meter with continuity test and diode test on same setting

The diode is good with those indications. That .566 is the drop the meter measures at whatever current it's producing.

Ed

In the diode test mode the meter puts a fixed current through the diode (usually 1mA) and the meter displays the voltage drop of the diode. The display should show 0.566 and not just "566". If it shows "566" then you are probably still in continuity mode instead of diode mode, in which case it's trying to show some meaningless resistance value. You usually have to push a button (usually a different colour unmarked button) to switch between continuity and diode mode. Many meters will show either a diode symbol on the screen when in diode mode, and an "audio" type symbol when in continuity mode.

Dave.

I don't know.

But my DMM has a diode position, and the ".7v" is typical or theoretical and is bound to not be exact. I do know on my meter that it does what is needed, show whether a diode is good, and will show a different reading for diodes with a different voltage drop. So germanium and schottky diodes will show quite different readings when measured, enough so it's clear which are which.

A good diode will show infinite in one direction, since it can't conduct that way, and some finite reading the other way.

Try some known diodes, and get used to what your meter shows with them, so you can get an idea of what to expect for unknown diodes.

Michael

Internally a DMM uses one of it's scales as a direct voltage reading to represent ohms. In cases like you're seeing, it's the net results of that scale reading the average .6 volts you get with diodes which most likely in your meter is translating to 566 Ohms. that does not mean it's actually 566 ohms.. you just got to be aware of the component you're testing and not assume ohms.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

  SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
  THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

no such button to switch from continuity to diode test ... I read some smallish diodes that are known to be good getting smaller readings on those than bigger ones so logically it's not measuring voltage drop ... hm. wish I had the manual ...

I am willing to bet you a dollar that it is displaying millivolts of diode drop (usually up to about 2000, before it switches to the open circuit, off scale display). The smaller diodes may be Schottkys or germanium, which have lower forward drop. If you can borrow a second meter, you can measure the test current, and the millivolts across the diode under test, for a comparison.

--
Regards,

John Popelish

--
Why not force a small current (say 1mA) through the diodes, read the
voltage drop across them with the voltmeter, and then compare that
reading with what you get on your diode/continuity test?  I bet
it\'ll be close.

man so much to learn, I had discounted germanium diodes because the number on the meter was "160" which apparently could be a schottky (part I was unaware of) ... I'll have to do more testing.

thank you

Which brand and model of meter are you using? If it really is a combined diode/continuity mode then the display is probably in millivolts. But this is quite indeed, most meters do not work like this.

Dave.

. . .

--
Quite indeed indeed! ;)

it really is unbranded, I got it years ago from web-tronics.com as a freebee when I bought a bunch of parts for some (probably failed) project. (long time electronics geek wanna be, fairly common for CS guys I think) the only marking on it at all is "CCL " followed by a 9 digit number. not helpful.

I still have to do some tests but I probably don't have time tonight.

Well, no, not 566 and not translating to ohms. The reading is .566, and it is displaying volts, not translating into ohms. It is displaying the voltage drop across the diode at whatever current the meter is producing.

Ed

And if you can't get a second meter, just use a resistor with the meter you have. Divide voltage (displayed on meter) by the resistance to see what the test current is.

Tried the following on my own meter (Extech model 380282), which also has a "diode + continuity" combined setting:

With a 1k resistor, got 0.669 volts. It's really 0.985 k-ohms, so the test current seems to be 0.68 mA (that's 0.669/0.985, and I'm not sure how accurate the 3rd digit is)

Next I tried it with 100 ohms (actually measures 100.3). Now the meter beeps, and the reading is 0.085 V. So for this lower resistance the test current is 0.85 mA, somewhat larger than before.

So we can conclude that:

1. The "test current" is not a rigid fixed value, but is somewhat less than 1 mA, and works good enough for diode/continuity tests.
2. My meter beeps to indicate "good continuity" somewhere between 100 and 1000 ohms.

The OP might try this on his meter and let us know the result. (taking his meter readings in mV)

Mark

Do we have an english problem here ? or are you just one those that must insist on starting something. Most Auto range meters will display that as .566 in the K scale range designated for DIODE test. Why do you think I texted it that way? .566 would be 566 ohms on these scales.

I won't get into it any deeper on the technical end of it as why , because it's obvious the major audience does not care about a trivial matter in this case and understandably so.

If it wasn't your intention to instigate, then I apologize.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

  SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
  THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

You have gotten me to doubt my experience and assumptions on this. I have two cheap digital multi-meters, a GE2524 and a DT-830B.

I used each one to measure the open circuit voltage and short circuit current from the other on its diode test range. I then calculated a possible internal current limiting resistance.

