# Transformer is not essential in half wave rectifier

My textbook for basic electronics states that

Transformer is not essential in half wave rectifier (advantage of half wave rectifier)... But why?

And there's another statement saying that half wave rectifier has disadvantage that

It have low ripple frequency which implies that there is more ripple that results in poor quality DC. More fluctuations results in more % ripple.

Now here I am totally confused whether the ripple frequency should be high or low that is if ripple frequency is low then how come there will be more ripple? Or the text is misprinted?

I am new to electronics and request an easy answer.

Thank you for your precious time.

• A rectifier doesn't rely on a transformer whether it's half wave or full wave. Maybe you should photograph the page in your book and provide a link to the book. Commented Feb 17, 2022 at 11:38
• We don't know the context which this relates to. Surely a half wave rectifier can exist with or without a transformer. It just depends what is the purpose of the circuit, and in case of power supplies, sometimes you need a safe suppy and sometimes you don't. Commented Feb 17, 2022 at 11:40
• More ripple isn't the same as more ripples. A smaller number of big ripples is probably worse than a large number of small ripples. Commented Feb 17, 2022 at 12:10
• Strange statements in your book or applies to a specific example. Please do some simulations of the three popular configuration, single secondary with half-wave rectification, same with full-wave and center tapped secondary with two diodes. Commented Feb 17, 2022 at 13:06
• @Curiousminded A half wave rectifier will only pass through a voltage on one half of the AC cycle. That would be one "ripple" per 1/50 or 1/60 second (depending on your local AC frequency). A full wave rectifier passes a voltage on both half cycles. That gives you ripples at 1/100 or 1/120 second, with shorter gaps between them. Commented Feb 17, 2022 at 19:58

tldr: The book is right

I'm not sure how I should take the first statement. It's true as in "an private jet is not essential in traveling from country to country". There are transformers used to transform one AC voltage to an AC voltage (lower, higher, same... everything is possible). And than there are rectifiers which are used to get a DC voltage from an AC voltage. They are often used together but are not essential to each other.

The second statement is also true. For this we better dive a little bit into what rectifiers do.

A half-wave rectifier is just a diode. A diode lets current pass in one direction and blocks it in the other direction. Among many other things, we can use that to rectify AC. We're just letting the current pass in one half-wave of the AC and blocking it for the other direction.

From a 50 Hz sine wave, we will pass only the positive part. Look at the sine wave and just cut off everything below the zero line. So we get a positive bump 50 times per second. To smooth this out, we can use capacitors. But we're only charging that capacitance 50 times per second. The rest of the time it is being discharged and losing voltage in the process, leading to big output ripple.

There will probably be another chapter on the bridge rectifier, which helps reduce some of the issue of the half-wave rectifier, but that's not the question here.

There was a time, many decades ago, when some inexpensive appliances like table radios would get the needed DC voltage for the vacuum tubes by rectifying the AC power from the wall directly without the benefit (or safety) of a transformer. The only way this could even begin to be safe was if:

1. One side of the AC power was 'neutral' i.e. connected to ground back at the breaker panel. This wire was connected directly to chassis ground and signal ground on the appliance.

2. A single diode was used as a half wave rectifier on the 'hot' wire.

This is absolutely not safe. It will never be seen on new appliances and is not recommended for any design, even home DIY.

As for the ripple depending on frequency, the author was probably referring to the fact that lower frequencies are more difficult to filter, hence the ripple will be higher after any filtering in the power supply.

There are lots of good books on electronics. It seems the one you're reading isn't one of the good ones.

• Although this doesn't answer the question, I think it's a very important point to make. While a transformer isn't necessary for electronics to work, it's absolutely necessary for electronics to work safely from mains voltage. Commented Feb 17, 2022 at 16:26
• For the classic "All-American 5" radio it was more dangerous than you say. Outlets and plugs were not polarized, so there was no way to force the chassis to connect to neutral. But even if you could, then the chassis would be hot with the radio turned off, because the power switch broke the connection between the line and the chassis. The chassis would then be connected to the hot side of the line through the tube heaters. The switch worked that way because to save money it was part of the volume control. Switching the non-chassis side would inject hum into the high z audio capacitively. Commented Feb 17, 2022 at 20:11
• @horta don't forget that it has to be an isolated transformer and not an "autotransformer" or anything that does not galvanically isolate primary and secondary. Most low voltage transformers use isolation but not all of them do. Commented Feb 18, 2022 at 9:23
• There are "appliances" still on sale which rectify AC mains directly without a transformer. They are LED light bulbs. They are safe because they have no electrical component which one touches. Everything inside the insulating plastic case is considered live, just as everything inside an old-fashioned filament bulb is considered live. (Plastic is less fragile than glass, though! ) Commented Feb 18, 2022 at 10:39

My textbook for basic electronics states that transformer is not essential in half wave rectifier

Not quite: -

I think your book may be saying that if you are rectifying regular AC mains $$\\color{red}{\text{(high voltage)}}\$$ then a transformer is not required.

Now here I am totally confused whether the ripple frequency should be high or low that is if ripple frequency is low then how come there will be more ripple?

A half wave rectifier only produces an output when the input AC waveform is of the correct polarity therefore, the basic output frequency is 50 Hz for a 50 Hz AC supply. A full wave rectifier will produce an output that rectified twice in one cycle of AC therefore, the output frequency will be 100 Hz for an AC frequency of 50 Hz.

Adding an output capacitor means that the capacitor gets replenished with energy twice as often for a full wave rectifier as it does for a half wave rectifier. This means a lower ripple (peak-to-peak) at the output.