I'm designing a power supply to deliver 6V DC with a zener diode acting as the regulator. I found out that there are two types of transformers that can be used. They can be center tap or a bridge. In center taps, we will use 2 diodes, while in bridge, we need 4. What are the pros and cons of each type and which is best suitable for my purpose? I think it's worth getting to know the advantage and disadvantage of using one or the other.
The full bridge has the disadvantage that you have two diode voltage drops, which may be up to 2 V. I don't think the cost of the two extra diodes will matter much.
I wouldn't recommend the center tap though: you have a lower voltage drop because only 1 diode conducts at a time, but you'll also use only half of the transformer. So where for the full bridge you could use for instance a 10 VA transformer for the center tap you'd need a 20 VA type.
The center tap should be avoided as much as possible. If the two diodes voltage drop for the full wave rectifier is a problem, you can probably use Schottky diodes.
A center tapped secondary allows for more efficient rectification since there is only one diode drop in series with the output. However, it requires a larger, heavier, and more expensive transformer since only half the secondary is used at any one time.
A single secondary winding allows for a cheaper, smaller, and lighter transformer, but there are two diode drops in series with the output.
A center tapped secondary is particularly useful when you want to make positive and negative supplies of the same magnitude with a common ground. Now both halves of the primary are always working and you still get only the single diode drop in series with each supply.
A better way
The above answered your question directly. However today using a transformer to directly scale down the wall power is rarely done. Nowadays it's cheaper, lighter, and smaller to full wave rectify the wall power, then electronically chop that at high frequency to drive a transformer that produces the output voltage. Due to the high frequency (could be 100s of kHz), the transformer can be physically much smaller for the same power level. It also makes the output ripple much easier to filter, usually to the point that the rectified output with a reasonable size capacitor accross it can be used directly as the DC. With feedback to the chopping circuit on the input, you can achieve regulation. This is the basis for most common AC to DC switching power supplies. The savings due to the smaller transformer outweigh the extra circuitry.
In my opinion, it depends on the power you are trying to achieve. For minimal power requirement, any of the two options will work just fine. Its like a compensation: while center tap will use just two diodes, it will require twice the power rating of the transformer a full bridge would use to achieve the same power output. The bridge however, uses a smaller transformer, but two extra diodes, resulting of course in more voltage drops (of course this goes without saying). Its a trade off between size and cost. For small power ratings, going the switch mode direction(as suggested above) is going to be a whole lot of added complexities. For higher powers however, I would recommend switched power supplies due to their higher efficiency and smaller size.