My First Project
Aaah my first electronic circuit…the fruit of my frustration at the local faculty’s attempts to deny me the joy of the op amp early in my btech. We had to do a paper on voltage regulators. I had studied and gone thru quite a few books to get mine ready. and it was quite exhausting. I had gone the whole hog and did even op amp voltage regulators which were ….oh how I hate the phrase. .”Out of syllabus” the problem of which was compounded by two conflicting diagrams in two textbooks, one by Boylestad and another by Remanan. Anywhere I took the doubt I was given the “no op amps till fourth semester” reply by all lecturers. I decided to take it upon myself to get to the truth myself. After learning of course that 741 op amps were Rs 6 each and not obscenely expensive. And I needed something to step down the 12 turntable motor voltage supply from my old Panasonic hi Fi. Most people would have used an lm317 and gotten it over with .unfortunately my research wasn’t that exhaustive. I decided to build the step down voltage regulator. But being a keen audiophile and having read to no end about noise hum and distortion, decided to back up my bets with the proven ripple rejection of a monolithic regulator. A 7806 was chosen for the task because 6 v was the min voltage a 741 worked at. The reference was…an LED. I understand it was unconventional, but understand me….it was an extremely ‘green’ engineer then who thought that zeners existed only up to 4.5 volt ( I wonder where I got the idea) and that the zeners below that value were not actually in the right breakdown mode.(SURPRISE!! Zeners below 4.5 are the real zeners, those above work on avalanche mode and are more stable) then I had an option to use a potential divider from the 7806 output. compared to that a 2 v from a forward biased led looked far better.( I had thought about using an ordinary diode also…but I had seen a log graph of v/I for all diodes and ordinary diodes didn’t look good compared to an extremely linear performance by led’s. forward biased transistor junctions gave a good performance but in those days I felt that it was a perverse use of a transistor .and thus it was decided. I was going to build it
This circuit, if you’re new to electronics is very good because it constitutes both types of voltage regulators. The op amp one is similar to what the IC 7806 looks from the inside. The design of the 7806 part was fairly straightforward as the datasheet came with an elementary diagram and design suggestions. The op amp design was what it came down to.
THE OP AMP
For those of you who don’t know what an op amp is, it s a circuit block that compares two voltages and gives an output which is proportional to the difference. Typically the constant of proportionality called the gain should be infinite. Most op amps achieve 10^5 and is for all practical purposes assumed to be infinite. How we get to make any use of this gain is by making it (heh heh) eat its own output and thus improve it. How it happens we shall see later. Oh and the process is called feedback. Another thing you will find out about sooner or later in electronics. in this case, the two inputs are the voltage from the LED (2v) and a fraction of the output taken by the potential divider formed by r2 and r3.the op amp gives an output proportional to the difference. Suppose the diode is at 2 v and the output of the potential divider is at slightly higher value (it should also be at 2 v. if it is higher it means that output voltage is higher. and the circuit being made to be a regulator would make its creator…meJ verry sad if it didn’t .but I’m a good creator and the circuit does it…how?
Ill tell you.)The slight increase is amplified by the op amp. But notice that the circuit is wired such that output is proportional to V diode –V divider. This is because the diode is connected to the positive input and the potential divider to the negative input. NOTE: you NEVER and I mean never connect the output of an op amp to its positive input unless you really know what you’re doing (and what you’re doing is an oscillator or a latch of some sort).anyway the small difference gets amplified …but negatively and fed to the output transistor. This transistor is here in what of you know as the common collector configuration. Input is at base and output at emitter leaving the collector to be common.
The difference causes the transistor to decrease current and reduce voltage. See how the output, when fed in the right way causes output to improve. What you just have seen is what is called feedback or more precisely negative feedback at work…TA-DAAAAA!
What you know now is more valuable in understanding an op amp than anything else. I know it helped me make sense of op amps and also a shocking fact that the foreign (Boylestad) text was wrong! Now comes the design part there are only three resistors to be designed here the one in series with the led and the two forming the potential divider.
This is the part where you use your instincts. There are no single correct values. Only a range from which we choose. For example the current limiting resistor for the led. Ideally it would be as low as possible if we want the led to be bright but then our current source is limited and we don’t want to blow up our led! Aim for a few mA. For the potential divider the opposite case is true. We can make the resistor values as high as possible as all we need is a voltage and no current is required. If we had an ideal op amp it would have been the thing we would have done. But it is not because it has an input impedance of only 47 k( ideally should be infinite to prevent loading…of the circuit in this case the potential divider) so here also an a rule of thumb we agree to get resistors in the k ohm range .but the values are up to your discretion, just enough such that the output of the potential divider is 2v at output voltage = to voltage you want. So I am all ready to plug the walkman into the circuit. It works fine…I used an SL100 as the power transistor. You can use any power transistor…you know the bigger ones. I use it on headphones for one night and no troubles result. The next day I connect the walkman output to my Hi Fi and guess what …all sorts of bangs and pops. I’m stumped and desp! Finally a cool head and several checks later it turns out that my walkman is the problem. Its output ground is a few volts ABOVE 0 V so I cant connect both of them together without letting some serious short circuits…it is ort of like connecting the input and output of a bridge rectifier to the same C.R.O because there also the ground points are not the same. Thankfully it turns out that it is just a dc component that was cured but adding an amplifier stage to it. I would have used an op amp there also but since I needed to run it off a car battery during power cuts I used a power amplifier IC TDA 2822 sort of like an op amp but with lesser gain so that you need not add anything to reduce the gain as you would to an opamp. This one gives a fixed gain of about 100 as opposed to 10^5 for an op amp. the capacitors are there to block dc…(RC coupled amplifier remember. only here the r is inside the IC)and the r c at the output is called a zobel network…no need to learn that now just something used at the output of most audio amplifiers to improve High freq stability. WHEW! That’s about it for today. Any doubts?….feel free to comment!
posted by kickassso, 11:59 PM