AN920

Much easier said than done. :) The LF pole created by the slow TIP31A will cause many problems that won't be easily cured. As it stands now the design is unstable and may oscillate with certain loads. The problem is that the transistor creates a LF pole and the output capacitor another LF pole. With two poles close together you are looking at stability issues. Lowering the gain of the error amp for more bandwidth in order to get stability is an option, but will degrade , PSRR and higher frequency noise rejection. Stability is real reason for concern that needs attention.

Good find! Saves time doing calculations, but does not calculate Rs which is important.

Using a series crystal on the output will act as a very narrow BPF at the crystal series frequency Fs.

After drawing the diagram for the PSU I could not resist doing some tests. I wanted to see the transient response during a step load change. As the design went through a couple modifications to increase output, it was a good idea to look into these changes and the effects on performance. I simulated only the voltage regulation stage (current limit circuit removed), with three different drivers. Error amp is TL081 1) BD139 (Ft 100+ MHz) 2) TIP31A (Ft 3 MHz) 3) 2N2219 (Ft 200 MHz) Tests were performed at 22 Volt out and a constant load current of 100mA. The current was then stepped up by pulsing a MOSFET and another load resistance. The total switched current was about 1 Amp. Below are the results. The blue trace is the pulse control signal to the MOSFET. The yellow trace is the output load voltage response. Violet trace is the error amplifier output. We can see a possible problem with using the TIP31A as a driver. It struggles to catch up with the error amplifier output and causes overshoot and ringing. The problem seems to be that the transistor is just too slow. The phase response plot of the system confirms this as a rapid phase shift at a fairly low frequency that could cause this problem.

It is difficult to comment without the whole picture. The missing parts are the crystal data. It is possible to use other ratios as long as you can get enough gain and enough negative resistance to satisfy oscillation. The article explains in detail most of the other circuit design values but none on the crystal oscillator? Many people get crystal oscillators to work by trial and error, not that I am suggesting that that was the case here. By making C20 much larger the loop gain will drop off more rapidly with frequency which will attenuate harmonics. The designer may have experimented with values that gave the lowest harmonics with just enough gain to oscillate at 14.3MHz Another crystal of the same value in series on the output will produce a very good sine wave output with low harmonics.

This should help you, and others to find faults in this PSU. Voltages to expect with three different conditions. 1) PSU with almost no load (2.2k) set to 12V 2) PSU with 1A load (22 Ohm) set to 23V 3) PSU with current limit turned down to about 500mA, from condition (2). Current limit LED is on. Measurements taken with input GND as reference and not out "-" !

Something strange in your circuit. Make sure you don't have any stupid mistakes, solder bridges, wiring errors etc.

I look after myself! :)

Can you adjust the output voltage with U3 removed?

Yes "-" U3 seems faulty. pin 6 should be positive as the non inverting input dominates.

Sorry, I meant voltage between GND-pin2 and GND-pin3

What is the voltage on U3 (pin 2,3)?

I was very happy when my parents got me my first computer. It was a Sharp MZ-80A! Still have it. http://www.old-computers.com/museum/computer.asp?st=1&c=172 I would spend many hours on this thing, sometimes programming right through the night. I wrote my very first chess program on this. Took months, but learned a lot about peeking and poking and machine level instructions!

May be a LM86-18N

Here is a simple experiment to see the effect of negative restance from a reversed biased transistor. It will happen at a certain bias level. If you copy my circuit then the same values should work for you. We would expect the voltage to decrease when we add resistor R2 as we will be drawing more current through R1. The opposite happens. The voltage increase by 1 volt. Read more on Google about negative resistance to understand this. This won't work using a circuit simulator.

http://www.gallawa.com/microtech/magnetron.html

Use the darlington I used or just a common bipolar like a 2N3904 or 2N4401. Power darlington's internal structure is different with low value resistors to help speed up switching time.

You may find old issues through your local library. Here are a few listed on this page. http://www.nleindex.com/index.php?pID=HTDI&sID=BrowseIndex&tID=E/99

This transmitter dates back from a 1965 electronics magazine. Circuit looks very much the same as many circuits on the web today. Some things never change.

DC motors have a lot of pull-out torque. Using a gear box enhances that.

You will have to calculate the moment of inertia. You need the radius of your disc. M.O.I = (mass x radius^2)/2 From here you can calculate the required torque requirement. It will help a lot if you know: 1) The load torque 2) Required rotation speed (rpm) 3) Time allowed to get up to speed (sec)

http://pdfserv.maxim-ic.com/en/ds/DS1869.pdf

In a recent article it was calculated that the savings for an average household would be only 0.1% of the total monthly electric bill. For industrial applications with lots of motors it will pay over time.

I don't know about power darlingtons. I implemented the small signal darlington in a design in this thread. http://www.electronics-lab.com/forum/index.php?topic=11442.msg62103;topicseen#new

Ok, I was wrong. They do work, and quite well. Just hooked up one here in the lab.