Joshua..most of these questions involve basic formulas like Vpeak = VRMS x 1.414, which you should have been taught. UNDERSTAND them and commit them to memory as you will need to use them. When you are dealing with rectifiers the above is the most used, and most useful one, but there are others. When you full wave rectify a voltage and measure it with a voltmeter it will display the average value which is Vpeak x 0.637 or if you're working back to the AC input it's VRMS x 0.9. This is for full wave rectification, half wave and three phase rectifiers have different values.Now - question 8: Calculate the peak voltage rating of each half of a centre tapped transformer used in a full wave rectifier circuit having an output DC voltage of 120V.This is the circuit concerned:
The DC voltage will appear across points C (-) and D(+). This is given as 120V, we can assume this is an average value unless otherwise stated. We can also assume ideal diodes for this; if they want you to factor in the voltage drop of the diodes they will usually tell you.At the top of one half cycle point A will be positive, D1 will conduct, point B will be negative and D2 will be reverse biased. At the top of the next half cycle it will be reversed, B will be positive, D2 will conduct, A will be negative and D1 will be reverse biased.So point D will have a full wave rectified voltage on it from each winding of the transformer in turn. For a full wave rectifier VDC = Vpeak x 0.637 We know that VDC is 120 so we can say that120 = Vpeak x 0.637 I am not here to teach math, so you can rearrange the formula and work out Vpeak.You need to be able to do these calculations for a few reasons - to work out what transformer voltage you need, and to specify your diodes. Say that A is positive and B is negative. Point D will have the same voltage as point A. Point B will have the same voltage but negative. So your diodes need to be able to withstand 2 x the peak voltage you have calculated - this is called their Peak Inverse Voltage (PIV). At 50Hz this is easy - the well known 1N400x diodes can withstand up to 1000V (the 1N4007), but for switching power supplies the same rules apply and the special diodes for them have much lower PIV ratings. So these calculations do have a practical use.For most of the rest of the questions you can easily solve them by applying the formulas above. Just write down what you have and what you need to find and plug the values into the applicable formula.This is pretty easy stuff. Wait till you get to transistors
