Experiment: identifying a diode, LED, resistor and capacitor

What is this experiment?

The aim is to pick out a diode, an LED, a resistor and a capacitor from a mixed collection of such items.

Both the JEE Main 2027 and the NEET (UG) 2026 syllabuses list this under Unit 20, Experimental Skills, as item 18: “Identification of diode, LED, resistor, a capacitor from a mixed collection of such items” (JEE wording). The 2026–27 NCERT textbook does not describe the method: its Chapter 14, still the 2025–26 reprint, ends with rectifiers in Section 14.7. So this page follows the NCERT Laboratory Manual (Physics, Class 12), Activity 8 (pages 167–173), with its Activities 4 and 9 on the multimeter.

Apparatus and the multimeter

• A multimeter, and the mixed collection of components (Laboratory Manual, page 167). The Manual prefers an analog multimeter; with a digital one, read “deflection” as “reading” (page 169). A pointer shows a capacitor’s brief kick plainly; a digital display, updated a few times a second, can miss it (reasoned).
• A multimeter works as an ammeter, a voltmeter or an ohmmeter (Laboratory Manual, Activity 4, page 150). An analog multimeter shows its readings with a pointer moved by a microammeter (Wikipedia, “Multimeter”).
• In the resistance mode, a cell inside the multimeter is connected and drives a current through the component; the meter senses that current, with its dial marked in ohms (page 150). On an analog meter this scale is non-linear, with its zero at the extreme right (pages 151 and 153).
• Which lead is positive? For measuring voltage the red probe is used for positive and the black for negative (page 153). In the particular analog meter described by NCERT, resistance mode connects the black lead to the positive terminal of the internal cell and the red lead to the negative (Laboratory Manual, Activity 9, page 175). For that meter, a diode is forward biased when the black lead touches its p-side. Other meters can differ: establish the test polarity and voltage from the instrument manual.
• NCERT Laboratory Manual slip (pages 168, 176 and 178): elsewhere the Manual calls the red lead “positive” in resistance mode, and its Table A 9.1 (page 176) lists a junction as forward biased with the red lead on the p-side. That follows the socket labels (red + for voltage), not the cell inside, which page 175 puts on the black lead. On an unfamiliar meter, test a diode whose band you can see: when it conducts, the lead on the unbanded (p) end is the positive one (reasoned).
• Some digital multimeters have a diode-test setting, which shows the forward voltage drop of a conducting junction (Wikipedia, “Multimeter”). Which lead is positive on it is given in the meter’s manual.

Testing each component with a multimeter

1. Look first (Laboratory Manual, pages 167–169). Count the terminals: four or more, on a chip (a black rectangular block), mean an IC, though some ICs, such as the 7805, have only three (page 167); three terminals may mean a transistor, which the Manual confirms with the meter (page 168); two mean a resistor, a capacitor, a diode or an LED. Three colour bands followed by a silver or gold band mark a resistor (page 169). Outside the supplied collection many other two-terminal devices exist, so markings and part numbers are stronger evidence than terminal count alone (reasoned).
2. Set the meter. The Manual uses resistance mode on the highest range (page 169): that range usually sends the smallest test current, and with a capacitor its larger resistance makes the kick last longer and easier to see (reasoned). Touch the metal ends of the two leads together and set the pointer to zero ohm with the zero-adjust knob (page 170). Beyond the Manual: when testing a diode, the test voltage must not exceed the diode’s maximum allowable voltage (NEETS Module 7, page 1-32). So find your meter’s test voltage in its manual, and if it has a low-voltage setting, such as diode-test mode, use it for diodes and LEDs (reasoned).
3. Test both ways. Touch the leads to the two terminals of the component and watch the deflection; then repeat with the component terminals reversed (Laboratory Manual, page 169). Our safety advice (reasoned): first discharge a capacitor through a suitable resistor and confirm near-zero voltage; test a polarised (electrolytic) capacitor in its marked polarity only, with the meter’s positive lead (black, on the NCERT meter, page 175) on its + terminal, rather than reverse-biasing it to see a kick; and reverse an LED only when the meter’s test voltage is within the LED’s reverse rating (step 2).
4. Watch for two more things: light from the component, and a deflection that appears for a moment and then dies away (pages 169–170).
5. Record each component, marked with a letter, in two tables: one by the number of terminals, one by how it conducts (Tables A 8.1 and A 8.2, pages 169–170).
6. Find the ends once you know what it is. A silver ring on a diode marks its n-side; on an LED the longer pin is usually the p-side (page 173). For the NCERT meter with black positive in ohms mode, the conducting orientation puts its p-side on black (page 175).

