Incandescent Bulb Inrush Current: Circuit, Measurements and Online Simulator

To build this Incandescent Bulb Inrush Current circuit you need a 4.5 V battery, a toggle switch, a digital multimeter and a 6 V 0.5 W bulb. A 6 V, 0.5 W bulb on a battery, with a multimeter in series: a cold filament has a tenth of its hot resistance, so it draws a surge at switch-on. Make the 4 connections in the wiring table, press Run, and and flip the switch: the current starts high and falls as the filament heats and glows. Flip it off and on again quickly to see a smaller surge.

Open this circuit in the free Electronics bench simulator

Parts list

Qty	Part	What it does
1	4.5 V battery	A DC source with a little internal resistance. + is the longer terminal.
1	Toggle Switch	An on/off switch between its two terminals. Flip it while the circuit runs.
1	Digital Multimeter	Turn the dial: V (across a part, COM and V), A (in series, COM and A), Ω / continuity / diode (on an unpowered part), Hz. The A range has a 10 A fuse — put it across a supply and it blows.
1	6 V 0.5 W bulb	A filament bulb. Cold, its resistance is a tenth of its hot value, so it draws a surge at switch-on and glows up over a few tens of ms.

Wiring

#	From	To
1	4.5 V battery +	Toggle Switch 1
2	Toggle Switch 2	Digital Multimeter A
3	Digital Multimeter COM	6 V 0.5 W bulb 1
4	6 V 0.5 W bulb 2	4.5 V battery -

Step by step

Open the electronics bench. Open the BitForBytes Embedded Lab in your browser. A circuit with no microcontroller runs on the analog bench: no install or account is needed.
Add the parts. Add a 4.5 V battery, a toggle switch, a digital multimeter and a 6 V 0.5 W bulb from the parts palette (Instruments, Power, Passive and Semiconductors).
Wire 1. Connect 4.5 V battery + to Toggle Switch 1.
Wire 2. Connect Toggle Switch 2 to Digital Multimeter A.
Wire 3. Connect Digital Multimeter COM to 6 V 0.5 W bulb 1.
Wire 4. Connect 6 V 0.5 W bulb 2 to 4.5 V battery -.
Run and measure. Press Run and flip the switch: the current starts high and falls as the filament heats and glows. Flip it off and on again quickly to see a smaller surge.

Lab sheet

// Electronics bench: filament inrush
//
// Hot resistance = V² / P = 36 / 0.5 = 72 Ω → 83 mA once lit.
// Cold, it is about 7 Ω: the first instant draws several times that.

What to try

Press Run and flip the switch: the current starts high and falls as the filament heats and glows. Flip it off and on again quickly to see a smaller surge.

Frequently asked questions

How do I wire the Incandescent Bulb Inrush Current circuit?

Make 4 connections: 4.5 V battery + to Toggle Switch 1; Toggle Switch 2 to Digital Multimeter A; Digital Multimeter COM to 6 V 0.5 W bulb 1; 6 V 0.5 W bulb 2 to 4.5 V battery -.

What parts do I need for the Incandescent Bulb Inrush Current?

A 4.5 V battery, a toggle switch, a digital multimeter and a 6 V 0.5 W bulb.

Can I simulate the Incandescent Bulb Inrush Current online for free?

Yes. The BitForBytes Embedded Lab solves the circuit in time, the way SPICE does (nodal analysis with real diode, transistor and capacitor models), and gives you a bench power supply, function generator, oscilloscope and multimeter to measure it — free, in the browser.

What does this circuit show?

A 6 V, 0.5 W bulb on a battery, with a multimeter in series: a cold filament has a tenth of its hot resistance, so it draws a surge at switch-on.