DIY voltage regulator: To do a good job, first use the right tools

● This is the full view of the interior. It’s ugly, right? In fact, as long as the things made by DIYers are reliable and practical, it’s fine.

● The biggest thing is a custom-made toroidal transformer, which has the advantages of small size, high power, low magnetic leakage, and a higher price. The parameters are 28V 100W, which is equivalent to 28V3.5A. The black part on the right is fixed with a tie, which is the main filter capacitor, with a withstand voltage of 63V10000UF.

● Some people may ask, why is 63V used here when the previous voltage is 28V? In fact, the previous voltage value is AC voltage. After rectification, the maximum voltage fluctuation is 28V*1.414=39.592V. Generally speaking, the voltage of electrolytic capacitors is divided into 25V-35V-50V-63V. In this case, choosing 50V withstand voltage is the most economical and practical. However, we DIYers may not always have capacitors with suitable voltages, so I increased the withstand voltage value of the selected capacitor. I happened to have a 63V10000UF capacitor, so I used it. Although the cost will increase if components are used in excess, their reliability and life will be much longer.

● The thing with a small heat sink welded on its foot is a 600V3A rectifier bridge, which is equivalent to 4 diodes bridged. Because the current passing through is relatively large, a heat sink is added. If the chassis shell is aluminum, you can also directly apply thermal grease and lock it directly on the shell to dissipate heat, which will lower the temperature.

● Further down, the black heat sink is the main circuit of the voltage-stabilized power supply. Of course, this heat sink is a bit small. If you like fanless operation, the volume of the radiator should be about 4 to 5 times larger.

● Because I need to output a relatively large current of about 2A, and I want to spend as little as possible while ensuring the simplicity of the circuit and the performance, so here I chose the LT1083CT three-terminal adjustable voltage regulator from LT Company of the United States. Of course, it is a disassembled one, and a brand new one is very expensive...

5 Circuit part

● The circuit is very simple, and its basic circuit needs to be slightly modified. The input voltage is about 40V DC, the capacity of the input aluminum foil capacitor is 10000UF, the adjustable resistance of the ADJ end uses a 10K ohm 3296 precision adjustable potentiometer, and the output uses a 35V22UF tantalum capacitor, because the impedance of the tantalum capacitor is quite small and the high-frequency characteristics are also quite good, which can be equivalent to the output performance of a 1000UF ordinary electrolytic capacitor, or even better.

● The output voltage of the three-terminal adjustable regulator is determined by the ratio of R1 and R2. Usually the value of R1 is 100 ohms to 120 ohms, and the output voltage is always less than the input voltage.

● Let's look at its output characteristics. Our power supply is about 40V DC, and the output is 24V DC, with a voltage difference of 16V in between. According to the chart, as long as the heat dissipation is good, the transformer I use can work at full power. In fact, I should reduce the voltage difference, because the higher the voltage difference, the greater the heat generated by the voltage regulator. This is where I did not do well. When I customized the transformer, I did not calculate the parameters properly, and I casually reported 28V...

● A hole was opened on the top, and an 8015 11-blade silent fan was installed. It is from NMB (Japan Meipei), and is powered by 24V DC. In order to reduce noise, an LM7815 was installed to reduce the power supply voltage, reduce the speed and noise, and isolate some fan interference. Because the metal back plate of the three-terminal regulator is usually connected to its voltage output pin (not absolutely), insulators and insulating heat dissipation silicone pads are used to insulate and conduct heat from the radiator to prevent short circuits.

6 Testing

● The final step is the full power test. According to the actual use, the current is about 2A, and then gradually decreases to below 1A. Then, I can directly connect a power resistor to the 24V end. The resistor is a 25W 10 ohm ceramic core wound power resistor. The current on the resistor is 24V/10 ohm = 2.4A. The actual power consumption is 24*2.4=57.6W, which is far more than the resistance power. So let's water cool it, just throw a cup of water. After working for an hour, the water temperature is hot, and the temperature of the internal regulator and rectifier bridge is acceptable. Basically, the power supply is successful.

● Well, my task is completed, but the power supply for our DIYERs cannot be so simple. For example, a voltmeter can be connected to the output port and an ammeter in series, so that the actual voltage value and current consumption can be monitored; the 3296 potentiometer can be replaced with a larger multi-turn adjustable potentiometer and installed on the panel, so that the output voltage can be adjusted. For this circuit, the output voltage can be infinitely adjustable within the range of 1.25V~35V. The output can even be reduced to 0V by changing it. If the current demand is not large, you can replace it with a cheaper three-terminal regulator, such as LM317T, which has the same output voltage range as LT1083CT, a maximum current of 1.5A (needs proper heat dissipation), and the pin definition is the same. You can also make a battery charger based on its voltage regulation performance. It can even be used as an audio amplifier, although its sound quality is not very good... Open your mind, there are still many places to play!