How each one responds

• Resistor: an equal deflection in both directions (Laboratory Manual, page 169). It obeys Ohm’s law, so the reading does not depend on which way the current flows.
• Diode: a deflection with no light one way, and no deflection the other (page 169). Its forward resistance is low and its reverse resistance high (NCERT page 337). Taking the two readings is the standard check of a diode: the larger value is the reverse resistance. A small signal diode has a reverse-to-forward ratio of several hundred to one; with adequate test voltage and good contact, two high readings suggest an open diode; two very low readings suggest a short (NEETS Module 7, page 1-32, US Navy, public domain).
• The “resistance” of a diode depends on the meter and the range. Ohmmeters differ in internal resistance and in the charge of their cells, so each sends a different current through the diode (NEETS Module 7, page 1-33). A diode is not ohmic, so V/I differs at each current. The displayed apparent resistance is therefore range-dependent; consult the meter’s test-current specification rather than assuming a universal range rule. A diode that passes this preliminary check can still fail in a circuit, because the meter works at a lower voltage, so the check is not conclusive (page 1-33).
• LED: a deflection with light one way, and much less or no deflection the other (page 169). The light is the difference from an ordinary diode. On a suitable low-current test setting the glow can be faint: shade the LED and look closely. An LED needs more forward voltage than a silicon diode, about 1.6–2.0 V for red and 2.5–3.7 V for blue (Wikipedia, “Light-emitting diode”), so a blue or white LED may not light from a meter’s 3 V cell (page 174; reasoned).
• Capacitor: no deflection either way, except that a large capacitor gives a momentary deflection; the Manual records it as an initial deflection that decays to zero (pages 169–170). A capacitor blocks a steady (dc) current (page 172). The meter’s cell charges it through the meter’s own resistance, and as the capacitor’s voltage rises the charging current, and the deflection, falls to zero (reasoned). It dies away over a few time constants, τ = RC, the same product as in a rectifier’s capacitor filter (see Example 4). A charged capacitor may show little initial kick on a repeat test. Discharge it through an appropriately rated resistor and confirm near-zero voltage before retesting; do not directly short an unknown charged capacitor.

Precautions and sources of error

• Clean the leads of each component before measuring its resistance (Laboratory Manual, page 170).
• Check the zero: with the metal ends of the leads touching, the meter must read zero ohm; if not, set it with the zero-adjust knob, or the readings are unreliable (page 170).
• Do not touch the metal ends of the leads (the Manual says either of them, page 170): with a finger on each, your body’s resistance is in parallel with the component’s and changes the reading.
• Test out of circuit or with at least one lead disconnected, and never remove or insert a diode while voltage is applied (NEETS Module 7, page 1-32). Our safety advice (reasoned): turn circuit power off and verify it is off, and discharge capacitors through a suitable resistor and confirm near-zero voltage first.
• The Manual uses the highest resistance range (page 169). Our safety advice (reasoned): use a range whose test voltage and current suit the component (step 2); a digital capacitance range is better for identifying small capacitors.

