Full wave rectifier with a smoothing capacitor

The DC voltage across the capacitor will be the same, the difference: one way it will be referenced to neutral, ie close to ground, and the other will have 230V superimposed on the capacitor terminals.

If an electrolytic capacitor then diode polarity will be very important.

What are you trying to do?

I'd assumed Adam meant a bridge rectifier as in

Correct...

Yup.

In fact if you are designing DC powered kit that runs from an external supply, then sticking a bridge rectifier on the input even though you are only ever expecting DC rather than AC, is a way to ensure your kit will never see the input polarity reversed if used with the wrong PSU.

I saw "full wave rectifier" and thought "single diode". My bad, I should try reading a little harder.

In which case L-N reversal will make no difference.

Your link is spot on.

I did that with a piece of battery powered kit I used to build to go with my guitars. It used an off-the-shelf box with built-in battery connectors that didn't have anything physical to prevent you putting in the battery the wrong way round. Rather than use a (much) more expensive box, I added a bridge rectifier into the circuit. Belt. Braces. Piece of string in pocket.

Nick

Correct.

PA

A single diode only passes 50% of the mains, a bridge of a full-wave passes 100% - so less ripple on the output - or a cheaper capacitor results.

PA

A single diode only passes 50% of the mains, a bridge or a full-wave passes 100% - so less ripple on the output - or a cheaper capacitor results.

PA

Surely you only need a single rectifier connected the right way round in either the +ve or -ve line. With a bridge rectifier three of the diodes are effectively redundant (unless one goes short-circuit), and you've got a double voltage drop across the diodes to take into account. That might be significant for the equipment - a bit like trying to use NiCads instead of alkaline-manganese batteries.

Even if you're trying to cater for the extremely rare short-circuit mentioned above, you still only need two diodes.

But with a bridge it works whichever way round you put the battery. Not that it is usually worth it.

If you want reverse voltage protection, while minimizing voltage drop, a P-channel FET is better, maybe with a zener to protect the FET from a reverse voltage that is also too high e.g.

If you do use a full wave bridge rectifier, there will be a total voltage drop of 1.2 V across the rectifier.

There will be a maximum current rating for the diodes and not only that, once you use P=VI, you may find in high current applications you may need to heatsink the rectifier to dissipate the power across it to keep it within its SOA (Safe operating area)

Some high current rated rectifiers have an electrically isolated metal case with a hole drilled through it for mounting to a heat sink.

like this example:

Similarly, if using a smoothing capacitor across rectified mains, you'd need a 400 V rated one.....

It's not always mains AC. Sometimes an appliance provides something like

After rectification you may need to also reduce the voltage with something like a voltage regulator. Which AIUI is basically resistance and hence power inefficient.

The question I had is might a half wave rectifier be better (than a full wave rectifier) to convert 24v ac to 5v dc, i.e give the resistance less work.

In the end I gave up and decided it was better to use a USB wall plug transformer, but it was something I have pondered upon since.

Switch mode regulator can be a pulse width modulated output for 90% efficiency.

I was replying to John's point about protection from a reversed-polarity DC supply. In that case we are not concerned with ripple, etc - purely the problem of connecting the DC +ve of the supply to the -ve of the equipment. That could be due, for example, to a coaxial plug with a low-voltage DC supply having the reverse connection to the inner and outer from the usual power supply plug.

That is true, but although it might be convenient, it wasn't the point I was trying to make in relation to equipment protection against a reverse-polarity power supply.

I think that it might be used by decent manufacturers of quality equipment, where the cost of a few extra diodes in a bridge (pennies) would make little difference to the bottom line, and should be good PR.

ARW presented the following explanation :

You are correct!

Assuming the AC is coming from an isolating transformer, no practical difference at all.

The OP mentions "LN reversal" so this thread is about mains. He also mentions a "smoothing capacitor" - so it is AC Mains he is asking about.

So, your reply is OT - but that's OK.

Wall plug USB PSUs are so cheap, you've made the right choice.

PA