In a power supply design a 12 V x 1 A transformer is ordered, but I only have one of 12 V x 800 mA. Can I use it? What should I change in the circuit?
You do not need to change anything on the circuit. Just remember that the maximum current your power source will provide will not be 1 A, but only 800 mA, dictated by the characteristics of the transformer. Likewise, if you have a 1.5 A transformer you can also use it, but be careful not to require more than 1 A from the source, as the transformer can supply this current but perhaps the rectifier and regulator circuit does not support. Similarly, depending on the circuit, the voltage of the transformer may be slightly higher. A 15 V transformer may be used in a source that requires a 12 V only and it should be noted that the regulator integrated circuit or the transistor will tend to warm up a little more when the source is supplying its maximum current.