# Is a 2A transformer too much for a power supply that outputs 13V to 15V DC

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.

• This might help electronics.stackexchange.com/questions/34745/… Commented May 31, 2014 at 8:18
• 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. Commented May 31, 2014 at 9:48
• 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. Commented May 31, 2014 at 15:26