GE2524 2.64 V 1.31 mA 2.02 kohms DT-830B 2.92 V 1.18 mA 2.47 kohms

Then I measured a diode junction with each meter while measuring the test current and voltage drop with the other meter. Here is the meter reading (with no decimal point showing), junction voltage and junction current and calculated diode resistance at that operating point.

GE2524 487 .518 V 1.06 mA 488 ohms DT-830B 526 .514 V 0.97 mA 529 ohms

So it looks like both these meters read out whole ohms, not millivolts, as I have been assuming, on the diode test scale.

The main difference between the diode test scale and the

2000 ohm range is that that scale supplies a much lower open circuit voltage:

GE2524 295 mV DT-830B 141.5 mV

Either of those ranges will allow pretty good resistance measurements without forward biasing silicon PN junctions very much.

Thank you for inspiring me to get to know some of my test equipment better.

--
Regards,

John Popelish

I don't think so. We simply disagree. I think what you posted is wrong, and apparently you think what I posted is wrong.

or are you just one those that

Nope. I dislike the posts where the issue is lost and ad hominum junk takes its place.

Probably not relevant whether it is auto ranging or not, but the op did not say he has an Auto range meter. He said he had an unbranded meter he got from Webtronics and the only marking on it at all is "CCL " followed by a 9 digit number. What he said makes the next paragraph relevant.

I have what is probably the same meter, which I got from them the same way he did - as a freebie when he ordered a bunch of parts. The one in my hand right now has CCL031017886 on it, which I assume is the serial number. I have at least one more from them, also received for free when I ordered a bunch of stuff.

What I think of your reason for texting that way is not relevant.

On the Webtronics meter, assuming it is the same as his, the reading has no decimal point. It shows up as 566. When you use a second meter in parallel with the diode testing meter and set the second meter to read voltage, it reads .566, or very close to it. My readings were 570 on the Webtronics, and .575 volts on the second meter set to volts. Incidentally, the second meter in diode test reads .572 with the decimal point, and puts a V on the screen to indicate it is displaying voltage.

Probably true. Considering the information value of the reading, however, is not something that should be so easily dismissed. On the one hand, we have information that says, if you treat it as resistance, this diode has

566 ohms resistance. On the other hand, we have information that says, if you treat it as voltage, this diode drops .566 volts. In both cases, the value is dependent on the current in the circuit - an unknown. But .566 volt drop is much closer to the actual drop that will appear when the diode is in most circuits, than 566 ohms. For example, look at a 12 volt circuit where you need 100 mA through a diode. The math says the damn thing would drop 56.6 volts if you use the 566 ohm figure, which is clearly impossible. A drop of .566 is a lot closer to the truth.

Your disclaimer "you just got to be aware of the component you're testing and not assume ohms" is right on target.

That wasn't my intention, but I don't see any reason or need for you to apologize.

Ed

Ed,

A simple test to decide whether the meter is reading resistance or voltage would be to run the diode test on a couple of resistors. Something in the ballpark of 1k, but use two different values in case one resistor gives identical numbers for voltage and resistance at your particular test current.

Mark

Whoops, I shouldn't post before having morning coffee. As long as the test current is not exactly 1 mA, you can do the test I described with a single resistor. The problem with 1 mA is, you'll get the same reading whether it's giving ohms or mV (or, k-ohms and Volts).

Mark

John, the test is flawed. When you add a meter in series to measure current, the added meter changes the current supplied by the diode testing meter, due to the internal resistance of the added meter (relatively large on the low mA scales). That automatically upsets the "calibration" such as it is, of the reading on the diode testing meter. You may happen to get a reading on the diode testing meter that agrees, but you would need to make multiple readings across a range of resistances that always agreed to make a proper conclusion.

Also, it sounds like you used only 2 meters. You would need

3 to be sure your setup is measuring what you think it is. From your description: "Then I measured a diode junction with each meter while measuring the test current and voltage drop with the other meter." You can't measure both current and voltage simultaneously with the other meter. When you remove it from a series connection for measuring current, to a parallel connection for measuring voltage, you change the current in the circuit.

There are two ways you can test that come to mind that do not upset that calibration:

1) Diode test normally and measure the voltage across the diode with a second meter. 2) Diode test, except use a low value resistor, like 150 ohms.

I've performed test #1 a number of times in the past, and the reading on the second meter (which is voltage drop) has always been close to the reading on the first.

I just performed test #2. My 150 ohm resistor read 220 on the Webtronics meter in diode test. (I believe that is the same meter the op has.) On the ohmmeter setting, it reads 149.2

I then tested the same way with an Alfa 2360 meter. It reads 149.8 ohms on the ohmmeter setting, and .154 volts on the diode setting.

Surprisingly, when I performed the test on the cheap $3.00 meter from Harbor Freight, it read

149 on both diode test and ohms.

Ed