12 Volts to 5 Volts Switched Mode Power Supply
My telepresence vehicle motors run off a 12Volt security battery but my electronics needs 5 Volts.
So I started to order components to build a switch mode power supply to drop 12 Volts to 5 Volts without a heat sink.
But then I saw Biff's posting
In order to run a 5 Volt MP3 player or similar in a car you would buy a power supply like the black plastic item below.
So these are mass produced and are for sale at well below the component-plus-delivery cost.
They use a MC34063 chip with the following specification
| 1.5A STEP-DOWN/STEP-UP/INVERTING DC-DC CONVERTER Control Circuits The MC34063A Series is a monolithic control circuit containing the primary functions required for DC–to–DC converters. These devices consist of an internal temperature compensated reference, comparator, controlled duty cycle oscillator with an active current limit circuit, driver and high current output switch. This series was specifically designed to be incorporated in Step–Down and Step–Up and Voltage–Inverting applications with a minimum number of external components. Refer to Application Notes AN920A/D and AN954/D for additional design information. •Operation from 3.0 V to 40 V Input •Low Standby Current •Current Limiting •Output Switch Current to 1.5 A •Output Voltage Adjustable •Frequency Operation to 100 kHz •Precision 2% Reference |
Top and bottom devices - 7dayshop.com USB Charger Adaptor (Car 12v Cigar Lighter to USB) - SUPER SPECIAL - £1.49 delivered
Middle device - Hama USB Charger Adaptor (Car 12v/24V Cigar Lighter to USB) - 014060 - SPECIAL ! - £3.99 Delivered
Both use the same regulator. The cheaper one (top and bottom) has smaller value capacitors.
The cheaper one lacks the transistor and two colour led of the middle unit.
For each device the output voltage was over 5V so I replaced the resistor (R1 below) between pin 5 and earth with a 350 ohm mini potentiometer and fixed resistor in series
- 1K for the Hama and 1.5K for the cheap one - adjust for 5 V output.
The output on a scope was noisy on the cheap unit at 500mA - adding a 470 microF capacitor across the load cleaned it up.
The Hama has an external transistor that I have not investigated.
The output as delivered is cleaner than the cheaper unit. The LED is green for no load, orange for any small current, red for current limiting.
The Hama seems to current limit at about 400 mA - all of this could be changed - the chip is the same in both units.
The cheaper unit seems to follow the circuit below (the component values are a bit different) and seems OK to 600 mA
To buy the components would cost much more!
(I guess it would be easy to set the voltage to say 3.3 / 6 / 9 volts as needed for other applications)
From the datasheet -
