# Centre tapped secondary winding

In the construction of a full wave rectifier, why is there centre tapped secondary winding in the transformer? OR What's the role of centre-tapped transformer in a full wave rectifier?

Think of the centre tap as a 0V reference; in fact it is usually the ground point on the low voltage side. Then you have two windings, supplying AC in opposite phases. Each winding supplies a half wave rectifier (one positive pulse per cycle), the result is a full wave rectifier (two positive pulses per cycle).

It is slightly more efficient than a single winding and bridge rectifier, because there is only one diode drop between the AC and DC supplies.

But wait - there's more! During the "other" half of the cycle, when a winding isn't providing a positive pulse, it can supply a negative pulse via another diode to provide a negative supply of the same voltage.

So it is a particularly convenient way of supplying +12/-12V (or other voltages) to power analog circuitry.

• if there was no centre tapped secondary winding then what would be the result? – Samama Fahim Mar 19 '13 at 15:29
• Instead of +12/0v/-12V you would have a single 24V supply fed by a bridge rectifier, with no central 0V point. Useful for some purposes but much less convenient for analog designs. – Brian Drummond Mar 19 '13 at 15:31
• what's diode drop? – Samama Fahim Mar 19 '13 at 16:32
• With the silicon diodes usually used in power supplies, each diode has about 0.7V across it, which means 0.7V less for your circuit. With a bridge rectifier, there are always 2 diodes conducting, so you lose 1.4V instead of 0.7V. – Brian Drummond Mar 19 '13 at 16:37
• One downside to the center-tapped topology as opposed to the single-winding bridge topology is that the diodes must block twice the output DC voltage. Probably not an issue at lower voltages, but at higher voltages and powers it can really hurt. – Stephen Collings Mar 19 '13 at 18:03

Apart from the other reasons, as only two diodes are required it can be cheaper than a single winding and diode bridge.

• Can be, but it's a more complex transformer, which is generally more expensive, so the costs will tend to balance out. – Brian Drummond Mar 19 '13 at 16:41