Vehicular 5V Power Supply

Users of our VFD-420 serial display frequently use it in automotive applications. Unfortunately, designing a bullet-proof 5-volt regulated power supply for use in the harsh vehicular power environment can be difficult.

An article in the June 1999 issue of Nuts & Volts magazine tackled this very problem. Here, reprinted with their permission, is the circuit from that article.

Power Supply Circuit

Schematic of the power-supply circuit. Click to enlarge.

Pinouts of the voltage-regulator ICs

Parts Lists

There are two parts lists for the circuit. Depending on the current draw of your project, you can select a switching regulator (up to 1000mA output), or a less-expensive linear regulator (up to 400mA). For our VFD-420, the switching regulator is the right choice, since the linear regulator would be operating close to its max output (from a heat-dissipation standpoint).

Parts List 1: Switching Supply (1A output)

All part numbers refer to Digi-Key (1-800-digikey)

• C1,C2--100µF, 50V electrolytic capacitor (part no. P1353-ND)
• C3--0.1µF, 50V monolithic ceramic capacitor (part no. P4923-ND)
• D1--silicon rectifier diode, 6A, 100V (part no. 6A1MSCT). Note: it's permissible to use a smaller diode, 2A or more.
• D2--zener diode, 36V +/-5%, 5W (part no. 1N5365BMSCT)
• D3--zener diode, 6V +/-5%, 5W (part no. 1N5340BMSCT)
• J1--Auto accessory (cigarette lighter) plug (part no. ZA5073-ND)
• R1--50-ohm, 1W resistor (part no. ALSR1F-50-ND)
• U1--5V, 1.5A integrated switching regulator (part no. 78ST105HC)
• fuse--2A, 250V normal-blow fuse (part no. F119-ND)

Parts List 2: Linear Supply (400mA output)

All part numbers refer to Digi-Key (1-800-digikey)

• C1,C2--100µF, 50V electrolytic capacitor (part no. P1353-ND)
• C3--0.1µF, 50V monolithic ceramic capacitor (part no. P4923-ND)
• D1--silicon rectifier diode, 6A, 100V (part no. 6A1MSCT)
• D2--zener diode, 36V +/-5%, 5W (part no. 1N5365BMSCT)
• D3--zener diode, 6V +/-5%, 5W (part no. 1N5340BMSCT)
• J1--Auto accessory (cigarette lighter) plug (part no. ZA5073-ND)
• R1--not used
• U1--5V, 1.0A linear voltage regulator (part no. NJM7805FA-ND)
• Heatsink--aluminum heat sink, bolt to U1 (part no. HS191-ND)
• fuse--1A, 250V normal-blow fuse (part no. F115-ND)

For More Information

The switching regulator used in the high-current version is a product of PowerTrends Inc. The author based his circuit on a PowerTrends application note, "Vehicular Power Adapter Using ISRs."