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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:

schematic

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?

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  • \$\begingroup\$ I'm guessing there's no load on the output ...try and use it and watch the voltage collapse (probably to 22V or so) \$\endgroup\$
    – user16324
    Sep 27, 2016 at 18:12
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    \$\begingroup\$ Look for a low drop-out regulator. They're available with drop-out voltages (minimum difference between input and output voltage) as low as 300 mV. What's available for 24 V output, I'm not sure. And watch out for degraded line regulation as the in-out voltage difference approaches the drop-out voltage. \$\endgroup\$
    – The Photon
    Sep 27, 2016 at 18:14
  • \$\begingroup\$ Is the mains voltage nominally 220 volt in your country? Or a variation of the same question: Are you sure that the transformer is 220V? If it's for example designed for 240 or 230V and your mains is 220, you'll see a proportionally lower output voltage. \$\endgroup\$
    – pipe
    Sep 27, 2016 at 20:29

4 Answers 4

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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:

schematic

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.

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  • \$\begingroup\$ Is it possible to reduce the voltage coming out of the 7824 to 20V or 22V (using resistors like LM317)? If yes, is it wrong? \$\endgroup\$
    – DimChtz
    Sep 27, 2016 at 19:44
  • \$\begingroup\$ @DimChtz The LM317 would probably be a perfect choice for your transformer. It's rated to supply no more than \$1.5\:\textrm{A}\$, which is above what I wrote about. So that's good. It may require up to \$3\:\textrm{V}\$ of headroom, though. So that would suggest a maximum regulated rail of about \$21\:\textrm{V}\$. Keep in mind the dissipation required! \$\endgroup\$
    – jonk
    Sep 27, 2016 at 19:47
  • \$\begingroup\$ Would Schottky diodes for the bridge rectifier help, with their lower forward voltage drop? Although the OP might be better off purchasing a transformer with a 24 V secondary at that point (or even 2 x 12 V secondaries for flexibility in re-use). \$\endgroup\$ Sep 27, 2016 at 20:06
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    \$\begingroup\$ @AndrewMorton Schottky diodes will cost more, have lower peak reverse voltage ratings, leak more, may be larger for the same ratings, etc. So I suppose you could go there. But you'd have to carefully check the specifications. But it will only buy you just a little bit of extra margin -- a minimum unregulated voltage of about 24.8V, at a guess. \$\endgroup\$
    – jonk
    Sep 27, 2016 at 20:31
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If the secondary of the transformer is supplying 20V AC you should have 20*sqrt(2) volt, being a little more then 28V, on the input of the 7824. That should be enough to make the stabelizer work correctly. Something must be wrong with the 2,2 mF capacitor being 2200 uF. Replace the capacitor with a new one and try again. It is also possible that the transformer is not supplying 20V AC under load.That however is not visible from here.Consider also the voltage drop over the bridge. There is not much room left.

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  • \$\begingroup\$ Thanks for your answer. I changed the capacitor and now I get 26V before the regulator (still not 28V). \$\endgroup\$
    – DimChtz
    Sep 27, 2016 at 19:39
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    \$\begingroup\$ Well, 20*sqrt(2) minus two diode drops, and then a little bit more for other losses. 26 volts sounds about right. \$\endgroup\$
    – pipe
    Sep 27, 2016 at 20:18
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20V AC gives a peak voltage of 28.3 volts and, after feeding through the diode bridge will be more like 26.8 volts DC. Under load this might droop and, if your AC supply drops by 10% you are in trouble so, how about using a flyback regulator: -

enter image description here

Or maybe one of the following if you want a few more watts: -

enter image description here

enter image description here

Having pointed you at a design you can probably get similar things to this on ebay for under $10.

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If you leave the circuit as is your regulator will perform poorly and your output will be lower than 24 V. In addition the transformers output voltage will drop with higher currents and therefore decrease the output voltage further.

Solutions:

  • Use a different transformer with higher output voltage
  • Use a buck-boost regulator instead of a linear series regulator (buck-boost or SEPIC)
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