

I just spotted a glaring technical error in the next to last
sentence; can you tell what it is?

 

[John Larkin]

On Thu, 27 Mar 2014 21:05:14 -0400, Phil Hobbs
<hobbs@electrooptical.net> wrote:

On 3/27/2014 11:25 AM, John Larkin wrote:
On Thu, 27 Mar 2014 18:16:15 +1100, "Phil Allison" <phil_a@tpg.com.au> wrote:


"John Fields"
"Phil Allison"

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

** You over snipped the post and CHANGED the meaning !!!

The OP said he was going test the *VOLTAGE* of his Extech meter.

I asked " with what " - because he seems to have only one meter.

Capice ???


.... Phil



Put the meter on the diode test range.

Connect it to an electrolytic capacitor for an hour maybe. 100 uF ballpark.

Disconnect the cap, switch the meter to voltage, and measure the cap voltage.



It's quite reasonable to measure the open-circuit voltage coming from
the diode test circuit, to get an upper limit for the voltage
compliance. If it's 1.5V open circuit, you aren't going to be measuring
many LEDs.

My 25-year-old Fluke 87 is 3.5V open circuit, but it won't measure
diodes past about 2.2V.

Cheers

Phil Hobbs

My 8845A is 5, 9, and 12 volts OC (onto a 10M handheld) on various ohm
ranges, and 6 volts on the diode check range.

It measures a white Cree led as 2.59 volts, and lights it up pretty
bright. Short-circuit current is 1 mA, apparently a constant-current
source.

Probably, newer DVMs are designed to check LEDs.




--

John Larkin Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Maynard A. Philbrook Jr.]

In article <7m3bj91kjksudlthedibu14djgja9a3mq4@4ax.com>,
jfields@austininstruments.com says...

Consequently, I posted a circuit which would allow him to vary the
current through the diode while watching the drop across it on the
meter supplying the current.

What's wrong with that?

---
Oops...

I just spotted a glaring technical error in the next to last
sentence; can you tell what it is?

A dog always smells their own stench first!

Jamie