Greetings, homemade radio amateurs!
Everyone knows about voltage stabilizers, these are 78Lxx series microcircuits, as well as adjustable LM317 and the like. They are often used in electronics because of their convenience – just one element allows you to reduce and stabilize the voltage at the required level, no matter how the input voltage jumps, it will always be unchanged at the output. However, most of these stabilizers have a drawback – a sufficiently high voltage drop, this leads to the fact that the difference between the input and output voltage must be at least 1.5 – 2V, which is very significant in low-voltage designs. Sometimes this leads to the fact that the use of a stabilizer is generally impossible: for example, if you want to obtain a stable voltage of 3.3V (the standard value for many circuits) from a single lithium-ion battery. The normal operating range of the battery is between 4.2V when fully charged and 3.3V when discharged. However, if a stabilizer designed for 3.3V is put on the battery output, then it will not work to discharge the battery below 3.45V, the stabilizer simply will not give, and thus part of the battery capacity will remain unused. This is where low dropout linear regulators come to the rescue, the so-called LDO (Low drop output). They are often produced in SMD cases, are not so common on the market and are much more expensive than conventional stabilizers. Alternatively, you can assemble such an LDO stabilizer with your own hands, this will require 4 transistors and a small handful of parts. Of course, it will take up more space than a factory ready-made microcircuit in an SMD case, but sometimes it can be very useful, given that every radio amateur has extremely common parts for assembling it. The diagram is presented below.
The input voltage is applied to the left side of the circuit, while, as indicated in the diagram, the difference between the input and output voltage can be only 0.05V, but this will be true only with a very low-power load. With an increase in the current consumption of the load, the minimum required input-output voltage difference will also increase. A nice feature of the circuit is that if the input voltage drops below the set output voltage, the circuit will freely pass current, so the consumer will not turn off.
The output voltage can be set in the range of 2-8V, it is varied using a 2.4 kΩ resistor marked with an asterisk in the diagram. It would be best to install a trimmer for convenient adjustment, the optimal value of the trimmer will be 5-10 kOhm. Transistors VT1-VT3 on the diagram are any low-power NPN structures – each has KT315, or KT2102, or BC547 in the bins. The transistor in the upper part of the circuit is a power transistor – the current consumed by the load will flow through it, so you should use a transistor with a maximum current of at least 3A, KT816 or its analogues. The maximum operating current of the circuit is 0.5A; at this current, the power transistor should be placed on a small radiator so that its heating does not exceed a dangerous level. The rest of the circuit does not have any features, it is assembled from conventional output resistors and capacitors.
In order not to etch the printed circuit board, the author offers an alternative option – using a homemade cutter to mark a small piece of PCB into rectangular spots, on which the circuit elements will then be soldered. Such a cutter is made from an unnecessary cloth for a hacksaw for metal – a piece of cloth about 10 cm long is taken and a tip is made at one end of it, with which it is very good to scratch insulating tracks on the copper surface of the PCB. Wire wrapped around the handle of the cutter for a comfortable grip.
This method of making boards is ideal if, for example, you need to quickly check the operation of a simple circuit, or simply if there is no pickling solution for copper at hand. The picture below shows the received board. After the grooves are cut in the right places, the copper patches are tinned, it will not be superfluous to ring the entire board for short circuits between the patches – they should not be there.
Then the parts are soldered to the board in accordance with the diagram, the process is shown in the photos below .
Thus, a very interesting adjustable voltage regulator with low voltage drop has turned out, the main feature of which is the use of the most affordable and cheapest components. It can be useful, for example, in devices with battery or rechargeable batteries, where every tenth of a volt is important. Happy build!
Greetings, homemade radio amateurs!