I'm confused about the orientation of some diodes I measured recently as the measurements seem to contradict the markings on the diodes.
The manual for my multimeter gives the following info and instructions on diode measurement . . .
When explaining how to determine if a diode is good: "If one reading shows a value and the other is overrange (O.F) (after the meter leads have been reversed), the device is good."
Then, under Notes: "The values that appear during a diode check show the actual forward voltage (max. 2.0v). If the voltage exceeds 2.0v, O.F appears. This means the diode check cannot be made using this meter."
This seems to contradict the instructions for determining if a diode is good.
It notes that diodes are usually marked with a stripe on the cathode (-) side with the other side being the anode (+). It then gives instructions to determine the polarity of an unmarked diode:
"Connect red test lead to one side and black to the other then measure and note the voltage. Reverse the leads and note the second reading. The side of the diode where the meter shows a higher voltage using the red test lead is the anode (+) side."
When I measured a UF4007 - which is marked, of course - I got .459v with red probe on what should be the anode side. With the red lead on the striped cathode side, the meter reads O.F, which would indicate a reading of over 2.0v. Since >2.0v is more than .459v, this would indicate that the diode is marked incorrectly. To add to the confusion, the diodes are packaged in strips with tape on the end of each lead. The tape on the striped "cathode" side (supposedly negative) is red while the tape on the unmarked "anode" side (supposedly positive) is white.
So, are the diodes marked incorrectly or are the meter instructions wrong?
I know I've used a few of these in the past - mostly as replacements for seleniums and maybe in a hybrid bridge on a PT without a center tap - and now I'm wondering if I put them in correctly. As I recall, the voltages in those situations measured OK and the amps seemed to work fine.
What would happen if the diodes were installed backwards? Would the circuit work?
. . . Charlie
The manual for my multimeter gives the following info and instructions on diode measurement . . .
When explaining how to determine if a diode is good: "If one reading shows a value and the other is overrange (O.F) (after the meter leads have been reversed), the device is good."
Then, under Notes: "The values that appear during a diode check show the actual forward voltage (max. 2.0v). If the voltage exceeds 2.0v, O.F appears. This means the diode check cannot be made using this meter."
This seems to contradict the instructions for determining if a diode is good.
It notes that diodes are usually marked with a stripe on the cathode (-) side with the other side being the anode (+). It then gives instructions to determine the polarity of an unmarked diode:
"Connect red test lead to one side and black to the other then measure and note the voltage. Reverse the leads and note the second reading. The side of the diode where the meter shows a higher voltage using the red test lead is the anode (+) side."
When I measured a UF4007 - which is marked, of course - I got .459v with red probe on what should be the anode side. With the red lead on the striped cathode side, the meter reads O.F, which would indicate a reading of over 2.0v. Since >2.0v is more than .459v, this would indicate that the diode is marked incorrectly. To add to the confusion, the diodes are packaged in strips with tape on the end of each lead. The tape on the striped "cathode" side (supposedly negative) is red while the tape on the unmarked "anode" side (supposedly positive) is white.
So, are the diodes marked incorrectly or are the meter instructions wrong?
I know I've used a few of these in the past - mostly as replacements for seleniums and maybe in a hybrid bridge on a PT without a center tap - and now I'm wondering if I put them in correctly. As I recall, the voltages in those situations measured OK and the amps seemed to work fine.
What would happen if the diodes were installed backwards? Would the circuit work?
. . . Charlie
Neither.
Sounds to me like it is working/testing as it should.
In other words, when the diode is forward-biased (with red lead on anode), you should get a reading of between 0.31 to 1.99 (depending on diode type). When reverse-biased (with black lead on anode), the reading should be OF or something indicating that it is not conducting, with reference to diode conduction.
Sounds to me like it is working/testing as it should.
In other words, when the diode is forward-biased (with red lead on anode), you should get a reading of between 0.31 to 1.99 (depending on diode type). When reverse-biased (with black lead on anode), the reading should be OF or something indicating that it is not conducting, with reference to diode conduction.
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It means the meter applies 2V to the diode to test it. If the diode can't turn on even with 2V across it, you will get no reading even though the diode may be good. Not a problem with rectifier diodes, but many LEDs don't turn on until the voltage is more than 2V, so you would not be able to measure them.
During a diode check, the meter measures the voltage drop of the diode. Forward Voltage would be your 0.459V reading. Reverse Voltage would be much greater than 2V, so therefore the over-range reading. The tape coloring of red just indicates the cathode and has no meaning or reference as to the color of your probes. From your measurements, as far as I can tell, your readings and the diode markings are correct.
