Powering the multimeter from a 1.5 volt battery

Among radio amateurs and professionals digital multimeters are very popular due to their multifunctionality. As a rule, a nine-volt “Krona” battery is used for their power supply, which has a noticeable self-discharge, a small capacity and a higher price in comparison with other elements. The proposed device for supplying a digital multimeter from one AA element with a voltage of 1.5 volts will allow avoid these shortcomings in the work and simplify the operation of the device.
Multimeter power from a finger-type battery
There are many different schemes on the Internet for converting a voltage of 1.5 to 9 volts. Each has its pros and cons.This device is made on the basis of A. Chaplygin's scheme, published in the magazine “Radio” (11.2001, p.42).. The difference between this version of the converter is the location of the battery and the voltage converter,in the cover of the multimeter case, instead of creating a compact power supply unit instead of the "Krona" battery. This allows you to replace the AA element at any time, without disassembling the device, and, if necessary, disconnect the converter (Jack 3.5 connector) with the automatic switching on of the Krona backup battery located in its compartment. In addition, when manufacturing a voltage converter, there is no need for miniaturization of the product. It is faster and easier to wind a transformer on a ring of larger diameter, better heat dissipation, a free circuit board. Such an arrangement of nodes in the case cover does not interfere with work with a multimeter. This converter can be made in any suitable case and be used in a wide variety of devices that require power from a nine-volt Kron battery. These are multimeters, clocks, electronic scales and toys, medical devices.

Voltage converter generator circuit

A step-up DC-voltage inverter with good output with a minimum of incoming elements is proposed.The diagram is shown in the figure.
The two-stroke pulse generator is assembled on transistors VT1 and VT2.The current of positive feedback flows through the secondary windings of the transformer T1 and the load connected between the + 9 V circuit and the common wire. Due to the proportional current control of transistors, the losses on their switching are significantly reduced and the converter efficiency is increased to 80 ... 85%. Instead of a high-frequency voltage rectifier, the base-emitter transitions of the transistors of the generator itself are used. In this case, the base current becomes proportional to the current in the load, which makes the converter very economical. Another feature of the circuit is the disruption of oscillations in the absence of load, which can automatically solve the problem of power management. The current from the battery, when there is no load, is practically not consumed. The converter will turn on itself when it is required to power something on and off when the load is disconnected. But since in most modern multimeters the function of automatic power off is introduced, in order to avoid reworking the multimeter circuit, it is easier to install a converter power switch.

Manufacturing a voltage converter

Generator base of pulses is transformer T1. Magnetic circuit of the transformerT1 is a ring K20x6x4 or K10x6x4.5 from ferrite 2000NM. You can take a ring from an old motherboard.
Ferrite ring
Transformer winding order. 1. First you need to prepare a ferrite ring. • In order for the wire not to cut through the insulation gasket and not to damage its insulation, it is advisable to dull the sharp edges of the ferrite ring with a fine-grained sandpaper or nailfile. • Wind the insulating spacer on the ring core to prevent damage to the wire insulation. For insulation of the ring, you can use lacquer, tape, transformer paper, tracing paper, dolcano or fluoroplastic tape. The power of the multimeter from the battery 1.5 volts
2. Winding of transformer windings with the ratio of 1/7: primary winding - 2x4 turns, secondary winding - 2x28 turns of insulated PEV wire -0.25. Each pair of windings is wound simultaneously in two wires. We fold the wire of measured length in half and with the folded wire start tightly winding the required number of turns on the ring. Transformer winding To avoid damage to the insulation of the wire during operation, if possible,use an MGTF wire or other insulated wire with a diameter of 0.2-0.35 mm. This will slightly increase the dimensions of the transformer, lead to the formation of a second layer of the winding, but ensures the uninterrupted operation of the voltage converter. • First, the secondary windings lll and lV (2x28 turns) of the transistor base circuit are wound (see transducer diagram). • Then, in the free space of the ring, just in two wires, the primary windings l and ll (2x4 turns) of the circuit of the collectors of the transistors are wound. • As a result, after cutting the loop of the beginning of the winding, each of the windings will have 4 wires - two on each side of the winding. We take the wire of the end of one half of the winding (l) and the wire of the beginning of the second half of the winding (ll) and connect them together. Similarly we enter the second winding (lll and lV). It should look something like this: (the red pin is the middle of the bottom winding (+), the black pin is the middle of the top winding (common wire)). Transformer Assembly • When winding windings, the windings can be fixed with glue "BF", "88" or colored insulating tape with a different start color and end of the winding, which will help in the future collect the transformer windings. • When winding all the coils, one direction of the winding must be strictly observed,as well as mark the beginning and end of the windings. The beginning of each winding is marked on the diagram with a dot at the output. If the phase winding is not observed, the generator will not start, as in this case the conditions necessary for generation will be violated. For the same purpose, as an option, it is possible to use two colored wires from a power cable.

Assembling a voltage converter

For work in converters of small power, as in our case, transistors will do A562, KT208, KT209, KT501, MP20, MP21. You may have to choose the number of turns of the secondary winding of the transformer. This is due to the different magnitude of the voltage drop across the p-n junctions for different types of transistors. Transistors should be selected based on the allowable values ​​of the base current (it should not be less than the load current) and the reverse emitter-base voltage. That is, the maximum allowable base-emitter voltage must exceed the required output voltage of the converter. To reduce noise and stabilize the output voltage, the converter is supplemented by a node of two electrolytic capacitors (for smoothing voltage pulsations) and an integral stabilizer7809 (with a voltage stabilization of 9 volts) according to the following scheme:
The converter is assembled according to the scheme and we solder all incoming elements on a textolite board cut from a universal circuit board, sold in radio goods, by means of mounted installation. Board dimensions are selected depending on the size of the selected transistors, the resulting transformer and the installation location of the converter. The input, output, and common bus of the converter are derived from a flexible stranded wire. The output wires, with a voltage of + 9V, end with a jack 3.5 connector for connecting to a multimeter. The input wires are connected to a cassette with a 1.5 volt battery installed.
Converter 1.5v - 9v
Multimeter power from a finger-type battery
Multimeter power from a finger-type battery
AA battery (1.5V) is installed in a double cassette from a portable receiver.
Multimeter power from a finger-type battery
One place is occupied by a battery, another place is to install a power switch and fix the entire cassette through a textolite strip, in the case of a multimeter.
Placement of the converter in the case of a multimeter
Setting up the converter. Checking the correctness of the converter assembly, connecting the battery and checking with the device the presence and magnitude of the voltage at the output of the converter (+ 9V). If the generation does not occur and there is no output voltage, check that all the coils are connected correctly. The dots on the converter circuit indicate the beginning of each winding. Try swapping the ends of one of the windings (input or output). The converter is able to work while reducing the input voltage to 0.8 - 1.0 volts and get a voltage of 9 volts from one galvanic cell with voltage of 1.5 V.

Modification of the multimeter

To connect the converter to the multimeter, it is necessary to find free space inside the device and install a jack for the 3.5 jack plug or a similar connector. In my multimeter M890D, there was an empty space in the corner to the left of the Kron battery compartment. The case for an electric shaver is used as a case for a multimeter.
Placement of the connector
Prepared: Smirnov I.K.

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