Adjustable 7805 and 7905 Voltage Regulator Circuit

Let’s build a cheap adjustable dual regulator circuit that uses a 7805 and 7905 voltage regulator IC as the main components. This circuit is also a great way to understand the basics of OP-AMP circuits.

This 7805 and 7905 voltage regulator circuit can supply a voltage ranging from +5V to +25V and -5V to -25V, which means that it can supply both positive and negative power. Both rails are also operated separately by different optentiometers, hence the ‘dual’ in the name.

Why should we build this circuit?

Suppose that we are testing an OP-AMP IC, such as uA741, LF351, OT071, et cetera. Usually, you would just use two 9V batteries. Simple enough, right? 

But the energy in the batteries is limited, so you will have to buy a new pair of them at some point. And there’s a high chance that your new batteries might not have the exact same electrical properties as the old pair, which may introduce unnecessary variables to your testing.

Designing Circuits And Estimating Components Easily

Based on our rough requirements, we can create a simple block diagram, as shown below.

First, we know that we use 7805 and 7905 voltage regulator ICs and two potentiometers(VR1, VR2) for adjusting an output voltage as a simple variable power supply circuit.

So, we will be using this 28V DC unregulated power supply circuit.

The AC voltage flows into the circuit through the S1 ON/OFF switch and passes F1 to protect against overload current. Next, the AC voltage flows to the primary coil of the transformer (T1) to be converted to a lower AC voltage.

Vsec = 27.5V ÷ 1.4 = 19.64V

Therefore, it is recommended to use a 20V secondary coil, or you may use an 18V transformer like in our prototype circuit. But the actual voltage we measured was 18.89V, causing the Vin to drop to 26.46V, which is still close enough to what we need.

The Rectifier: we will use four 1N4007 (1A 1000V) diodes connected together in a bridge form.

Next, we can calculate the capacitance of the C1 and the C2. Both act as a DC filter, giving us a smooth DC voltage.

For this circuit, we need to keep the noise as low as possible. Let’s say we are testing a preamplifier circuit using an OP-AMP IC such as uA741. But instead, we hear noise mixed in with the music we are listening to. The noise can occur from a variety of points, but it typically travels through the power supply.

The solution is to add decoupling capacitors. The 7805/7905 datasheet requires the addition of output capacitors (C5 and C6), for which we chose a 22uF 50V electrolytic capacitor for each. 

However, their high-frequency response is poor, so this may not be sufficient. So we should also add C7 and the opposing C8. The noise will see C7 as a very low-resistance resistor, so it will flow through it to the GND, and the DC voltage is immediately cleaner.

Now, we put all the components together, turning it into a complete 7805 and 7905 voltage regulator circuit, as shown in the diagram below.

• 0-60V Dual power supply using LM317, LM337 
• +15V CT -15V Dual Regulator circuits
• 12V Dual Power supply using 7812 and 7912

How to build

This project consists of quite a few electronic components and details. I think that we can solder all components onto the perforated PCB.

Testing

We tested this circuit by turning VR1 (positive voltage) and VR2 (negative voltage) completely counterclockwise and measuring the voltage, which should be at +5V and -5V, respectively. 

Then, turn clockwise continuously. It will read the voltage increase according to our voltage adjustment. Until it reaches all the way, it will read the highest voltage, which is around +24.5V and -24.5V.

Next, adjust VR1 and VR2 until you get +18V and -18V, which gives a total voltage of 36V, as we want. Then, connect two loads (two 100Ω 1W resistors), and it will have the same voltage level. Changed or decreased only slightly. And L7805 and L7905 had a slight temperature increase. It shows that it can supply a current of approximately no less than 180 mA.

And, when adjusting the voltage to +12V and -12V and changing the load to 47Ω 1W resistors, the voltage level dropped to +11.3V and -11.3V. It is considered that this circuit works well enough.

Conclusion

This simple circuit works pretty well, exactly as we need it, and is economical too. We can use equipment that we already have (or can easily find) to create. Additionally, there are other dual-adjustable power supply circuits that can be used with the OP-AMP circuits listed below.