Diode Test

[OpenCircuit]

40U60CT: https://www.diodes.com/assets/Datasheets/SBR40U60CT.pdf

When DMM POS. probe is placed on the common cathode and the negative probe is placed on either anode (terminals 1 and 3) the meter reads "1998" and "1998."
When DMM NEG. "                                                                                                                                                                                                      " "80 "    and  "80."

Could someone please explain this for me?

Thanks.

[helius]

Diode test forces a current through the device under test from + to -, and displays the voltage. It is used to find semiconductor PN junctions (P for +, N for -).
Diodes are just PN junctions (excluding Schottky for now). When current flows from the P side to the N side (the forward direction), it passes through with a small voltage drop (corresponding to some power wasted in the diode). When current is forced in the reverse direction, the diode resists it and the voltage drop will be very high. If sufficient current is forced in reverse, the diode will go into breakdown.
A reading of 80 means that at the test current, 80 [units] of voltage was dropped by the device. Normally a PN junction has a voltage drop of 0.7 V, so this is a strange reading. Maybe your DMM reads in hundredths of a volt?
The reading of 1998 is also strange, but a 3.5 digit meter cannot read any higher than 1999. If there is no actual OL range indication then it may display the maximum reading for voltages exceeding the range.

[OpenCircuit]

The manual does state "3 1/2 Digit LCD wth a maximum reading of 1999: https://www.manualslib.com/manual/1049274/Etek-10709.html?page=3#manual

Odd. I have repeated the above tests and sometimes get a result of "84" and "1" (switching probes) and then the next time I get the odd "84 and "1998" readings. I just don't know what to make of these.

Did a test on a new IN5406 and got "575" and "1". Does this indicate a drop of 57.5v drop? So, my meter knows exactly what it is putting out and then reads what has  made it through the other side of the diode and then displays the difference?

Tried another meter with limited credibility (CEN-TECH) and getting 81/1 (reversing probes) on all 4.

Is there another way I could test them?

[rstofer]

Pick a voltage, say 12V.  Run it into the anode (unmarked) end of the diode.  Connect the cathode to a 1k resistor connected to ground.  A nice series circuit driving 12V into a forward biased diode and then through a resistor.  In theory, the diode will drop 0.7v (measure across it on the DC scale) so the current will be 12-0.7/1k = 11.3 mA.  Measure it using the low current scale.  + test lead to + battery, - test lead to diode anode.  Measure the voltage drop across the resistor (11.3V).

The cathode side of the diode has the horizontal bar in the symbol.  The anode (+) end is unmarked.
 

[OpenCircuit]

Here are my results:
DMM: Pos. probe to A (see diagram) and neg probe to C DMM: .14
DMM: Neg Probe to B and Pos probe to CDMM: 11.82

DMM: Across resistor is 11.81
DMM A B 11.96v
Tried Amp test on 20m setting and get 0.0.0. I do not trust this meter however. I do have a voltage drop of .14.

Diodes: good or bad?
Test: Valid or invalid?

Schematic:

[alsetalokin4017]

It always helps to actually check the Data Sheet.

"Ultra Low Forward Voltage Drop" of 0.55 v at 20 amps,   and a TO-220AB package.... with Anodes pins 1 and 3 and common Cathode pin 2 (and tab). The actual diode symbols are quite hard to see on these things sometimes.

And your Meter instruction manual or data sheet should tell you what "units" it reports in the Diode Check mode.

So if you connect the meter with the Positive (Red) probe to the Anode and the Black (negative) probe to the Cathode, you should read the the forward voltage of around 0.5 to 0.7 volts. More or less depending on lead resistance, the meter's test current, and the day of the week. Connected the other way round you should read "open circuit", however the meter reports that. The Cen-Tech for instance will read "1" on an open circuit or a diode connected with + probe to Cathode and - probe to Anode. 
If the diode is good, that is. If the diode is shorted, you will probably read some low to intermediate voltage which will be the same or similar in both directions. If the diode is open, you will read "1" or whatever your meter reports for open circuit, in both directions.

[OpenCircuit]

It always helps to actually check the Data Sheet.

"Ultra Low Forward Voltage Drop" of 0.55 v at 20 amps,   and a TO-220AB package.... with Anodes pins 1 and 3 and common Cathode pin 2 (and tab). The actual diode symbols are quite hard to see on these things sometimes.

And your Meter instruction manual or data sheet should tell you what "units" it reports in the Diode Check mode.

So if you connect the meter with the Positive (Red) probe to the Anode and the Black (negative) probe to the Cathode, you should read the the forward voltage of around 0.5 to 0.7 volts. More or less depending on lead resistance, the meter's test current, and the day of the week. Connected the other way round you should read "open circuit", however the meter reports that. The Cen-Tech for instance will read "1" on an open circuit or a diode connected with + probe to Cathode and - probe to Anode. 
If the diode is good, that is. If the diode is shorted, you will probably read some low to intermediate voltage which will be the same or similar in both directions. If the diode is open, you will read "1" or whatever your meter reports for open circuit, in both directions.

Bit of Greek in a datasheet for a beginner, but some excellent information inside like internal structure and terminal pinout.

The symbols clearly depict two simple diodes with a common cathode.

Seems you have more confidence in my meter than I. Difficult to interpret results from a meter that does not show a decimal while simultaneously reporting two very different results depending on the "mood" of the IC-or is it the mood of the DMM? I am unable to ascertain. For a beginner, the guess work must be removed. Manual states "approximate voltage drop will be displayed" for diode mode. The piezo buzzer works with a slap and a bang when it wants to. See page 13 for all you will ever want to know about diode testing. This meter is my first problem to get through it seems. Struggle between keeping cost down and sufficient quality for a beginner anyway. Based on a reading of "575" a beginner can only imagine the voltage loss; .575, 5.75 57.5 or maybe even 575.

[rstofer]

Look at the graph on page 2 of the datasheet, lower left corner.  You will have a voltage drop of 0.5V only when you have a current of 20 AMPS!
You meter is putting just a small current through the diode and, if the graph is correct, this will result in a VERY small voltage drop, on the order of 0.1V  This is a very strange diode and it's no wonder you have odd readings.  If your meter said 0.08V, I could believe it, having read the datasheet.

For comparison, look at Figure 4 here:
http://www.vishay.com/docs/88503/1n4001.pdf

Note that the minimum Vf is 0.6V and it increases with current.  That's why everybody just jumps at 0.7V drop.

And, yes, your meter is less than optimal.  But that doesn't mean its wrong.

FWIW, I have never played with such a device and I have no idea what I'm talking about.  I just look at the graphs and smile.

[Kleinstein]

The usual meters read voltage in mV, when in diode test. The actual current with the test varies between meters, usually around 0.1 - 1 mA.
With the very large schottky diode it seems to be 80 mV in forward direction and 1980 mV in revers - schottky diodes tend to have a rather high leakage current.

[macboy]

Normally, with a silicon diode, you will always measure a very high resistance in the reverse direction, usually resulting in over-limit on the DMM, whether that's "OL" or "1   ", or whatever. This is due to silicon rectifier's very low reverse leakage current.

Schottky diodes are much more leaky. This can result in a lower apparent reverse resistance. This particular diode's specs look like some proprietary variation of the Schottky, as it has significant reverse leakage current. That is the reason for the reverse reading that isn't simply "over range". Your reading looks normal, there is nothing wrong with your meter, nor your technique, nor your diode.