​Diode Measuring Voltage Drop

[Trader]

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"​In general, a ​digital multimeter with a maximum test voltage of 4V is good enough for electronics work but an even higher upper limit for the test voltage is always preferred." https://www.approachlabs.com/best-digital-multimeter-for-electronics-work-and-for-the-electronics-hobbyist/

My DMM can measure Voltage Drop at a maximum of 1.8V.  I can measure Red LEDs only (1.77V), for the other colors the LED turns on a weak light and the DMM shows "OL".  I wonder, this capability of measuring Voltage Drop above 1.8V, it is really important? Since the regular silicon diode drops only 0.7V, when will be necessary to measure Voltage Drop above 1.8V? Thank you so much.

[hamster_nz]

Different LEDs have different forward voltages, either because they use 'exotic' materials to generate the desired colors, or because they internally are multiple LEDs in series.

The meter measures up to 1.8V for testing darlington transistors, which have twice the normal base-emitter voltage of a standard silicon transistor.

If measuring such LEDs was generally important, it would be a standard feature.

I would most likely make a small test circuit to enable measurement if it was important.

[magic]

You will need it for blue LEDs

If you are happy with your DMM as-is then no problem. Lower open circuit voltage may actually be safer for some unusually sensitive components.

[Trader]

Thank you for explaining. Some multimeters reviews check how many Diodes in Series can be read. My DMM can read up to three 1N4007 in series, my concern is if some circuits will be necessary to read more.

My DMM shows 2.95V in Diode mode, I realized that some DMMs show from 3.3V (can read 5 series-diodes) up to 6V. For what application/situation is this necessary? Maybe to test other components? (transistors?)

[magic]

Finding differences between things is what reviewers do to earn their living, if you know what I mean

If you find more than three 1N4007 in series, you can always test them on by one. And the most 1N4007 I have seen in series so far was two.

LEDs might be a legitimate reason to desire about 4V testing voltage. I wouldn't want more than 5V because that can damage BE junctions of transistors, if applied in reverse. Some RF components can supposedly be damaged even by 4V already. It all depends on what you need.

If you you are looking for a pretext to buy a more expensive DMM, the Test Equipment Anonymous thread may be a better place to ask. It truly all depends on what it is that you need, nothing more. Stop caring what others do.

[jmelson]

My DMM can measure Voltage Drop at a maximum of 1.8V.  I can measure Red LEDs only (1.77V), for the other colors the LED turns on a weak light and the DMM shows "OL".  I wonder, this capability of measuring Voltage Drop above 1.8V, it is really important? Since the regular silicon diode drops only 0.7V, when will be necessary to measure Voltage Drop above 1.8V? Thank you so much.

Well, that's a crummy meter.  It delivers enough voltage to light the LED, but won't give you the reading!  My meter will light a white LED at about 3.5 V, and give me the correct reading.  I think it goes up to 3.8 V or so, open circuit, and shows that reading, too.

Jon

[David Hess]

Most of my multimeters diode test to about 3 volts but newer ones tend to be higher to support blue LEDs.

The meter measures up to 1.8V for testing darlington transistors, which have twice the normal base-emitter voltage of a standard silicon transistor.

All Darlington transistors include a low value base-emitter shunt resistor on the output transistor so at normal test currents, the second base-emitter junction looks shorted anyway.

[greenpossum]

The other thing to realise is that even if the DMM can put out the voltage, the test current will be much smaller than normal use so the voltage drop won't be typical, it will be lower. To do it right you should use a constant current supply to mimic the actual use and measure the voltage across the device.

[bdunham7]

There was an recent discussion on this that may help.

https://www.eevblog.com/forum/projects/is-it-safe-to-test-for-continuity-with-multimeter/msg3139178/#msg3139178

The answer is you need multiple multimeters! 

