An off-line power supply

An EMI-free , environmentally friendly power supply built from a portable power tool's battery charger to charge a Li-ion battery. The post An off-line power supply appeared first on EDN.

Electronics Sep 30, 2025 0 2 Add to Reading List

One of my electronics interests is building radios, particularly those featured in older UK electronics magazines such as Practical Wireless, Everyday Electronics, Radio Constructor, and The Maplin Magazine. Most of those radios are designed to run on a 9-V disposable PP3 battery.

Wow the engineering world with your unique design: Design Ideas Submission Guide

Using 9 V instead of the 3 V found in many domestic radios allows the transistors in these often-simple circuits to operate with a higher gain. PP3 batteries are, at a minimum, expensive in circuits consuming tens of mA and are—I suspect—hard to recycle. A more environmentally friendly solution was needed.

In the past, I’ve used single 3.6-V lithium-ion (Li-ion) cells from discarded e-cigarettes [1] with cheap combined charger and DC-DC converter modules found on eBay. They provide a nice, neat solution when housed in a small plastic box, but unfortunately generate a lot of electromagnetic interference (EMI), which falls within the shortwave band of frequencies (3 to 30 MHz) where a lot of the radios I build operate. I needed another solution that was EMI-free and environmentally friendly.

Solution

One solution is to eliminate the DC-DC converter and string together three or more Li-ion cells in a battery pack (B₁) with a variable linear regulator (IC₁) to generate the required 9 V (V₁) as shown in Figure 1. Li-ion cells, like all electronic components, have tolerances. The two most important parameters are cell capacity and open circuit voltage. Differences in these parameters between cells in series lead to uneven charging and ultimately stressing of some cells, leading to their eventual degradation [2]. To even out these differences, Li-ion battery packs often contain a battery management system (BMS) to ensure that cells charge evenly.

Figure 1 Li-ion battery pack, with 3 or more Li-ion cells, and a variable linear regulator to generate the required 9 V.

As luck would have it, on the local buy-nothing group in Ottawa, Canada, where I live, someone was giving away a Mastercraft 18-V Li-ion battery with charger as shown in Figure 2. The person offering it had misplaced the drill, so there was little expense for me. Upon opening the battery pack, it was indeed found to contain a battery management system (BMS). This seemed like an ideal solution.

Figure 2 The Mastercraft 18-V Li-ion battery and charger obtained locally.

Circuit

The next step was to make a linear voltage regulator to drop 18 V to 9 V. This, in itself, is not particularly environmentally friendly, as it is only 50% efficient, and any dropped battery voltage will be dissipating as heat. However, assuming renewable power generation is used as the source, this would prove a more environmentally friendly solution compared to using disposable batteries.

In one of my boxes of old projects, I found a constant current nickel-cadmium (NiCad) battery charger. It was based around an LM317 linear voltage regulator in a nice black plastic enclosure sold by Maplin Electronics as a “power supply” box. The NiCad battery hadn’t been used for over 20 years, so this project would be a repurpose. A schematic of the rewired power supply is shown in Figure 3.

Figure 3 The power supply schematic with four selectable output voltages—6, 9, 12, and 13.8 V.

In Figure 3, switch S1 functions as both the power switch and selects the output voltage. Four different output voltages are selectable based on current needs: 6 V, 9 V, 12 V, and 13.8 V can be chosen by adjusting the ratio of R2 and R3-R6 as shown in the LM317 datasheet [3]. R2 is usually 220 Ω and develops 1.23 V across it, the remaining output voltage is developed across R3-R6. To get the exact values, parallel combinations are used as shown in Table 1.

Resistor #	Resistors (Ω)	Combined Value (Ω)
3	910, 18k, 15k	819
4	1.5k, 22k, 33k	1.35k
5	2.2k, 15k	1.92k
6	2.2k	2.2k

Table 1 Different values of paralleled R3 to R6 resistors and their combined value.

A photograph of the finished power supply with a Li-ion battery attached is shown in Figure 4.

Figure 4 A photograph of the finished power supply with four selectable output voltages that can be adjusted via a knob.

Results

Crimp-type spade connectors were fitted to the two input wires, which mated well with the terminals of the Li-ion battery. Maybe at some point, I will 3D-print a full connector for the battery. With the resistor values shown in Figure 3, the actual output voltages produced are: 5.96 V, 9.03 V, 12.15 V and 13.8 V. While these are not the actual designed values due to the use of preferred resistor values, it is of little consequence as the output voltage of disposable batteries varies over their operating time and there is of course a voltage drop due to cables. With this power supply, though, the output voltage of the power supply will remain constant during this time, even as the output voltage of the Li-ion drops due to its discharging.

Portable power

Although the power supply was intended for powering radio projects, it has other uses where portable power is needed and a DC-DC converter is too noisy, like sensitive instrumentation or some audiophile preamplifier [4].

Gavin Watkins is the founder of GapRF, a producer of online EDA tools focusing on the RF supply chain. When not doing that, he is happiest noodling around in his lab, working on audio electronics and RF projects, and restoring vintage equipment.

Related Content

References