Series Circuit Troubleshooting

Observe the circuit

Identify the parts and the switch

V_T (supply)
R1
R2
R3
SW1

The schematic and the board are the same circuit. Each node has a test pad: VCC, TP1, TP2, TP3 and GND. Any part can fail: the supply, SW1 or a resistor. Some circuits are healthy.

Calculate the totals

Find R_T and I_T

R1 + R2 + R3 = R_T

V_T ÷ R_T = I_T

Calculate each voltage drop

Predict V_R1, V_R2 and V_R3

I_T × R1 = V_R1

I_T × R2 = V_R2

I_T × R3 = V_R3

Find the fault

Measure the board with the Field Meter

Plug the black lead into COM and the red lead into V Ω. Turn the dial to V⎓.
Measure V_T first, from VCC to GND. Is the supply giving what it should?
Then hunt for the fault. Drag each probe onto a test pad; red goes on the pad nearer VCC.

Every reading is counted. A good troubleshooter finds the fault in a few readings, not by measuring everything. Try the walk: black on GND, then red on VCC, TP1, TP2 and TP3 in turn. QUICK REFERENCE explains it. Par, a fair number to find the fault, is 5 readings (7 when there are two faults). Readings so far: 0.

ACROSS EACH PART · V⎓ WITH SW1 ON

	PADS	CALCULATED	MEASURED

ADDITIONAL READINGS

A reading is logged on its own when the meter is steady. Also useful: Ω across a resistor with SW1 OFF (OL means open, about 0 Ω means shorted), and I_T (SW1 OFF, red lead in A, dial on A⎓, probes on VCC and TP1).

Diagnose

Name what is wrong, and prove it

	CALCULATED	MEASURED

Real parts are never exact: within about 5% of your calculation is normal.

Is there a problem with this circuit?

Repair and verify

Fix the circuit, then prove it works

What you are measuring

Field Meter TRUE RMS

Auto Manual HOLD MAX

− 0.000 V

Series circuit rules

A series circuit has only one path for the current, so the current is the same through every part. The supply's voltage divides between the parts, each taking a share in proportion to its resistance.

I_T = I_R1 = I_R2 = I_R3 = …
R_T = R1 + R2 + R3 + …
V_T = V_R1 + V_R2 + V_R3 + … (Kirchhoff's voltage law)

Switches, wires and connections that are working drop almost nothing: about 0 V across each.

Ohm's law

V = I × R I = V ÷ R R = V ÷ I

Use it for the whole circuit (V_T, I_T, R_T) or for one part. A resistor's share of the supply is V_R = I_T × R.

Tolerance: close enough is normal

Real parts are never exact. A gold band on a resistor means it can be up to 5% off its color code, and a supply is a little off its setting too. A reading within about 5% of your calculation is normal. A reading far outside that points to a fault.

Fault symptoms

Fault	Voltages, power on	Resistance, power off
An open: a resistor, a switch that will not close, a broken wire or connection	The whole V_T across the open part, 0 V across every other part. No current flows anywhere.	OL across the open part
A short across a part	About 0 V across it. More than calculated across the others, and more current.	About 0 Ω
A resistor the wrong value (overheated: its resistance has risen)	More than calculated across it, less across the others, less current.	Far from its color code
A low supply	V_T below the supply's rating, and every drop low by the same share.	Every part reads normal

Two faults: an open anywhere still stops all the current, so it can hide a second fault. Two opens can each read 0 V on their own, because the other open is in series with the meter; measure across a span that contains both and the whole V_T appears. After a repair, measure everything again: a second fault can show up only once the first is fixed.

Search smart

Measure the source first. If V_T is wrong, nothing else will be right.
The walk. Put the black probe on the supply's − side (ground) and leave it there. Move the red probe from the + side along the circuit, one point at a time. Each reading is the voltage still left at that point, and it should step down a little at each resistor. A big step shows where most of the voltage is dropped; no step across a resistor means it is shorted, or no current is flowing.
Split in half. Measure across half the circuit. If that half is wrong, the fault is in it. Split it again.
Confirm before you replace. With the power off, check the part on Ω. Replacing a good part wastes time and money.

Repairing

Replace only the parts your readings prove are bad.
With the power on, measure the supply and every part again. Every reading should match your calculations. If one doesn't, the circuit is not fixed yet.

Measuring with a multimeter

Voltage: black lead in COM, red in V Ω, dial on V⎓, power on. The probes go across the part, in parallel with it. Red on the side nearer the supply's + reads positive.
Current: the meter has to be in series, so the current flows through it. Turn the power off, open the circuit (a switch turned off is a ready-made gap), move the red lead to A, turn to A⎓, and bridge the gap with the probes, red on the + side. Move the red lead back to V Ω when you finish. Across a part, an ammeter is a short.
Resistance: power off, dial on Ω, probes across the part. An ohmmeter sends its own test current, so a powered circuit gives a meaningless reading. Anything wired straight to the supply stays live even with a switch off.

Significant figures

Match the precision of your least precise value: 12 V ÷ 470 Ω = 0.026 A (two significant figures, the same as 12). For a meter reading, write down exactly what the display shows.

Troubleshooting checklist

Calculate what each reading should be: R_T, I_T, and the drop across each part.
Measure V_T, then search for the fault with as few readings as you can.
Compare measured with calculated, and confirm on Ω with the power off.
Diagnose: name the fault and the reading that proves it.
Replace the bad part.
Measure every part again to prove the repair worked.