If you want a Dual Variable power supply circuit that covers the most appliances. This circuit may be what you are looking for. With the following features.
1. On signal mode—can supply DC voltage of 0V to 60V.
2. On Dual rail mode—Can provide DC dual variable on positive, negative and ground, are +/- 0-30 volts.
3. Can supply current out the maximum of about 1.5 A.
4. Use the most popular IC are LM317T and LM337. So, do not worry about finding the devices, and it is certainly easy.
Before construction, we should learn the properties of important devices, regulator IC.
Choosing Regulator IC
The regulator IC is used to regulates DC voltage. Which in this circuit, we use an IC number of LM317T for the positive voltage and the LM337 circuit for the negative voltage. Both ICs have the same features. Just only it is the different polarity. Thus, describes specific number LM317T only.
Figure1: 0-60 volt DC variable power supply circuits using LM317&LM337
The LM317T is the adjustable regulator IC, can adjust the output voltage of 1.25V to 37V. There are 3 pins and the body is the same as the regulated IC, 7805.
But it can provide to 1.5 A and gets input voltage of 3V to 40V. It is easy to apply and these features have advantages more than regular IC 78xx series a lot.
In addition, Internal the LM317 has full protection, the short-circuit protection, input over voltage protection, overload protection. Another interesting feature is the elimination of the ripple.
The output voltage (Vo) determined from the formula.
The R1 is the resistance constant from 120 Ohms to 240 Ohms. We can adjust resistor-R2 from a minimum value (0 ohms) to the value as we want. If R2 is 0 ohms minimum output the voltage of about 1.25V.
Related: LM317 Calculator
Making LM317 start at 0 volts
In electronic circuits require a constant voltage. The output voltage of LM317T may not need to reduce to 0V. However, in the experiments, we may be necessary to start the voltage at 0V.
It has this weakness. The minimum output voltage across between Adj and the ground is 1.25V.
But here is how you can make the output is 0V, it is easy to do that.
By creating a reference voltage is lower than the zero 1.25V. It is -1.25V. Then, take this voltage to the Adj. So you can adjust the output voltage to 0V.
The negative voltage regulated IC, LM337Tis similar to LM317. If the Adj connects to the ground, get lowest output voltage is -1.25V.
Which if you want to adjust the output voltage is 0V. You need to connect Adj of LM337 with the positive reference voltage, +1.25V.
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How Dual variable circuit works
In figure 1 is this complete circuit. The Diodes D1-D4 are rectifier ACV from 24 volts of the transformer into DCV of about 33V, in both positive and negative voltage.
The capacitor C1 and C4 are a voltage filter from the bridge diodes to smooth.
The R1, D5, and D6 are made the reference voltage of +1.25 volts to the LM337T to adjust the voltage to start at 0V.
The R3, D10, and D11 make voltage -1.25V to LM317T. It can also adjust the beginning voltage is 0V.
The D7, D8, and D12, D13 protects voltage backward voltage from the output. Which may make IC Which may cause damage to the IC.
The C2 and C5 are connected to reduce the noise signal from adjustable the potentiometer (VR1, VR2) and makes the voltage at the output to smooth up.
Judicially built and tested before applications
This project has a few amounts of devices. We can assemble onto the perforated board. You need to check and check the circuit for error. When you are sure all the wires and components are installed correctly.
After examining successfully assemble the devices. Then, try to apply AC power. If everything is not wrong. Hold the DC voltmeter measure the positive output. And then Rotate VR1, you can read the voltage from 0-30 volt.
If everything is correct, then move the lead of the meter to measure the negative. And then adjust the VR2, you can read voltage from 0-30 volts.
Everything is ready for applications. However, if the malfunction, disconnect the transformer. Then looking for the error.
Building Dual Variable power supply circuit
This project is not used many components so can assemble on the universal PCB board. But can make a Single-sided PCB layout as Figure 2. And see the wiring and various components layout can view the example in Figure 3. But be careful the polarity of the electrolytic capacitors and Diodes and both IC1-LM317, IC2-LM337 correctly and hold proper the heatsink.
The LM317T is positive voltage regulator ICs that low watts because are the plastic case or TO-220 model.
The LM317K is higher watts because of the metal case TO-3 model.
The LM337T is negative voltage regulator ICs in TO-220 model.
See: LM317K pinout
Be careful polarity component
Some components have the polarity. For example, the electrolytic capacitors, diodes, LM317, LM337, etc. If you incorrect them. Your circuit does not work. Event them damage.
Also Dual variable power supply
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