To connect lights in series, you wire the positive terminal of the power source to the anode of the first light, the cathode of the first light to the anode of the second, and so on, finally returning the last cathode to the source's negative terminal. In a series circuit, current remains constant while voltage divides across each load. This walkthrough traces a practical 24V DC control panel circuit using three standard 2V LED pilot lights and a current-limiting resistor, detailing exact terminal mappings, diagram symbols, and multimeter verification steps.

Safety & Code Caveat: Never wire standard 120V AC home room lights in series. The National Electrical Code (NEC) requires parallel branch circuits for general illumination. If one bulb fails in a series AC string, the entire circuit goes dark, and the remaining bulbs may experience overvoltage. This guide focuses strictly on low-voltage DC indicator applications.

Diagram Symbols and Circuit Assumptions

Before tracing the physical wire path, we must decode the schematic symbols and establish our baseline assumptions. A series circuit is defined by having only a single path for current to flow from the source, through every component, and back to the source. According to All About Circuits, the total resistance is the sum of all individual resistances, and the current is identical at every node.

Here is what the standard symbols mean in this specific drawing:

  • DC Voltage Source (V1): Represented by alternating long and short parallel lines. The long line is the positive terminal; the short line is the negative (ground/common).
  • LED Diode (D1, D2, D3): A triangle pointing toward a vertical line, with two small arrows pointing away indicating light emission. The triangle side is the anode (positive); the line side is the cathode (negative).
  • Resistor (R1): A standard zig-zag line (US symbol) or rectangular box (IEC symbol). It is non-polarized and can be installed in either direction.
  • Nodes (Dots): Solid dots where wires intersect indicate an electrical connection. Crossing lines without a dot mean the wires are insulated from each other.

Circuit Assumptions & Math:
We are using a 24V DC power supply. Our loads are three identical standard red 5mm LEDs. According to the SparkFun LED Tutorial, a typical red LED has a forward voltage (Vf) of 2.0V and a target forward current (If) of 20mA (0.020A).

  • Total LED voltage drop = 3 × 2.0V = 6.0V.
  • Remaining voltage for the resistor = 24V - 6.0V = 18.0V.
  • Required resistance (Ohm's Law: R = V / I) = 18.0V / 0.020A = 900 ohms.
  • Nearest standard 5% resistor value = 910 ohms.
  • Resistor power dissipation (P = I²R) = 0.020² × 910 = 0.364W. We will use a 0.5W (1/2W) resistor to provide a safe thermal margin.

Node-by-Node Wiring Trace (Source to Load)

This textual trace follows the current path from the positive terminal, through the loads, and back to the DC ground. Strip 1/4 inch of insulation from your 22 AWG solid-core wire for each connection.

  1. Node 0 (Source Positive): Connect a red wire from the 24V DC power supply's positive (+) output terminal to the left lead of the 910-ohm, 1/2W resistor (R1). Because resistors are non-polarized, either lead works, but we establish left-to-right flow for consistency.
  2. Node 1 (Resistor to LED1): Connect a red wire from the right lead of R1 to the anode (positive) of the first LED (D1). Physical identification: The anode is the longer leg on a new 5mm LED. If the legs are trimmed, look for the smaller metal anvil inside the LED epoxy dome; the larger internal post is the cathode.
  3. Node 2 (LED1 to LED2): Connect a short jumper wire from the cathode (short leg / flat side of the lens) of D1 to the anode (long leg) of the second LED (D2). The current has now passed through the first junction, dropping 2.0V.
  4. Node 3 (LED2 to LED3): Connect a jumper from the cathode of D2 to the anode of the third LED (D3). Polarity is strictly maintained; reversing any LED in this chain will block current flow for the entire series string.
  5. Node 4 (LED3 to Source Negative): Connect a black wire from the cathode of D3 to the negative (-) terminal of the 24V DC power supply.
  6. Ground Path Verification: The black wire at Node 4 serves as the DC common/ground return path. In a floating DC system, this is not bonded to earth ground, but it completes the circuit loop back to the source's internal chemistry or transformer winding.

Terminal and Pin Mapping Table

Use this spec-sheet table to verify your physical breadboard or panel mount layout against the schematic nodes. Ensuring correct terminal mapping prevents reverse-bias damage to the LEDs, which typically break down at around 5V in reverse.

