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For a class project (basic electronics class 1st year) I have to build a basic power supply. The proffesor told the class that the power supply must output 13V to 15V. He said use a 20VAC to 25VAC transformer that outputs 1A to 1.5A. The problem is I can't find any transformer with that current rating. I don't have time to buy online because I have to build it in class monday (the project was given yesterday). I have to buy from a local store like radio shack. The problem is the only transformers that they have are 25V 2A or 12V 450mA. I'm still not knowledgable in electronics so I'm wondering if 2A is too much. As far as I understand current is not pushed is only pulled by a component, or am I wrong? Some info. I'm using a full bridge rectifier, I still haven't calculated the capacitor value. I'm also going to be using an IC 15V regulator.

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    \$\begingroup\$ This might help electronics.stackexchange.com/questions/34745/… \$\endgroup\$ – Cornelius May 31 '14 at 8:18
  • \$\begingroup\$ One thing that you should check is the no-load voltage of the transformer. Transformers output higher voltage when the current is low, so due to your time constraints, it might be a good idea to get a bit bigger heatsink for the voltage regulator. There's a rule of thumb for estimating the output voltage of the transformer, but I can't remember it right now. It's mentioned on this site several times though, so searching should provide results. \$\endgroup\$ – AndrejaKo May 31 '14 at 9:48
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    \$\begingroup\$ Note that with a 20 or 25 VAC transformer the rectified voltage will be MUCH greater than 13V. Vpeak = 1.414 Vac for sine wave. 20VAC gives 28VDC peak and 25 VAC gives 35 VDC peak. Under load this will drop but still be >> 13 VDC. If you draw say 1.5A and if DC supply loads to say 22 VDC dissipation in the regulator etc is (22-13) * 1.5A = 13+ Watts. Regulator etc will need heatsinking. \$\endgroup\$ – Russell McMahon May 31 '14 at 15:26
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A home work question! But you declare it as such, and you already understand the concept - in this scenario (a voltage source) current isn't "pushed", it's "pulled" by the load. So the short answer here is that you'll be OK buying the 25 V, 2 A transformer.

A different way of thinking about this is for both voltage and current, the listed rating is what is/can be supplied. Therefore a 25 V, 2 A transformer will have 25 VAC on the output and the maximum current which can be drawn is 2 A. If the load draws less than 2 A, the no problems. More current is beyond what can be supplied, which will mean the transformer will overheat, with the follow-on consequences you might imagine.

You can't use the 12 V transformer because - assuming you're using a linear regulator - you need to supply a higher voltage to the regulator than what it regulates to. Exactly how much higher is a question for reviewing the data sheet for the device.

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  • \$\begingroup\$ Thank you for the fast response. I just wasn't completely sure. Thanks for the help. \$\endgroup\$ – user2227702 May 31 '14 at 8:38
  • \$\begingroup\$ Quite late to the party here, but a 12V transformer might be viable--if that's 12V under load, and you use low-Vf diodes for your rectifier (or use a two-diode rectifier with a center-tapped transformer) and you use a low-dropout regulator. Rectified 12V AC will give 12√2 volts, or about 17V, leaving you with 2V of headroom for your regulator dropout and rectifier voltage loss. This might be preferable even if you want to limit losses while still using a linear power supply. \$\endgroup\$ – Hearth Oct 2 '19 at 0:49

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