Another 9V battery power supply for multimeter

Today, I was browsing the Internet and found another article about a replacement circuit for the 9V power supply of a multimeter. I have excerpted it here for your reference. This can be said to be the simplest circuit I have ever seen. You can try it.

I always feel that these replacement circuits are not very good, because the battery is not durable, because many multimeters now have self-shutdown, and self-shutdown takes a long time to shut down, so during the waiting time, a lot of power is consumed, making the battery not durable. I once made a circuit powered by a 1.5V battery, and I forgot to turn it off several times, wasting a lot of electricity. The battery is not durable and it ran out of power in a month, so it is better to switch back to a 9V laminated battery.

Part of the original text is as follows:

1. Working principle of the circuit
The circuit of the multimeter substitute power supply is shown in Figure 1. The circuit consists of five components: transistor VT, step-up transformer T, diode VD, capacitor C and power supply GB. The transistor VT and step-up transformer T form a transformer feedback oscillator. When there is load current passing through the output end of the power supply, the base current of the transistor VT passes through, and the circuit oscillates; otherwise, there is no base current and the battery does not consume current, so this circuit does not have a power switch.

2. Component selection and installation and debugging
VT: PNP type low power triode, such as 2N3906, β>200.
VD: 1N4148 type switching diode.
C: 1uF/16V.
T: Step-up transformer, using Φ10mm magnetic ring as the skeleton, primary winding L2 with Φ0.15mm enameled wire for 16 turns, secondary winding L1 with Φ0.08mm enameled wire for 140 turns. Before winding, you can make a small shuttle with plastic or bamboo pieces. Cut a small fork at each end, wind the enameled wire around the shuttle, and then wind it, as shown in Figure 2.

Figure 2 Step-up transformer production diagram
The printed circuit of the multimeter substitute power supply is shown in Figure 3. The printed circuit board of the power supply can be made by knife engraving according to the size shown in the figure, without punching holes, and all components can be directly welded on the copper foil surface. The battery is installed on the circuit board, and the positive and negative poles are made of elastic phosphor copper sheets as a clip, which is welded to the corresponding position of the printed board to fix. The shell is similar in size to the laminated battery and can also be directly installed in the multimeter box.

Figure 3 Printed circuit of multimeter power supply
After the whole circuit is soldered and checked, it can be powered on for debugging. First, connect a 3 kΩ/0.125W resistor to the voltage output end, and use the DC voltage range of the multimeter to measure the voltage across the capacitor C to check whether it is around DC 9V. If the output voltage is low, the position of the leads at both ends of the transformer L2 winding can be appropriately changed. The power supply has good performance in long-term use. It should be noted that the capacity of the nickel-cadmium battery should be checked regularly to replenish the power in time.