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Build a three-resistor DC voltage divider, calculate its current and voltage drops, then use a multimeter to check the results and test Kirchhoff’s Voltage Law (KVL). This beginner lab uses a nominal 6 V source and resistors in series; its measurements also show why real circuits do not always match ideal calculations exactly.
What this lab demonstrates
A series voltage divider distributes a source voltage across resistors in proportion to their resistance. In a simple two-resistor divider, with R1 from the source to the output and R2 from the output to ground, the unloaded output is:
Vout = Vin × R2 / (R1 + R2)
The resistor below the output node matters: it is the resistance between the output and the reference node. For three series resistors, the drop across any one resistor is VRn = Vsource × Rn / (R1 + R2 + R3). These ratio formulas assume a series path and no significant load on the measured node.
The exercise combines resistance, current, and voltage measurements with Ohm’s law, voltage-divider calculations, and KVL. The project’s suggested resistor range is 1 kΩ to 100 kΩ, a practical range for this low-voltage demonstration. The original DC Lab project also builds the circuit on a breadboard, a terminal strip, and with free-form leads.
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Parts and equipment
- A nominal 6 V battery or a low-voltage, current-limited DC supply.
- Three resistors, preferably between 1 kΩ and 100 kΩ.
- A digital multimeter with resistance, DC-voltage, and DC-current functions.
- A breadboard, terminal strip, or jumper wires and clips; plus test leads.
- A notebook or spreadsheet for recording measured and calculated values.
A basic, correctly fused meter is sufficient for this experiment. Use a low-voltage source, and check that its current limit is appropriate before powering the circuit.
Safety before wiring
- Turn off or disconnect power before changing connections or measuring resistance. Never use resistance mode on an energized circuit.
- Measure current by inserting the meter in series. Never connect an ammeter directly across a battery, supply, or resistor: its low-resistance current path can cause a short.
- Check that the red lead is in the correct current jack and that the selected range can handle the expected current. Afterward, turn off power and return the lead to the voltage jack.
- Keep probe tips and jumper wires from bridging the source terminals. Check polarity before taking signed voltage readings.
1. Measure the resistors
With the resistors disconnected from power—and ideally isolated from other components—set the meter to resistance mode. Measure each one and record the results as R1, R2, and R3. Use these measured values in your calculations rather than assuming the printed or color-code nominal values are exact. Tolerance, meter accuracy, lead resistance, and contact quality can all create small differences.
2. Build the series circuit
Node 3 (+Vsource) ── R1 ── Node 2 ── R2 ── Node 1 ── R3 ── Node 0 (0 V / negative)
There must be one continuous path through all three resistors, with no unintended bypasses. A solderless breadboard is quick to change, but check which holes share a connection: the layout of connected rows and power rails varies. A terminal strip can make connections easier to follow, while clip-and-jumper wiring is quick but less mechanically secure. Whatever method you choose, keep the node labels consistent with the diagram.
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Before powering on: check that the three resistors are in series, that the supply polarity is correct, that no wire joins the positive and negative terminals directly, and that the meter is not connected in current mode across the supply.
3. Measure the source voltage under load
Power the circuit. Set the meter to DC voltage and measure across the source terminals, with one probe on each terminal. Record this loaded reading as Vsource. A battery labelled 6 V is nominally 6 V; its actual voltage can differ and may sag slightly under load. Use the measured value in the calculations instead of assuming exactly 6.000 V.
4. Calculate total resistance and current
For series resistors, total resistance is the sum:
Rtotal = R1 + R2 + R3
The same current flows through every resistor in this single series path:
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I = Vsource / Rtotal
For example, if R1 = 5 kΩ, R2 = 3 kΩ, R3 = 2 kΩ, and the source measures 6 V:
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With a 6 V source and resistors in the suggested range, current can be in the milliampere or microampere range, depending on their total resistance.
5. Measure current safely
Turn off the source and open the series path at a convenient point. Put the meter in the gap so the circuit current flows through it; the meter must be in series, not across a component. Select an appropriate DC-current range, confirm the lead is in the current jack, then power the circuit and read the result. Turn power off before removing the meter or changing its connections.
The measured current should be reasonably close to Vsource / Rtotal, but may differ because of resistor tolerance, source resistance, the meter’s burden voltage, wiring and contact resistance, or display resolution.
6. Calculate and measure voltage drops
Calculate the expected drop across each resistor using either Ohm’s law or the divider ratio:
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For the example above, the predicted drops are 3.0 V across R1, 1.8 V across R2, and 1.2 V across R3. They add to the 6.0 V source. The fractions across the resistors are 50%, 30%, and 20%, respectively, because their resistances are 50%, 30%, and 20% of the total.
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To measure a drop, leave the meter in DC-voltage mode and put its probes across the resistor’s two ends. Voltage is measured in parallel, not by breaking the circuit. For the example, the readings should be near 3.0 V, 1.8 V, and 1.2 V. Reversing the probes changes the displayed sign, not the physical drop.
Changing the source voltage scales all three drops proportionally, while changing a resistor changes its share of the total. Keeping resistor ratios constant keeps the ideal percentages constant, provided the divider is not significantly loaded.
7. Test Kirchhoff’s Voltage Law
KVL says that the algebraic sum of voltage changes around a closed loop is zero. The signs matter: a reading taken in the opposite direction is negative. Keep the probes oriented in the stated direction and record the sign shown by the digital meter rather than reversing probes just to make each number positive.