My bench multimeters tend to use higher test currents and voltages for ohms, diode and continuity, but are more precise for checking components.  I suspect this is pretty much standard nowadays.  One of them I have has selectable diode check voltage range (5/10V) and test current (1ma/100uA) and I was actually able to test a 6-LED array at 7.8V without using a power supply.  But that same meter, even at 5V, might be too much for in-circuit testing.

Surprisingly, of the ones I checked, the ultra-cheap (free at Harbor Freight if you watch for coupons) D830 seems to be the safest* for in circuit testing, having open circuit voltages of only 250mV on most ohms ranges, ~2.6V on 200R and diode check.

*safest for the circuits, the meter is not particularly safe for people when used on live circuits...

[bdunham7]

The other thing to realise is that even if the DMM can put out the voltage, the test current will be much smaller than normal use so the voltage drop won't be typical, it will be lower. To do it right you should use a constant current supply to mimic the actual use and measure the voltage across the device.

Many good meters use a standard 1mA test current, which corresponds to the datasheet forward voltage rating for many diodes.  Of course, RTFM.

[greenpossum]

Many good meters use a standard 1mA test current, which corresponds to the datasheet forward voltage rating for many diodes.  Of course, RTFM.

Of course that assumes that there is even a FM to read for said meter.

[bdunham7]

Of course that assumes that there is even a FM to read for said meter.

I would argue that no manual would exclude it from the 'good' category.

In any case, even the 'free' D830 clone has one:

https://manuals.harborfreight.com/manuals/63000-63999/63604.pdf

but it doesn't list the diode test current, so of course I had to check (this is another reason everyone needs at least 3 DMMs) and.... it's about 1mA.  D830 is lookin' good!

[Trader]

My bench multimeters tend to use higher test currents and voltages for ohms, diode and continuity, but are more precise for checking components.

Surprisingly, of the ones I checked, the ultra-cheap (free at Harbor Freight if you watch for coupons) D830 seems to be the safest* for in circuit testing, having open circuit voltages of only 250mV on most ohms ranges, ~2.6V on 200R and diode check.

*safest for the circuits, the meter is not particularly safe for people when used on live circuits...

My DMM voltages: Ohms (219mV), Diode and Continuity (2.93V).

[madires]

All Darlington transistors include a low value base-emitter shunt resistor on the output transistor so at normal test currents, the second base-emitter junction looks shorted anyway.

I beg to differ. Some do, but not all. For example, V_BE of a BC516 or BC517 is typically about 1.15 V (datasheet states max. 1.40 V for V_BE_on).

[magic]

In any case, even the 'free' D830 clone has one:

https://manuals.harborfreight.com/manuals/63000-63999/63604.pdf

but it doesn't list the diode test current, so of course I had to check (this is another reason everyone needs at least 3 DMMs) and.... it's about 1mA.  D830 is lookin' good!

Haha, you don't know even half the truth about your DMM.

The approximate forward voltage drop of the diode will be displayed in mV.

This wording very much reminds me of something.

Connect an alkaline/NiMH/similar battery to the meter and switch alternately between '2000mV' and 'diode'.
 

[bdunham7]

Haha, you don't know even half the truth about your DMM.

The approximate forward voltage drop of the diode will be displayed in mV.

This wording very much reminds me of something.

Connect an alkaline/NiMH/similar battery to the meter and switch alternately between '2000mV' and 'diode'.
 

Yep, they've tweaked it to approximate the voltage drop on a standard diode.  They didn't lie!  In the sub-$5 DMM category you have to expect some limitations...

[David Hess]

All Darlington transistors include a low value base-emitter shunt resistor on the output transistor so at normal test currents, the second base-emitter junction looks shorted anyway.

I beg to differ. Some do, but not all. For example, V_BE of a BC516 or BC517 is typically about 1.15 V (datasheet states max. 1.40 V for V_BE_on).

Usually even small signal Darlintons have the base-emitter shunt resistor to remove charge from the output transistor but maybe those are exceptions.  For small signal devices, the test current may be high enough that the output emitter junction conducts anyway.  A graph of the voltage versus current would reveal the resistance if it is there.