Can't test LED with HP34401A

[wmundstock]

Hi All.

I got an old HP 34401A that is quite in decent shape. I have been learning, testing, cleaning for the past couple of days and something caught my attention.

If I put it on diode test mode, I can test regular diodes and transistors just fine, but if I try to probe a LED it just shows as open. The led itself light up a just like with other multimeters, but it keeps reading OPEN.
Am I doing something wrong? Is this expected?
If so, I would like to learn why.

Thank you!

[TimFox]

Most voltmeters' diode-test mode comprises a current source feeding the terminals, and then measuring the voltage across the terminals.
Usual silicon PN diodes will to up to roughly 0.7 V with this test, so the full-scale setting is appropriate to that voltage.
GaAs and other LEDs have substantially higher forward voltage at typical test currents, so the diode may light but the meter is programmed to indicate off-scale or overload at typical voltages.

[BrokenYugo]

To expand on TimFox's post IIRC typical DMM diode mode is 3V 1mA, you can easily measure the open circuit voltage and short circuit current yourself with a second DMM. If you look at the I/V graph (or plot it yourself) of a typical blue/white LED you'll see what's happening, the diode will pass enough current to make it visibly light up, but not enough to drag down the current source and give a reading. It's a design tradeoff made long before such LEDs existed to avoid damaging delicate transistors.

[AVGresponding]

The open circuit voltage isn't necessarily helpful; the Fluke 87V has an ocv of over 7 volts but it won't read past 3 volts, which is its full scale.

The 34401A manual states full scale as 1.0000V, so it's not going to give a reading above that, regardless of what the ocv is.

[wmundstock]

Thanks for the responses.

From what I am reading, it seems that is "by design" especially because the meter was designed with other "requirements" in mind.

Here is what is bugging me:

I did a comparison between the Rigol and the HP multimeters.
I ran the same test for both as follows:

Measure voltage across probe when DUT is set to diode:
Rigol: 5.7V
HP: 6.85

Measure of Current in series while testing the LED:
Rigol: 1mA (in fact a bit less 0.995, maybe due to burden load)
HP: 1mA (in fact a bit less 0.995, maybe due to burden load)

So my conclusion is that even though both meters are generating in enough voltage/current to be able to measure, there must be a threshold setting and the voltage drop of a led is outside the range programmed in the meter. I think that is what TimFox was referring to.

so the diode may light but the meter is programmed to indicate off-scale or overload at typical voltages.

In fact, I just referred to the manual one more time, and it is actually stated but I did not know how to read it.

To Check Diodes
Test current source: 1 mA
Maximum resolution: 100 μV (range is fixed at 1 Vdc)
Beeper threshold: 0.3 volts ≤ Vmeasured ≤ 0.8 volts (not adjustable)

I am thinking this means that the beeper will beep as long as the voltage in which the DUT starts conducting is between 0.3 and 0.8.
In the curve tracer I can see that the led I am testing starts conducting at ~1.5(outside the threshold) were the diode conducts at around 0.5V which is in the threshold.

Did I get this right?

Not sure how to read the specs on the Rigol:
Test Current Source: 1 mA, 100 μA, 10 μA, 2 μA, 200 nA
Voltage Measurement Range: -0.2 V ~2.2 V, -0.2 V ~4.2 V, -0.2 V ~4.8 V, -0.2 V
~5.2 V, -0.2 V ~5.5 V
Max Resolution: 100 μV
Input Protection: 1000 V Input Protection.
Open-circuit Voltage: < 8 V

Thanks!

[tautech]

Get the right tool for the job.

[wmundstock]

Get the right tool for the job.

Thank you for the reply tautech, I do have other meters that are capable of doing this test, however, because I recently bought a 34401a, I wanted to test and learn its specifics .

[TimFox]

As you found in the manual, the programming for the 34401A will recognize a diode if 0.3 V < V < 0.8 V.
This allows the machine to distinguish between
  a short circuit with 0.3 V > V
  a diode with 0.3 V < V < 0.8 V  and
  an open circuit with 0.8 V < V
The physical voltage with V > 0.8 V at 1 mA will be limited by the design of the current source, but is roughly 7 V max.
These three possibilities are a reasonable set of conditions for a good or damaged silicon PN diode.

[Kean]

In fact, I just referred to the manual one more time, and it is actually stated but I did not know how to read it.

To Check Diodes
Test current source: 1 mA
Maximum resolution: 100 μV (range is fixed at 1 Vdc)
Beeper threshold: 0.3 volts ≤ Vmeasured ≤ 0.8 volts (not adjustable)

I adjusted your bold highlighting.  The text now in bold is the key thing.
In diode mode, the measurement range is fixed at 1VDC - so this meter cannot measure Vf beyond 1V.

[wmundstock]

Thanks for the replies, that clears out my questions.