Some multimeters use 9V block batteries which do not last long and are relatively expensive compared with other alkaline battery types. If the multimeter is used extensively one need to replace batteries pretty often. A more practical solution would be to power the multimeter from AA or AAA batteries and use a DC-DC converter to obtain 9V from 3V.
The converter is based on the power supply controller TL499A manufactured by Texas Instruments and the schematics follows the standard one from the data-sheet. The only difference is attaching a LF filter to pin 3 consisting from a 10 Ohm resistor and a 1μF capacitor. This idea is taken from a similar project published in June 2007 issue of Everyday Practical Electronics and it significantly reduces the voltage peaks at the output. The inductor is Murata 18R473C.
| Schematic | Layout | PCB | ||
The converter is assembled on approx. 1"x1" PCB. This is the maximum board size that fits into my multimeter after installing two AAA batteries instead of a 9V one. The picture below shows the board location in the case. Since TL499A draws some current even with no load, one should switch off the battery from the converter when the multimeter is not in use. I used the multimeter power switch for this purpose, which required to cut some wires on the multimeter PCB in order to completely disconnect the switch from the rest of the multimeter circuit.
| Battery and PCB in case | Back view | Measuring hFE | ||
Last modified:Mon, Jan 23, 2023.