Measuring Real Circuits
Use a multimeter like a technician: voltage, current, resistance and continuity, safely.
The question
Your calculations say the LED should get 11 mA. How do you check what’s really happening?
What you’ll discover: how to measure voltage and current in a working circuit, how to check batteries properly, and how not to damage your meter.
In simple terms
BeginnerAcross for volts, in line for amps
- Voltage is measured across a part: one probe on each side, with the circuit running. It’s like checking the water pressure difference between two points.
- Current is measured in line: the current has to flow through the meter, so you open the circuit and put the meter in the gap.
- Resistance and continuity are measured with the power off. The meter uses its own small battery.
Measuring current safely
- Switch the circuit off.
- Move the red probe to the meter’s mA socket and set the dial to the milliamp (mA) range. Use the mA socket only, never the 10 A socket: on many low-cost meters the 10 A socket has no fuse.
- Remove one wire in the circuit (for example, the jumper from the + rail to the resistor) to make a gap.
- Touch the probes to the two ends of the gap: red on the side nearer the battery’s +, black on the other. Switch on and read the current.
- When you’re done, move the red probe back to the V socket and set the dial to volts.
Go deeper
IntermediateChecking batteries properly
A battery can read normal voltage with nothing connected and still be nearly flat, because its voltage drops when it has to deliver current. Measure the battery pack’s voltage while the circuit is running: fresh alkaline AAs read about 1.5 V or a little more each; when each cell falls to around 1.1 V under load, it’s getting low, so replace the set soon.
Compare your measured LED current with your calculation. If they differ by more than about 10 to 20%, something’s worth checking: battery voltage, the resistor’s real value, or a loose connection.
AdvancedMeter loading and resolution
A digital meter on its voltage range has a high input resistance, typically around 10 MΩ, so it barely affects most circuits. In high-resistance circuits, though (a sensor divider using megohm resistors, for instance), the meter itself becomes a parallel path and pulls the reading low.
In current mode the meter adds a small burden voltage (its internal shunt resistance times the current), which slightly reduces the current you’re measuring. Every measurement changes the circuit a little; good technicians know by how much. Also note the meter’s resolution (its smallest step) and accuracy (often ±0.5% to ±1% plus a digit or two): a reading of 11.3 mA doesn’t mean exactly 11.3 mA.
Quick check
How is a meter connected to measure voltage?
Across the part, one probe on each side, with the circuit running.
How is it connected to measure current?
In line: open the circuit and put the meter in the gap so the current flows through it.
Why measure a battery’s voltage while the circuit is running?
A tired battery can read normal with no load but drop a lot when it has to deliver current.
Words to know
- In line (in series)
- Connected so the current flows through the meter.
- Across (in parallel)
- Connected to both sides of a part.
- Fuse
- A small part that melts to protect a circuit or meter from too much current.
- Burden voltage
- The small voltage a meter itself uses up when measuring current.
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.