the diode setting on multimeters

ok i realize again this is common knowledge for many but i just dont get why there arnt any units of measure when you connect a diode to the multimeter, and switch it to the setting with the "diode" symbol.

Do they vary from model to model?

its clear its an analog measurement, as i get a wide range of values when i connect the meter to different pairs of pins on the "scanner head" from a scanner. (those long tubular diodes that are transparent encased in the glass below where you place the paper)

Most are a simple go or no go indication.
If current flows it shows and if no current flows it shows open circuit.

Weedpharma

Normally the multimeter displays the forward voltage of the diode with a fixed current
applied (something like 1mA would be likely). This is enough to distinguish which way
round a diode is and more usefully whether its a pn-junction diode or a schottky diode
(which have lower forward voltages). Also the higher current a diode is the lower the
forward voltage (roughly speaking).

Annoyingly multimeters are normally only designed for silicon diodes, and can't measure
the forward voltage of LEDs on this setting, except IR emitters perhaps.

MarkT:
Annoyingly multimeters are normally only designed for silicon diodes, and can't measure
the forward voltage of LEDs on this setting, except IR emitters perhaps.

It is annoying; my Fluke doesn't measure LEDs. I'll try an IR one though.....

MarkT:
Annoyingly multimeters are normally only designed for silicon diodes, and can't measure the forward voltage of LEDs on this setting, except IR emitters perhaps.

Excellent answer - if only you could expurgate that horrid word wrap!

At least if the current lights the LED it is useful..

but i just dont get why there arnt any units of measure when you connect a diode to the multimeter, and switch it to the setting with the "diode" symbol.

Let's think about this question for a minute:

If you select "DIODE" scale, that presupposes that you KNOW what a DIODE is ?
DO YOU ?

If you select DIODE scale, it means you are trying to:

1- test a diode to ascertain if it is faulty (shorted or open)
2- trying to ascertain the forward voltage of an unknown diode.
3- test a led to see which is + lead and which is the - lead (the meter will show a forward voltage. My
Fluke 77 lights the 5 mm red led and displays 1.74 V. If the led lights, one would have to assume it
has reached it's forward voltage , would they not ?)
4- test a transistor
5- test a triac
6- test some other semiconductor component (ie: mosfet)

In all of the above cases, you are EXPECTING a VOLTAGE, are you not ?
Would it seem necessary to display units if there is only ONE type of UNIT you will EVER get on diode scale ?

raschemmel:
In all of the above cases, you are EXPECTING a VOLTAGE, are you not ?
Would it seem necessary to display units if there is only ONE type of UNIT you will EVER get on diode scale ?

Come on, OP started his post with

ok i realize again this is common knowledge for many

and… I know people who thought they’d get Ohm value ;D

no comment

Ohm, ohm on the range ...

:grinning: :grinning: :grinning:

3- test a led to see which is + lead and which is the - lead (the meter will show a forward voltage. My
Fluke 77 lights the 5 mm red led and displays 1.74 V. If the led lights, one would have to assume it
has reached it's forward voltage , would they not ?)

Is that the forward voltage of the diode being tested, or the forward voltage of the Fluke red LED?

Hi, its worth reading the DMM manual, Fluke 77, the display has a range in diode mode of 2.4V.
So any drop over 2.4V will be indicated as an Overrange.
To see what the test voltage or open circuit voltage of a meter in diode test mode, put another meter across the probes and measure the voltage.

Tom..... :slight_smile:

, except IR emitters perhaps.

well like i said i was getting the wide range of 4 digit values on the setting on one of those types of multimeters that like someone said, supplies an arbitary amount of current, (hence lighting up the led serving as an easy check for A&K) only when i connected it up to this scanner head from a cannon printer, (attached a photo)

im guessing seeing it has something to do with an optical device it might be an IR emitter to an extent yes?

Would it seem necessary to display units if there is only ONE type of UNIT you will EVER get on diode scale ?

yes, it SHOULD be necessary for the sake of non-ambiguity.

Ohm, ohm on the range …

::slight_smile:

scannerhead11.png

Hi, can you give us a picture of the device all in frame and in focus.
It looks like it is a flourotube type used to back light large LCD displays, what is the unit that it plugs into look like.
The supply will be a high voltage high frequency supply like use to battery power standard mains powered flourescent tubes. The tubes are more efficient at higher AC frequencies then 50 or 60Hz.

Tom.... :slight_smile:

3- test a led to see which is + lead and which is the - lead (the meter will show a forward voltage. My Fluke 77 lights the 5 mm red led and displays 1.74 V. If the led lights, one would have to assume it

Is that the forward voltage of the diode being tested, or the forward voltage of the Fluke red LED?

I thought it would be obvious I meant the 5 mm led I had the meter leads clipped onto as a UUT LED.
(Unit Under Test).
If I had phrased it "My Fluke 77 lights a 5 mm red led .." you probably would have asked the same
question. No the Fluke 77 doesn't have any leds. All it has is the LCD display. The red lead is what I was testing.

yes, it SHOULD be necessary for the sake of non-ambiguity.

It'a already non-ambiguous. The meter is on Diode scale. Maybe the meter should display this message
when you turn it on:

"HAVE YOU READ THE MANUAL YET ?"

im guessing seeing it has something to do with an optical device it might be an IR emitter to an extent yes?

Like the manual says, the Fluke 77 will test up to a forward voltage of 2.4 V so the scanner head must
have a V f that does not exceed that.

the "diode test" puts like 1mA of current on the leads with a max voltage of 3V
the value that apear on the display is the voltage that settles on the load at 1mA of drive

screwpilot:
the "diode test" puts like 1mA of current on the leads with a max voltage of 3V

Depending on the meter.

Most I know of have a 9V battery, and will therefore put up to about 8V on the terminals. The limitation is on what scale they are set at the time - usually 2V because they are "3½ digits"- so what voltage is over-range.

i too have a couple of 9V powered meters and trust me they limit the max voltage to around 3V for safety reasons
also you can put a 1k resistor on the terminals to know what's the current that your multimeter sources
3V are enough to test a mosfet (you can test a mosfet with a multimeter somehow) and not every multimeter can charge a mosfet's gate enough to close it because they will never output 8V or so

I submit that if you see a led light when you connect the meter to it on diode scale then the voltage displayed IS
the forward voltage.

screwpilot:
i too have a couple of 9V powered meters and trust me they limit the max voltage to around 3V for safety reasons

Hmm. You are right. Now that is a worry, because it means they will not test all LEDs, also because it implies that the voltage limiting circuit will draw more current than it needs to while in "diode test" mode.

Obviously the Fluke is different. :astonished:

Yep - tested - 3.84V on the Fluke 87. Can't beat design! And lit a high-efficiency white LED at 2.638V

I have a pack of really funny dodgy white LEDs here. A number do not light on the LED tester at 2 or 5 mA and read something like 0.3V on the Fluke, but if I give them a "burn" at 20 or 50 mA, they start working properly on the lower currents and read a suitable voltage.

modern red leds will lit decently with the diode test
it's also a good way to determine if your led is an IR one, it will display 1.1V ish