Practical: how to tell a diode, an LED, a resistor and a capacitor apart using only a multimeter?
Tanvi Deshmukh12th Sample
7 Sept 2026 · 1 min read
So our school lab test has the NCERT activity: 'identify a diode, an LED, a resistor and a capacitor from a mixed collection'. I'm the notes queen but I've never actually held most of these, so I made a table of what I expect and want someone to confirm it!!
| Component | Resistance range, one way | Resistance range, reversed |
|---|---|---|
| Resistor | some value | same value ? |
| Diode | low | very high |
| LED | low (glows?) | very high |
| Capacitor | starts low, then increases | same again ? |
The worry: a diode and an LED both show low in one direction and OL in the other. How do I separate them with just the meter and my eyes? Another: I read that the needle of an analog meter on a capacitor swings and then comes back. Is that what we should see on a digital meter too? And what do we do if the colours and markings are rubbed off?
Also how do I know which lead of the LED is positive, so I don't connect it the wrong way round and damage it?
6 answers & replies
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[+][–]Kabir Joshi11thSample· 1mo ago
Your table is mostly right. Details from actually doing this with a multimeter:
- Resistor: same reading in both directions and the value matches the colour-band value within tolerance. It is non-polar.
- Diode (and LED): put the meter in the diode/continuity mode if there is one. Forward (red probe on anode, black on cathode): low reading. Reverse: OL (infinite).
- LED vs diode: the LED is the one with a clear or coloured dome that lights up in forward bias, usually faintly with a meter's small current. A normal diode is a small glass or black cylinder with a stripe at the cathode end and it never lights.
[+][–]Rohan Sharma11thSample· 1mo ago
capacitor: first it shows low resistance, then the meter stays at 0 forever since the capacitor is conducting, so it's basically a wire. you can ID it that way 🤷 that's what I remember from our lab demo, though the teacher was explaining at the back and I couldn't hear much
[+][–]Rahul DubeyDropperSample· 1mo ago
Rohan, it goes the other way round in steady state. A capacitor has an insulator (dielectric) between its plates. When the meter is first connected, the battery inside the meter pushes a brief charging current, so you see a low resistance at the first moment. As the capacitor charges, the current dies away and the resistance climbs towards infinity (OL on a digital meter). If you reverse the leads, the charge you stored has to be reversed so you get a bigger swing for a moment, and again it climbs. A big electrolytic capacitor shows this clearly; a tiny ceramic one may give only a flicker. Electrolytic ones are polarised, the stripe on the body marks the negative lead.
[+][–]Tanvi Deshmukh12thSample· 1mo agoAuthor
thank you!! updating the table with 'rises towards OL' for the capacitor and adding the stripe/leg notes. Yay 💖
[+][–]Meera Patel11thSample· 1mo ago
For the LED polarity: the longer leg is the anode (positive) and the shorter leg is the cathode, and the plastic body has a flat edge on the cathode side. In a circuit always keep a resistor in series, otherwise too much current flows and it will burn out. LED has a much higher turn-on voltage than a silicon diode (around 1.8 to 3 V depending on colour, vs about 0.7 V), which is why a meter's tiny current only makes it glow faintly.
[+][–]Vivek Mishra12thSample· 1mo ago
If markings are rubbed off, do this order: (1) Measure both ways. Same reading both ways and steady = resistor. (2) Reading that starts small and rises = capacitor. (3) Low one way and OL other way = diode family. (4) Among those, the one that glows = LED. Doing things in this order, the lab test is easy. And try with new unknown pieces from my old radio kit too, you will learn quickly.
