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.
Electronic – Is a 2A transformer too much for a power supply that outputs 13V to 15V DC
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Related Solutions
You can easily make a 13V low current linear regulated power supply with the LM317. The LM317 is a programmable regulator that has the output voltage set via a pair of resistors that form a divider from the output to GND. A simple power supply would consist of these two resistors and two capacitors, one at the input and one at the output. The following picture shows the circuit for an LM317.
For a 13V output you will want an input voltage 16V or more. A typical current value to use for the IADJ is 50uA as can be determined from the part data sheet. For a 13V output you can use the formula in the picture to get the resistor values. If you use precision values of R1=274 ohms and R2=2550 it is possible to achieve an output of almost exactly 13V when the IADJ value is 50uA.
The transformer is wired as follows:
simulate this circuit – Schematic created using CircuitLab
Notice that the two 15V wires are 180° out of phase, resulting in a 30V(AC) between the outer two windings and 15V relative to the middle wire.
You mustn't cut the transformer's insulation as that is a safety hazard. The two 15V wires share a common ground (0V) wire, you cannot change that. You do have several choices how to use them, but to answer that you have to elaborate in your question on what your goal is.
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Best Answer
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.