Noob rectifier question. - Page 1
Forum section: electronics / beginners

author: medical-nerd
author_rank: Regular Contributor
timestamp: July 11, 2017, 11:44:00 am
content: HiyaI've been looking at full wave rectifier and bridge rectifier circuits - the output waveforms seem to be the same except that the voltage drop is 2x the diode drop in the bridge.Why then are bridge rectifiers used so often?? Are centre tapped transformers so more expensive or am I misinterpreting the waveform in the books?? I rarely see an amateur mains powered circuit containing a full wave rectifier.Additionally I see references describing the bridge as a full-wave bridge rectifier.I'm getting a little confused.Cheers

author: Benta
author_rank: Super Contributor
timestamp: July 11, 2017, 11:56:48 am
content: Simple answer: silicon is cheaper than copper. Two rectifiers and a centre-tapped transformer only utilizes the secondaries half of the time. This means you need to place more copper in the secondaries for the same average current - or live with higher losses and heat.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2017, 01:58:53 pm
content: I completely agree with Benta.In some applications the increased efficiency of a center tapped full wave rectifier is important enough to make up for the extra cost.

author: medical-nerd
author_rank: Regular Contributor
timestamp: July 11, 2017, 02:55:35 pm
content: So then you are both saying that a centre tapped full wave rectifier is more efficient because of the reduced voltage drop by using just two diodes and a bridge rectifier is cheaper because the secondaries are used for the full cycle so that a smaller transformer can be used?If " Two rectifiers and a centre-tapped transformer only utilizes the secondaries half of the time" how can it be that more efficient?Right, so the difficulty I have is conceptualising what is happening / the difference between them, since the book I have been looking at gives the same output waveforms for each, mentioning the increased voltage drop for the bridge.I realise that this will be a simple concept and will be driving you crazy at my stupidity - I just don't get it.Cheers

author: Benta
author_rank: Super Contributor
timestamp: July 11, 2017, 04:24:24 pm
content: Then let me phrase it differently. With a centre-tapped layout, you only have one rectifier voltage drop, with a bridge you have two. This is obvious.But if you look at the transformer, it's a different story.Let's say you have an normal EI transformer.Copper cross section of the primary is 1 cm2, secondary also 1 cm2.This is fine for a single secondary and bridge, and the secondary is supplying current continuously.But if you have a centre-tap transformer with two rectifiers, the secondary cross section will effectively only be 0.5 cm2, because you need space for two secondary windings, meaning thinner wire.Only one of the two halves supplies current at any time, meaning your resistive losses will be higher. Solution is either to live with the losses (= lower secondary voltage) or increase the size of the transformer.Especially on smaller transformers (<100 W), copper losses can be significant and higher than the extra diode drop.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2017, 05:02:22 pm
content: Quote from: medical-nerd on July 11, 2017, 02:55:35 pmExactly.QuoteIt is more efficient from the perspective of power loss at the expense of using a larger transformer. This is commonly done in switching power supplies where the transformer is already small and there is a premium on electrical efficiency.QuoteFrom the perspective of the input and output, they produce the same result.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2017, 05:13:34 pm
content: Quote from: medical-nerd on July 11, 2017, 11:44:00 amCommercial appliances nowadays rarely use a low frequency mains transformer. A switched mode power supply is the norm. A full wave rectifier bridge converts the mains voltage into a DC voltage, equal to the peak voltage minus two diode drops. It's then converted to a much higher frequency and stepped down to a lower voltage, using a tiny transformer. Quite often the secondary side has a centre tap and two diodes, rather than a full diode bridge. I presume this is because the extra copper required is tiny and the lower diode loss is worth it. Very low voltage, high power, high efficiency designs use MOSFETs, rather than diodes on the secondary, for even lower losses.

author: medical-nerd
author_rank: Regular Contributor
timestamp: July 11, 2017, 07:11:15 pm
content: Thank you all - I see a light flashing in the distance.I was expecting a difference in efficiency to be demonstrated in a difference in the output waveform, but now understand that this is incorrect.I understood the difference in diode voltage drops, but didn't know how important the transformer construction was in each. This is clearly very important in mass produced items with small profit margins and switch mode power supplies looking at maximizing efficiency.The circuits I have been looking at have been for home construction of items using 3 pin regulators to supply the voltages, invariably using bridge rectifiers. The only time I have seen center tapped transformers used is when dual rail supplies are required.I suppose that using bridge rectifiers is the norm for home construction now with toroidal mains transformers but full wave rectification was more common in older designs especially valve designs when toroids were not available and center/multiple tapped transformers were the norm.I have both varieties of transformer, so look forward to using both methods.Many many thanks to all.Cheers

author: Benta
author_rank: Super Contributor
timestamp: July 11, 2017, 07:57:23 pm
content: QuoteIt has nothing to do with toroidal vs. EI transformers. It has more to do with two other things:- in older days, rectifiers were bl**dy expensive, so it made sense to only use two.- tradition. When I look at older designs, US power supplies mostly use center-tapped transformers, where European designs use bridge rectification. Don't ask me why, perhaps an "Electronics Historian" is present?As an aside, most transformers today have dual secondaries, and you can connect them in series or parallel according to your fancy.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2017, 10:18:25 pm
content: Another thing to note about efficiency and transformer rating, is that the centre tapped rectifier configuration does not reduce the maximum power output by half but by a factor of 70%. This is because each half of the secondary winding only conducts current for half of the sine wave and has a rest 50% of the time to cool off. Therefore the safe peak power dissipation in the secondary winding can be double the original design value. Because P = I2R, the current can be root(2) = 140% higher, than the rated value, gives double the copper loss and the voltage is halved, as you're not using the whole winding in series, so half of 140% is 70%.Note that the above calculation assumes no smoothing capacitor is present. Adding a smoothing capacitor increases the copper losses considerably, as the current is drawn in pulses, near the peak voltage, so you should de-rate the transformer even more.