Lots of smoke, fried components and a loud bang or two as the power supply capacitors explode. 😱
Do not assume that the red lead is positive when doing this test. Lots of multimeters have the pos on black and the neg on red and you would think the diode is reversed.
Way to check is with another multimeter in voltage metering mode. 😉
Jan
Way to check is with another multimeter in voltage metering mode. 😉
Jan
Or just measure a battery, and connecting the red lead to the positive battery terminal should give a positive voltage reading,
if red means positive on the meter.
The red meter lead is almost universally positive when measuring voltage, but old analogue meters often swapped polarity when measuring resistance. This is basically because the meter box contained the voltage source, and it reduced switching. Some DMMs might maintain this historical anomaly, but it would be better if they did not. It sounds like the OP's DMM sticks to 'red is +ve'.
Thanks for the explanation on the meter readings. Yeah, my DMM has red as positive - which is why I found the red tape used in the packaging of the diodes to be confusing since it was on the cathode (negative) side.
None of my previous work that used these exploded, so I guess I installed them correctly!
Thanks . . . Charlie
None of my previous work that used these exploded, so I guess I installed them correctly!
Thanks . . . Charlie
One thing not mentioned by our band of old timers, is that many multimeters also didn't have more than a single, or double set of AA batteries in them to provide juice to measure resistance (and diode-checking). These days, there has been a push to use 2.7 volt lithium cells. (a single one) ... and of course, there was an era (which still exists at the low-end of digital multimeters) where "two and a half digits" was the precision of the analog-to-digital chip. -199 to +199. Sometimes they went from -299 to +299, but that tended to increase the cost of the LCD / LED display. Sometimes it was "3 and half" digits ... -1999 to +1999. That was a more expensive meter, and for the most part, few people needed the last digit for ordinary 'needling'.
The PROBLEM here is that things like LEDs have broken the diode-check function. Many new LEDs, especially "traffic light green" and "wicked blue" (and white!) have forward voltages of 3 or more volts. Even the old "2 AA cell" meters might not have the voltage to bust through the forward-voltage gap.
LASTLY ... there was a good reason to have the polarity of the leads "reversed" for resistance/diode measurement, and it wasn't to make the circuit cheaper. It was to identify the "positive pin" of diodes... when electronic techs were less concerned with words like "cathode" and "anode". By putting negative (electrons) on the red lead, a diode's cathode placed on the same lead would get a bunch of electrons that would jump across the gap to the black (positive, electron short) lead. Peering at the diode, you'd see the cathode-ring on the same side, and you'd say in your head, "ah. that's the positive lead".
This makes sense pragmatically, since multimeters ARE used way-more-often to determine orientation of diodes and rectifiers than they are, say, to test electrolytic capacitors (which in resistance-scale is just almost pointless).
As others here have quipped, know your meter, though, and don't make an assumption that its polarity is the "old fashioned, but helpful in practice" way. It could be more formal, and you'll not know that until you hook something up backward and have the sparks fly.
GoatGuy
The PROBLEM here is that things like LEDs have broken the diode-check function. Many new LEDs, especially "traffic light green" and "wicked blue" (and white!) have forward voltages of 3 or more volts. Even the old "2 AA cell" meters might not have the voltage to bust through the forward-voltage gap.
LASTLY ... there was a good reason to have the polarity of the leads "reversed" for resistance/diode measurement, and it wasn't to make the circuit cheaper. It was to identify the "positive pin" of diodes... when electronic techs were less concerned with words like "cathode" and "anode". By putting negative (electrons) on the red lead, a diode's cathode placed on the same lead would get a bunch of electrons that would jump across the gap to the black (positive, electron short) lead. Peering at the diode, you'd see the cathode-ring on the same side, and you'd say in your head, "ah. that's the positive lead".
This makes sense pragmatically, since multimeters ARE used way-more-often to determine orientation of diodes and rectifiers than they are, say, to test electrolytic capacitors (which in resistance-scale is just almost pointless).
As others here have quipped, know your meter, though, and don't make an assumption that its polarity is the "old fashioned, but helpful in practice" way. It could be more formal, and you'll not know that until you hook something up backward and have the sparks fly.
GoatGuy
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Well... you're not really sure unless you measured it with another meter. Measuring a battery only tells you what the polarity is in voltage metering mode, NOT in resistance metering mode. And in THAT mode, it was often swapped, as it seems to be the case with your meter. See also DF96's post above.
Jan
as said above. Some diodes can't be tested with an average DMM. That's why my wavetek LCR-55 has a separate test function for those. It says it's for diodes in microwave ovens, but that's just because the LCR-55 was marketed as a component tester for appliance repair technicians.
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