A Dollar Store Switch Mode Buck Power Supply

Karl Berger, W4KRL

While cruising the aisles of a newly opened dollar store I discovered an automotive cigarette lighter adapter with two USB connectors to charge a cell phone or other device. The nameplate claimed that it can provide 1 amp at 5 volts. I bought a couple with the intention of tearing one apart to see what was inside. My experience with other dollar store products led me to expect poor quality construction with unmarked parts and little hope that anything useful could be learned. The dollar store unit (DSU) turned out to have a few surprises. First, it was very easy to open even without tools. The case pried apart after unscrewing the ferrule that holds the fuse. Inside I found a small printed circuit board with a few resistors, disc and electrolytic capacitors, something that looked like it might be an inductor or transformer and an 8-pin DIP integrated circuit. The next surprise was faintly visible markings on the IC. It took a magnifying glass and some guesswork to decode the part number as MC34063, a versatile switch mode converter made by a number of companies with datasheets readily available on the Internet[1]. And then a real surprise - several of the datasheets had schematics for a 12 Volt to 5 Volt converter with parts values identical to the DSU!

The unit is useful as is. A lot of amateur radio stations these days use 12Vdc for their rig power and have 5V accessories. However some accessories often use 6, 7.5 or 9Vdc. A modified DSU provides an efficient and very inexpensive way to power lower voltage accessories from a 12V bus. It turns out that only a few simple modifications are needed to adapt the unit to another output voltage. The job is made even easier by using one of the design tools available on the Internet[2]. First, let’s modify the DSU then look into the theory and design.

The Modification

The amplified speakers for my station computer use a 9V battery so I modified the DSU to output 9V with 12V input. I also tried to reuse as many parts as possible. For example, R_SC in the DSU is a current sense resistor with a value of 0.22 Ohms. Equation 7 below tells us that this will limit the peak switch current to 1.36A. Equation 6 says that the peak switch current is twice the output current so maximum is 0.68A, not the 1 Amp nameplate rating but more than enough for most 9V applications. Let’s keep R_SC as is. To make a long story short, we are only going to change the voltage feedback resistors (R1 and R2) and the dropping resistor for the LED (R3). The design formulas calculate the minimum values for the inductor and output capacitor. You can reuse the stock components as long as they are larger than the calculated values. You can try changing the frequency to bring the calculated values in line with the stock components. This would require changing the timing capacitor C_T.

To modify the unit take off the fuse ferule and pry apart the case. Unsolder the fuse spring, the wired-in USB connector (JB) and the one mounted on the circuit board (JA), resistors R1, R2 and R3, and any wires remaining on the board.

The LED on my DSU was soldered to pads on the back side of the board so that it would not interfere with the USB connectors. To remount the LED to the thru-holes provided on the board, remove the LED and be sure to remember the polarity. There is probably a flat on the cathode side of the cathode side. Reinstall it in the thru-holes.

For 9V operation replace the resistors as follows: R1 = 1K, R2 = 6.2K, R3 = 1.2K. I didn’t have a 6.2K resistor for R2 so I used 5.6K. That reduced the output voltage to 8.25V which is no problem for most 9V devices.

While you are at it you should dress up any of the original components that may be poorly mounted or soldered. I routed the output wires through existing holes in the board to provide some strain relief.

Test your modified unit before going any further. My first test failed because I had the LED in backwards...