The USB port of a laptop or phone charger is a stable 5 V supply you can get anywhere. You can power a perfboard circuit without changing batteries, so it is often used for experiments with 5 V logic ICs and microcontrollers. This article designs a board that takes 5 V from USB power and supplies the perfboard power rail through a fuse, capacitors, and a power indicator LED. It also explains current limits and safety tips.
The basics of USB power
The power you use from a USB connector is the two wires VBUS (5 V) and GND. The 5 V is available even if you do not connect the data lines.
| Power source | Current available |
|---|---|
| PC USB 2.0 port | Usually a 500 mA limit (100 mA before connection is confirmed) |
| PC USB 3.0 port | About 900 mA |
| Phone charger | 1–3 A (as much as the load requests) |
A PC port may shut off or be damaged by overcurrent, so be sure to protect it with a fuse. A charger supplies a large current and a short is dangerous, so a fuse matters even more.
Ways to connect power
Micro-USB breakout
A small board that brings a Micro-USB connector out to 2.54 mm pins. Use only the VBUS and GND pins. It is cheap and plugs straight into perfboard.
Power-only USB-C breakout
To get 5 V over USB-C, you have to connect the connector's CC1 and CC2 pins to GND through 5.1 kΩ resistors each, to announce "I am a device requesting 5 V". It is convenient to choose a breakout that already has these resistors attached. A product without them gets no power from a charger.
Modifying a USB-A cable
You can also cut an unused USB cable and use only the red wire (5 V) and the black wire (GND). Wire colors are not always the same, so confirm 5 V with a multimeter before connecting.
The circuit
| Part | Value | Role |
|---|---|---|
| Fuse | Resettable PTC 500 mA (hold current 0.5 A) | Overcurrent protection. Resets automatically when the short is gone |
| C1 | 100 µF electrolytic | Stabilizes the supply. + side to 5 V |
| C2 | 100 nF ceramic | Removes high-frequency noise |
| R + LED | 1 kΩ + LED | Power indicator (about 3 mA) |
| Power switch (optional) | Slide switch | Cuts the whole circuit |
The connection order is USB VBUS → PTC fuse → power switch → 5V rail, with C1 and C2 and a series circuit of 1 kΩ and an LED between the rail and GND. This board has the same structure as the 5 V output section of Making 5V from a 9V Battery: 7805 Power Board. No regulator is needed, so it is far simpler.
Watch the voltage drop
A PTC fuse has a little resistance at normal current, so the voltage drops 0.1 to 0.3 V. 74LS-family ICs require 4.75 V or more, so if USB 5 V is 5.0 V and 0.2 V drops across the fuse, that is 4.8 V, which just barely meets it. If the voltage is low, check the following.
- Do not add a series diode. It drops 0.7 V, leaving 4.3 V.
- A long, thin cable drops more voltage. Use a thick, short cable.
- If 5 V falls below 4.75 V, consider something other than a 7805 approach (such as a boost module).
Perfboard build tips
- Solder the breakout connector at one corner of the board with a 2.54 mm pin header. Plugging it into a socket makes it easy to detach.
- Make the 5 V from the connector pass through the fuse first. Keep the wire before the fuse short.
- Send 5 V to the top rail of the board and GND to the bottom rail. Make both rails thick and short (Power Rails, Grounding, and Decoupling Capacitor Placement).
- Put 100 µF and 100 nF in parallel close to the power entry.
- Put the power LED at the end of the rail so you can see at a glance whether power is on.
Checking
- Before connecting power, check between the 5 V rail and GND with a continuity test. If it beeps, there is a short, so do not plug in the USB until you fix it.
- Connect USB and see whether the power LED lights.
- Measure the rail voltage with a multimeter. It should be 4.75 V or more.
- After connecting the load circuit, touch the PTC fuse to check that it is not getting hot.
- If you are curious about the current, connect the multimeter in current mode in series with the +5 V line and measure (How to Choose a Multimeter).
Safety rules
Next steps
- For experiments that need voltage adjustment, a bench power supply is convenient.
- If you use a 3.3 V device, read Connecting 3.3V and 5V Signals and match the voltage levels.
- Drive the finished 5 V board with the 555 timer LED blinker or the digit counter.
Summary
Build the USB power board as connector → resettable fuse → switch → 5 V rail, with 100 µF, 100 nF, and a power LED at the entry. USB-C needs 5.1 kΩ on the CC pins, and in 74LS circuits avoid a series diode so voltage drop does not take it below 4.75 V. Before the first power-up, always check for a short with a continuity test.