The Laptop was down for a couple weeks because my dealer couldn’t get me a replacement for the broken power supply. Although I had a friends laptop, the unreasonable delay got me quiet frustrated. In fact it pushed me so hard, I decided to build a ‘Linear’ power supply for temporary use. This meant I would have to design (kind of) one all on my own. The good thing is, I learned a lot and the bad thing, I still had to wait till I got the original PSU, to turn the laptop on.
So why not a Switching PSU instead of the Linear one?
- I have zero experience designing a switching power supply
- I do not have a ‘properly’ working DMM
- Linear one is much easier to build and I had components lying around
- Although I haven’t yet designed any Linear PS, I was pretty sure they would work real good
And off I went got a piece of paper and started scratching my head!
Linear Power Supply constructed using an op-ampFully assembled power supply. The heat sink is pretty big, but its no match for the large amounts of heat the transistor dissipate.
The scribbled diagram is not correct. It's purpose was to get a basic idea to work on.
The series regulator is a simple extension of the voltage follower circuit of an op-amp. In simple words, an op-amp always tries to adjust its output so as to make both the inputs equal (inverting and non inverting). (the concept of virtual ground is used here). Since an op-amp cannot provide much current a series dissipative element, an NPN transistor, is used. The one I used is a 5Amp TIP122 Darlington transistor.
If we increase the bias to the base Vbe of an NPN transistor voltage drop across it Vce decreases (as Vbe increase, Vce decrease). When the whole circuit is wired up and if the output voltage increases due to some reason, this should be compensated by an increased voltage drop across the transistor (Vce). For this the base bias Vbe should decrease. This means the output voltage should be multiplied with a scaling factor and negatively fed back to the base of the transistor. This is where the op-amp comes to play. The output through a potential divider is connected to the inverting pin of the op-amp.
To get a stable output voltage, we need a reference source to compare the output voltage to. For this purpose, a simple Zener diode and a series resistor is used. This acts as a pretty good reference source. The reference voltage is connected to the non-inverting pin of the op-amp. This is because, we need to get a positive correlation between reference voltage and the output.
5A NPN darlington Transistor TIP122-D