Sooner or later, any novice radio amateur is faced with the need to have a simple, reliable and inexpensive regulated power supply for testing your own crafts, and, of course, testing new ones "Patients". There are few options - either buy a ready-made unit with the required characteristics in the store or from a more experienced craft colleague, or assemble the device yourself from the materials at hand. Taking into account the prices of more or less high-quality SMPS with voltage regulation (on average from 15 to 80 y. e.) the conclusion suggests itself.
- 1 We do not want to buy, we want to create!
- 2 Accessories and scheme
- 3 Few nuances
We do not want to buy, we want to create!
One of the most simple and versatile options is the power supply on the LM 317. It is popular and inexpensive. adjustable linear voltage regulator, usually manufactured in TO-220 package. You can find out which leg is responsible for what, from the image below.
The main characteristics are:
- Input voltage up to 40 V.
- Output current up to 2.3 A.
- The minimum output voltage is 1.3 V.
- Maximum output voltage - Uin-2 V.
- Working temperature - up to 125 degrees Celsius.
- Stabilization error - no more than 0.1% of Uout.
A little more detail on the maximum current. The fact is that the LM 317 is a linear stabilizer. The “extra” voltage on it turns into heat, and the maximum thermal package of the microcircuit with an additional cooling radiator is 20 W, without it - about 2.5 W. Knowing the formula for calculating the power, we can calculate what kind of current can actually be obtained under different conditions. For example, Uin = 20 V, Uout = 5 V - voltage drop Upad = 15V.
With a thermal pack of 20 W, this means the maximum allowable current of 1.33 A (20 W / 15 V = 1.33 A). And without a radiator - just 0.15A. So besides radio components should attend to finding a radiator - Any more relaxed, from the old power amplifier, and the choice of power source must be approached wisely.
Accessories and scheme
You need quite a few details:
- 2 resistors: constant, nominal 200 Ohm 2 W (better is more powerful) and variable tuning 6.8 kΩ 0.5 W;
- 2 capacitors, voltage in accordance with the requirements, capacity - 1000... 2200 microfarad and 100... 470 microfarad;
- diode bridge or diodes designed for a voltage of 100V and a current of at least 3..5 A;
- voltmeter and ammeter (measurement range, respectively, 0... 30 V and 0... 2 A) - analog and digital will suit your taste.
- a transformer with suitable characteristics - at the output of no more than 25... 26 V and a current of at least 1 A - in power better pick with a good marginso that there is no overload.
- radiator with screw fastening and thermal grease.
- The body of the future power supply, into which all the details fit, and, importantly, with good ventilation.
- optional: screw clamps, adjustment knobs, crocodiles for leads, and other small things - toggle switches, operation indicators, fuses that will save the power supply from serious damage and make it more work with comfortable.
Just in case, we will explain separately why the transformer voltage is not more than 25 V. When straightening using a filtering capacitor, the output voltage rises by the root of two, that is, about 1.44 times. Thus, having a 25 V AC winding at the output, after a diode bridge and a smoothing capacitor voltage will be about 35–36 V dc, which is pretty close to the limit microcircuits. Keep this in mind when choosing capacitors and transformers!
As you can see, there is very little work - the wiring of parts can be performed even by mounting, without compromising quality, provided that all contacts are carefully insulated and the power supply is durable.
After assembly, do not rush to connect to the unit load - first check the voltage at the output of the diode bridgeand then run the unit at idle and use your finger to check the temperature of the stabilizer - it should be cool. Then connect the power from the unit to any load and check the voltage reading at the output - they should not change.
LM 317 has many analogues, both good and not so - be careful when choosing a product on the market! If the accuracy of adjustment is important, you can change the value of the tuning resistor to 2.4 kΩ - the output voltage range, of course, will decrease, but accidentally touching the handle will hardly change the output voltage - and sometimes it is very important! Experiment with different denominations to make your power supply convenient.
It is also necessary to observe the temperature mode - the optimum operation temperature of the LM 317 is 50... 70 degrees Celsius, and the more the chip heats up, the worse the voltage stabilization accuracy.
If constant large loads are assumed, say, powering power amplifiers or electric motors, it is desirable not only to fix the microcircuit on the radiator, but also increase the capacity of the smoothing capacitor up to 4700 uF and above. With a properly selected capacity under load, the voltage will not subside.
When you decide to acquire your own universal power source, think that it would be better for you to pay a decent amount for ready to make a decision or assemble the device with your own hands, using inexpensive components and amusing your own pride even if small, but, nevertheless, an achievement.
The cost of an adjustable power supply, made by hand, is small - from the cost of the chip itself (about 20 rubles) to 700–800 rubles when buying new parts in the store.