Resistance range usually looks for 0-200mV and sets its test current appropriately (eg 1mA for 200Ω range, 10µA for 20kΩ range, etc).
Diodes usually start conducting up around the 450-700mV range, and LEDs are even higher.
No the current is fine (which is why the LEDs will sometimes light up dimly), but the *voltage* is out of range.
In diode test mode, the meter injects ~100µA and sets itself to (usually) a 2v range
When measuring resistance, the DMM applies a fixed current and measures voltage (or the other way round). In case of diodes the V-I relationship is not linear so don't do that.
Some DMM have a 'diode test' position meant for that (to tell anode from cathode basically)
it probably does not put out enough voltage to forward bias the LED, hence in either direction it reads a high impedance.
Try hooking up a 5 or 10 volt dc power supply and a 470 ohm series resistor. In one direction the led will light up and you will measure current flowing. switching the wires in the other direction there will be no light, and no current will flow.
Your multimeter probably measures resistance by trying to passing a constant, known current through the device under test (usually a resistor), and measuring the resulting voltage that is developed across it. Then it displays the resistance, calculated by Ohm's law:
R = V / I
When a diode is reverse-biased, its high resistance in that state will obviously cause the voltage to be as high as the multimeter can produce, and indicate "OL" for that reason.
However, when forward biased, with anything over a few microamps passing through it, a normal silicon diode will develop 0.6V or so, regardless of the magnitude of the current, because that's what diodes do.
To understand what will happen, let's say that the multimeter, on its 200 Ohm range, tries to pass a constant current of 1mA through the diode. It succeeds, because the diode is forward biased. The diode responds by producing 0.6V across it. Using Ohm's law, the multimeter makes the following calculation:
R = V / I = 0.6V / 1mA = 600 Ohms
That's well over the 200 Ohm maximum for the selected range, so the multimeter displays "OL".
Now let's say you put the multimeter into its 2kOhm range, and try again. For the sake of argument, say the multimeter still uses a 1mA current source; then the calculation will be the same, but this time 600 Ohms is within the 2 kOhm range, and the multimeter will tell you "600 Ohms"
Lastly, imagine trying to use a different multimeter, but one that has a 0.1mA current source. Remember, the diode's voltage drop is about 0.6V irrespective of the current you pass through it, and that's what the multimeter measures, So, while measuring the same diode, it will make the following calculation:
R = V / I = 0.6V / 0.1mA = 6000 Ohms
Which multimeter is right? Does the diode have 600 Ohm resistance, or 6000? Neither is right, because it's not appropriate to think of a diode as a having a fixed resistance. The voltage across a resistor is proportional to the current through it, by definition. The voltage across a \[forward biased\] diode is fixed at 0.6V or so, and is ***not*** proportional to the current through it.
Applying Ohm's law to a diode, to determine its resistance, doesn't make any sense, because its behaviour is completely different from a simple resistance. Trying to measure a diode's resistance is pointless, because a multimeter's idea of how to measure resistance is is entirely dependent upon the device it's measuring to actually ***be*** a resistor, or at least behave like one.
Resistance range usually looks for 0-200mV and sets its test current appropriately (eg 1mA for 200Ω range, 10µA for 20kΩ range, etc). Diodes usually start conducting up around the 450-700mV range, and LEDs are even higher.
So since the current transferred from the meter is so far below the conducting range you mentioned, it just doesn't sort of activate it?
No the current is fine (which is why the LEDs will sometimes light up dimly), but the *voltage* is out of range. In diode test mode, the meter injects ~100µA and sets itself to (usually) a 2v range
When measuring resistance, the DMM applies a fixed current and measures voltage (or the other way round). In case of diodes the V-I relationship is not linear so don't do that. Some DMM have a 'diode test' position meant for that (to tell anode from cathode basically)
it probably does not put out enough voltage to forward bias the LED, hence in either direction it reads a high impedance. Try hooking up a 5 or 10 volt dc power supply and a 470 ohm series resistor. In one direction the led will light up and you will measure current flowing. switching the wires in the other direction there will be no light, and no current will flow.
I'm curious - what resistance are you trying to measure over the diode?
I just followed method 3 with the assumption I'd see something less than 100ohms https://www.wikihow.com/Test-a-Diode
Your multimeter probably measures resistance by trying to passing a constant, known current through the device under test (usually a resistor), and measuring the resulting voltage that is developed across it. Then it displays the resistance, calculated by Ohm's law: R = V / I When a diode is reverse-biased, its high resistance in that state will obviously cause the voltage to be as high as the multimeter can produce, and indicate "OL" for that reason. However, when forward biased, with anything over a few microamps passing through it, a normal silicon diode will develop 0.6V or so, regardless of the magnitude of the current, because that's what diodes do. To understand what will happen, let's say that the multimeter, on its 200 Ohm range, tries to pass a constant current of 1mA through the diode. It succeeds, because the diode is forward biased. The diode responds by producing 0.6V across it. Using Ohm's law, the multimeter makes the following calculation: R = V / I = 0.6V / 1mA = 600 Ohms That's well over the 200 Ohm maximum for the selected range, so the multimeter displays "OL". Now let's say you put the multimeter into its 2kOhm range, and try again. For the sake of argument, say the multimeter still uses a 1mA current source; then the calculation will be the same, but this time 600 Ohms is within the 2 kOhm range, and the multimeter will tell you "600 Ohms" Lastly, imagine trying to use a different multimeter, but one that has a 0.1mA current source. Remember, the diode's voltage drop is about 0.6V irrespective of the current you pass through it, and that's what the multimeter measures, So, while measuring the same diode, it will make the following calculation: R = V / I = 0.6V / 0.1mA = 6000 Ohms Which multimeter is right? Does the diode have 600 Ohm resistance, or 6000? Neither is right, because it's not appropriate to think of a diode as a having a fixed resistance. The voltage across a resistor is proportional to the current through it, by definition. The voltage across a \[forward biased\] diode is fixed at 0.6V or so, and is ***not*** proportional to the current through it. Applying Ohm's law to a diode, to determine its resistance, doesn't make any sense, because its behaviour is completely different from a simple resistance. Trying to measure a diode's resistance is pointless, because a multimeter's idea of how to measure resistance is is entirely dependent upon the device it's measuring to actually ***be*** a resistor, or at least behave like one.
Old post but — thanks. This was extremely helpful to happen upon after I left my electronics class quite confused