Car power supply for multimeter, 13/9 volts 8,9 milliamps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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Many car enthusiasts, among other things, carry an inexpensive M830 type multimeter with them, or keep it in the garage. Of course, the device is very necessary, especially if something breaks out of the electrics on the road, but although it is portable, it is more suitable for home storage.
The 9-volt battery is especially affected, which, after a drop from severe frost, to the temperature of the glove box warmed up by the stove, loses a significant share of its capacity. And the device needs such that at any time it is ready for work. So it comes to mind to power the multimeter from the "cigarette lighter", like a portable lamp.
Of course, everything seems simple, you can lower the voltage to 9 V with a stabilizer and connect it instead of Krona. But if at the same time we take measurements in the circuits of the machine, that is, in the circuits of the power source, then the readings will be unpredictable. Yes, and the multimeter itself can be spoiled by such treatment. It turns out that the power supply of the multimeter must be galvanically isolated from the electrical circuits of the car. This can be done using a pulse converter circuit with an intermediate transformer.
Since the current that the multimeter consumes is very small, the source may also be low-power. The figure shows a source circuit based on the K561LN2 CMOS chip. This chip has six inverters with high power outputs. Here, a master pulse generator with a frequency of about 100 kHz and an output buffer stage are made on it.
The master oscillator is built on the elements D1.1 and D1.2 according to the multivibrator circuit.
The generation frequency is set by the C1-R1 circuit. The remaining four inverters are assembled in parallel and represent the output buffer stage.
Parallel connection of elements allows you to get increased output power. The pulses from the outputs D13-D16 connected together are fed through the decoupling capacitor C2 to the primary winding of the RF transformer T1. From the secondary winding, the alternating voltage is supplied to the rectifier VD1-C3 and the parametric stabilizer R2-VD2, which maintains the output voltage at 9 V.
Transformer T1 is wound on a ferrite ring measuring 10x6x5mm from 2000NM ferrite. The edges of the ring must first be smoothed with fine sandpaper so that the winding insulation does not collapse on them. Then coat the ring with a thin layer of epoxy varnish. And after drying, wind two windings. The primary contains 30 turns, the secondary 60, both windings are wound with PEV 0,12 wire (or so). The windings are located on opposite sides of the ring.
Chip K561LN2. It can be replaced by a foreign analogue CD4049, but this is not a complete analogue, as it is made in a 16-pin package and differs in pinout. In general, you can also use a microcircuit with four inverters, for example, K561LA7, put two of them on the multivibrator, and two on the buffer, but what the output current results will be will be clear only from the experiment.
Diode KD102 can be replaced with KD103, KD522, 1N4148. Zener diode. D814B-1 - in a glass case.
Here you can use any zener diode for a voltage of about 9 V.
The output current of the converter is 8,9 mA. this is quite enough to power the multimeter type M830.
Author: Snegirev I.
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