Solved examples

Common mistakes to avoid

Quick revision summary

• Resistance mode, highest range (the Manual’s choice): test each two-lead component both ways and watch for light and for a brief kick; isolate and discharge components first, and keep a polarised capacitor to its marked polarity (our safety advice).
• Resistor: the same reading both ways. Diode: low one way, very high the other, no light. LED: the same, and it may glow if the test supply is sufficient. Capacitor: no deflection either way; a large one gives a brief kick that dies away as it charges.
• NCERT’s example analog meter has black positive in resistance mode; verify an unfamiliar meter. A diode’s band marks its n-side; an LED’s longer leg is its p-side.
• A good small signal diode has a reverse-to-forward ratio of several hundred to one; two low readings suggest a short, and two high readings suggest an open only if the test voltage and contacts are adequate.
• Precautions: clean leads, check the zero, do not touch both probe tips, test out of the circuit.

Questions students ask

Is identifying a diode, LED, resistor and capacitor in the JEE Main and NEET syllabus?

Yes, identifying a diode, an LED, a resistor and a capacitor is item 18 of Unit 20, Experimental Skills, in both syllabuses: the JEE Main syllabus lists “Identification of diode, LED, resistor, a capacitor from a mixed collection of such items”, and the NEET (UG) 2026 syllabus lists the same item. Unit 20 asks for “Familiarity with the basic approach and observations of the experiments and activities”. The NCERT textbook does not describe the method (its Chapter 14 is still the 2025–26 reprint); it is Activity 8 of the NCERT Laboratory Manual (Physics, Class 12), pages 167–173, which adds a transistor and an IC to the collection (page 167).

How do you check a diode with a digital multimeter?

A digital multimeter checks a diode best on its diode-test setting, beyond the NCERT text: this passes a small, limited current and shows the voltage across the junction instead of a resistance (Wikipedia, “Multimeter”). A typical small silicon diode shows roughly 0.5–0.8 V at the meter’s test current when its p-side is on the lead the meter makes positive, and an over-range reading with the leads reversed (0.7 V is not a universal upper limit); a red LED shows a larger drop and may glow faintly (reasoned). With suitable test voltage and sound contacts, near zero both ways suggests a short, and over-range both ways suggests an open; an LED may also read over-range because the meter cannot supply its forward voltage, as with an analog ohmmeter (US Navy, NEETS Module 7, page 1-32).

Can you test a capacitor with a digital multimeter?

Yes, a digital multimeter can test a capacitor in two ways, beyond the NCERT text: many have a capacitance range that reads the value directly, from a few picofarads up to some hundreds or thousands of microfarads (Wikipedia, “Multimeter”), and on a resistance range a large capacitor’s reading starts low and climbs to over-range as the meter’s current charges it (reasoned). Isolate it, discharge it through a suitable resistor and confirm near-zero voltage first; respect polarity on electrolytics; a reading that then stays low means it is shorted, and a capacitance well outside tolerance suggests a fault after confirming meter range and measurement conditions (reasoned). Ordinary multimeters seldom measure a capacitor’s leakage or equivalent series resistance, so one that shows the right value can still be faulty (Wikipedia).

How do you remember the resistor colour code?

The resistor colour code can be remembered by the NCERT Laboratory Manual’s (Physics, Class 12) term “BBROY GB VGW (B B ROY Great Britain Very Good Wife)”: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8 and white 9 (page 172). The Manual adds red (2%) and brown (1%) to the tolerance bands, beside gold (5%) and silver (10%), and notes that 2% and 1% resistors often have their values printed on them (page 172). So bands of blue, grey, orange and red mean 68 × 10^³ Ω = 68 kΩ ± 2%, anywhere from 66.64 kΩ to 69.36 kΩ (reasoned).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is each component tested with the multimeter leads one way and then reversed?

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   To see whether it conducts equally both ways, like a resistor, or in one direction only, like a diode or an LED (Laboratory Manual, page 169).

2. Q2. A two-lead component makes the pointer jump and then return to infinite resistance, whichever way the leads are connected. What is it?

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   A capacitor: it charges through the meter, so the current flows only for a moment (Laboratory Manual, pages 169–170).

3. Q3. Why should you not hold the metal ends of both multimeter leads while measuring a resistance?

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   Your body’s resistance is then connected in parallel with the component and changes the reading; the Manual says to avoid touching either metal end (Laboratory Manual, page 170).