Diagram Node Physical Component Terminal / Pin Wire Color Function & Polarity
Node 0 24V PSU / R1 PSU (+) to R1 Lead A Red Source Positive / Current Entry
Node 1 R1 / D1 R1 Lead B to D1 Anode (Long Leg) Red Current Limiting / LED Positive
Node 2 D1 / D2 D1 Cathode (Short Leg) to D2 Anode Orange Inter-stage Series Link
Node 3 D2 / D3 D2 Cathode to D3 Anode Yellow Inter-stage Series Link
Node 4 D3 / PSU D3 Cathode to PSU (-) Black DC Ground / Return Path
Bench Tip: When soldering 5mm LEDs in series for a permanent panel, use heat-shrink tubing over the Node 2 and Node 3 jumper connections. Bare wire jumps in a series string are a primary failure point if they short against the metal panel chassis, which would bypass the downstream LEDs and cause the upstream LED to overcurrent and burn out.

Verifying Connections with a Multimeter

Before applying full power, or to troubleshoot a dead string, use a digital multimeter (DMM) like a Fluke 117 to verify the circuit. Set your meter to the correct function for each test.

1. Continuity Test (Power OFF)
Set the DMM to the continuity/diode setting. Place the red probe on Node 0 and the black probe on Node 4. You should read a forward voltage drop of roughly 6.5V to 7.0V (the combined Vf of the three LEDs plus a slight drop across the resistor). If the meter reads 'OL' (Open Loop), you have a broken wire, a backwards LED, or a blown resistor. If it beeps continuously (near 0.00V), you have a dead short bypassing the components.

2. Voltage Drop Test (Power ON)
Energize the 24V supply. Set the DMM to DC Volts. Place the probes directly across each component to verify Kirchhoff's Voltage Law (the sum of voltage drops must equal the source voltage):

  • Across R1 (Node 0 to Node 1): Expect ~18.0V DC.
  • Across D1 (Node 1 to Node 2): Expect ~2.0V DC.
  • Across D2 (Node 2 to Node 3): Expect ~2.0V DC.
  • Across D3 (Node 3 to Node 4): Expect ~2.0V DC.

If D1 reads 0V but R1 reads 24V, D1 is shorted internally. If D1 reads 24V, the circuit is open at Node 1.

3. Current Verification (Power ON)
Set the DMM to DC Amps (move the red probe to the Amps jack). Break the circuit at Node 0 and place the meter in series with the power supply. You should read exactly 0.019A to 0.020A (19-20mA). Because this is a series circuit, measuring current at Node 2 or Node 3 will yield the exact same 20mA reading.

Frequently Asked Questions

Can you connect lights in series on a 120V AC home circuit?

No. Connecting standard 120V AC room lights in series is a severe code violation and practically non-functional. In a series AC circuit, the 120V source divides among the bulbs. If you wire two 120V bulbs in series, each only receives 60V, resulting in dim, flickering light. Furthermore, if one bulb burns out and breaks the filament, the entire circuit opens, and all lights go dark. The NEC mandates parallel wiring for branch circuits so each fixture receives the full 120V nominal (114-126V acceptable) and operates independently.

What happens to the brightness when you connect lights in series?

Brightness depends on the current, which is dictated by the total resistance of the series string. If you add more identical lights in series without increasing the source voltage, the total resistance increases, current drops, and all lights become dimmer. However, in our 24V DC walkthrough, we calculated the resistor specifically to maintain 20mA regardless of the fact that three LEDs are in series. If you were to add a fourth 2V LED to this exact circuit without changing the 910-ohm resistor, the total Vf would rise to 8V, the resistor would drop 16V, and the current would fall to 17.5mA, resulting in a noticeable dimming across all four LEDs.

How do you find a bad bulb when you connect lights in series?

The fastest way to find an open (dead) bulb in a series string is the 'voltage drop' method. Leave the circuit powered on. Set your multimeter to DC Volts and place the black probe on the known ground (Node 4). Move the red probe sequentially from Node 1 to Node 2 to Node 3. The node that suddenly reads full source voltage (24V) instead of the stepped-down voltage is the exact point where the circuit has broken. The component immediately downstream of that node is the dead bulb. This is much faster than pulling every bulb out to test them individually with a continuity tester.