I have a 220V to 20V – 2A transformer and I want to make a simple 24V regulated power supply out of it. So, using only components I already had I made the following:
simulate this circuit – Schematic created using CircuitLab
Firstly, since the voltage right before 7824 is 24.7V (due to the capacitor, and because it's unloaded guess) I thought it will be okay. Then I did a little research and found that the input voltage (to the 7824) should be at least 2.5V higher than the output. Obviously, I can change the 7824 to 7818 or lm317 and adjust output voltage to 20V or even replace the transformer with a bigger one. However, is there any other solution? Would it be a bad idea to leave it as it is?
Best Answer
For a \$2\:\textrm{A}\$ sustained output, and a minimum unregulated voltage of \$26.5\:\textrm{V}\$ and a ripple of \$2\:\textrm{V}\$, you will select diodes capable of peak currents of about \$12\:\textrm{A}\$ and a sustained level of \$4\:\textrm{A}\$. These will likely drop near \$1.3\:\textrm{V}\$ at their peak currents. So you will need a transformer rated at \$22\:\textrm{V}_\textrm{RMS}\$. Not \$20\:\textrm{V}_\textrm{RMS}\$. And a general rule, which in this case is close enough, you apply a factor of about 1.6 to the desired DC output current rating, so you'd want the transformer rated for \$3.2\:\textrm{A}\$, not \$2\:\textrm{A}\$.
(This rule comes from the fact that your peak currents generate more losses in the transformer than it was rated for. These peak currents only happen for a short time, so they do average out. But because they exceed the designed rating, the transformer's dissipation rating needs to be de-rated a bit. If you work through the math, it tends to come out close to a factor of 1.6. But an exact factor would require more detailed analysis -- in general, that analysis only buys you a tiny adjustment from that rough factor.)
Your transformer just doesn't cut it.
Your transformer is capable of providing \$1.25\:\textrm{A}\$ and a minimum voltage of \$24\:\textrm{V}\$, though. So if you can find and use a regulator which will work with that minimum input voltage to deliver something you want at its output, then you could still use your transformer. A possible circuit might look like this:
simulate this circuit – Schematic created using CircuitLab
Your bridge rectifier diodes would experience peak currents near \$8\:\textrm{A}\$, though. But with the transformer derating, as indicated at the output, and with appropriate selections for your bridge rectifier, it should be fine. The problem is that this won't help you achieve a regulated \$24\:\textrm{V}\$ output. Without any regulator at all, it's just scraping by to put out a minimum \$24\:\textrm{V}\$ unregulated output.