Why won't my meter check this?

LED's need more _voltage_ than a diode checker circuit provides. ...Jim Thompson

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| 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.

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

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"

** 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

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

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            -- 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

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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

** It uses a fixed current ( like 1mA) - then it reads the voltage. ** With what ?

** 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" "Phil Allison"

** 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"

** FYI:

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

.... Phil

"John Fields" "Phil Allison"

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

The OP's post says no such thing.

YOU did not FOLLOW the thread.

.... Phil

A dog always smells their own stench first!

Jamie

--
The better to fend off the smart fellers.

Jamie is a Larkin sycophant, _and_ infinitely incompetent... need one say anything more? ...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.

So by your own admission, you'd expect the audience to take that as a highly accurate non conflicting judgment?

I am not "Wag the Dog" but I sure see a few that think they are.

Jamie

Allegedly; the green LED is a pretty good 2V reference.

In the late 80'S one of the Thorn Consumer Electronics TVs used one for a reference on the CRT base panel.

I remember that sort of thing, I don't remember if they were said to be a good reference or just a handy low voltage "zener".

I am reminded of an article that was about a radio synthesizer, and the author suggested using a common switching diode as the varactor, since the runing range was fairly small. But then he warns "I had endless problems with 60Hz modulation, until I realized that the glass cased switching diode was under the desk lamp and was being modulated by it. A switch to a non-clear cased diode, or just a bit of electrical tape, solved the problem".

Michael

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It's just a diode working in quadrant I of the VI curve, so it's no 
better a "reference" than any other forward-biased diode, it just 
happens to conveniently drop 2V with a peculiar current forced 
through it.

Years ago I had a meter that I would forget and leave on, so I modified the circuit with a permanent magnetic latching relay, it would turn off automatically for me, and I simply added a push button when needed to turn it back on, via the other side of the magnetic relay..

I used a 555 timer into a scaler IC.

Jamie