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Add the signed readings. The total should be close to zero; a small residual is normal in physical measurements. A digital meter is useful here because it displays negative DC readings clearly.
Repeat around a different closed path, such as 1 → 2 → 3 → 1. The signed readings should again sum to approximately zero. A loop in this voltage-measurement exercise is a closed sequence of electrical nodes; it does not mean that a separate current flows along each probe path.
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8. Compare results and estimate error
Use a results table to separate what was measured from what was calculated. Record resistor values individually, since a nominal value alone may not predict the exact readings.
| Quantity | Measured | Calculated or expected |
|---|---|---|
| R1, R2, R3 | Record each measured resistance | Nominal values, if known |
| Source voltage | Loaded terminal voltage | Nominal source rating for comparison |
| Series current | Ammeter reading | Vsource / Rtotal |
| Each resistor drop | Voltmeter reading across that resistor | I × Rn |
| KVL loop sum | Algebraic sum of signed readings | Approximately 0 V |
For a nonzero calculated value, percentage error is:
% error = |measured − calculated| / |calculated| × 100%
For a KVL sum whose theoretical value is zero, do not use that percentage formula; instead report the residual sum in volts and, if useful, compare it with meter resolution and the source voltage.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Optional: verify the circuit in SPICE
The following illustrative netlist models the 5 kΩ, 3 kΩ, and 2 kΩ example with node 0 as ground:
Voltage divider v1 3 0 r1 3 2 5k r2 2 1 3k r3 1 0 2k .dc v1 6 6 1 .print dc v(1,0) v(2,1) v(3,2) v(0,3) .print dc v(2,1) v(3,2) v(1,3) .end
The sweep starts and ends at 6 V, so it represents a single DC operating point. The printed voltages let you check the drops and add signed readings around the listed loops. SPICE syntax varies: some simulators require different voltage-source syntax or print commands, and most expect ground to be node 0. The project page’s example is a starting point, not a guarantee of identical behavior in every simulator.
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An ideal simulation verifies the model, not the physical wiring. Unless you add them, it does not account for your meter, battery internal resistance, resistor tolerances, breadboard contacts, or wiring faults. Free circuit tools such as LTspice can simulate a simple divider; a paid simulator is unnecessary for this exercise.
Why real readings differ
A voltmeter has finite input resistance. When placed across a resistor, it forms a parallel path; the effect is usually small with a typical high-input-resistance meter and modest divider resistances, but becomes more significant as resistances rise. Any circuit connected to the output can load the divider too.
For a two-resistor divider with a load RL from output to ground, replace the lower resistor with the parallel combination:
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Rbottom,loaded = R2 || RL = (R2 × RL) / (R2 + RL)
Then:
Vout = Vin × Rbottom,loaded / (R1 + Rbottom,loaded)
Because the parallel resistance is lower than R2, the loaded output is generally lower than the unloaded prediction. Source voltage sag, resistor tolerance, meter burden voltage during current measurement, and contact resistance can also explain discrepancies.
Power check
For each resistor, calculate dissipation with Pn = I² × Rn. In the 0.6 mA example, R1 dissipates 1.8 mW, R2 1.08 mW, and R3 0.72 mW. Those are small values, but recalculate power if you use lower resistance or a higher supply voltage, and make sure the resistor rating is adequate.
Troubleshooting
| Symptom | Likely cause | What to check |
|---|---|---|
| Supply current limit activates or voltage collapses | A short, or an ammeter connected across the source | Turn off power; inspect the wiring and meter connection before trying again. |
| Current reads zero or unexpectedly low | Open series path, wrong meter jack or range, or blown meter fuse | Turn off power; check continuity of the resistor path and the meter manual/fuse. |
| All resistor drops are near zero | Power is off, the circuit is open, or probes are on the same node | Check source voltage and probe placement across each resistor’s two ends. |
| Drop is unlike the prediction | Wrong resistor order or value, incorrect node reference, loading, or wiring bypass | Recheck measured resistances, node labels, breadboard connections, and any attached load. |
| Negative voltage reading | Probe direction is opposite the chosen voltage direction | For ordinary drop measurements, swap probes if a positive display is convenient; for KVL, keep the direction and record the sign. |
| Readings drift or vary at high resistance | Meter resolution, leakage, contamination, or loose contacts | Check connections, keep the circuit clean, and consider lower resistor values within the source’s safe current limits. |
When a divider is—and is not—the right tool
A passive divider is useful for biasing a high-impedance node, scaling a sensor signal, or feeding an ADC input that draws little current. It is not automatically a voltage regulator. If load current changes appreciably, the load changes the effective lower resistance and shifts the output; a divider is also inefficient when it must continuously supply substantial current. For a stable output under varying load or source conditions, use an appropriate regulator or buffer instead.
Further experiments
- Change one resistor and predict the new voltage percentages before measuring.
- Attach a load resistor from the output node to ground and compare measured voltage with the loaded-divider equation.
- Replace one resistor with a potentiometer and observe how its adjustment shifts the node voltages.
- Compare nominal and measured resistor values, then repeat the calculations with each set.
- In SPICE, add a source resistance or load resistance and see how the output changes.
The lab’s central result is straightforward: in an unloaded series divider, resistance ratios determine voltage-drop ratios, Ohm’s law predicts the shared current, and signed voltage changes around a closed loop sum to approximately zero in a real measurement.
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