Full wave rectifier with a smoothing capacitor

Yes I was confused as normally one would use an isolating transformer in such a case. Also if you are inputting ac, the hum you hear will be 100 hz not 50hz.

I cannot think of any safe usage for this idea connected straight to the mains other than making your own electric execution chair perhaps. Brian

One diode will give input protection, but with the supply polarity reversed, the device won't work.

With a bridge, then it will work regardless of the polarity of the supply. Generally if using an unregulated supply with internal regulation, the supply voltage will have enough headroom to allow for a couple of diode junction drops.

Same applies to data. The Army's FAM - Field Alert Monitor - starts with a bridge.

Squaddy-proof !

PA

And the reason I posted is because I found a LN reversal on a socket during a EICR.

The tenant was adamant a rectifier would not work.

I was convinced it would (even after a bottle wine of when I posted).

So I asked the collective wisdom of you fine posters.

Yes, but obviously with an earthed public supply a full wave rectifier implies that neither + or _ terminal are anywhere near earth DC potential (and approx equally distant from it). Of course, as the live input is at the same

It also depends on where the protective devices are and where there L-N reversal is....

If the protective device is in the 13A plug on in the consumer unit before where the L-N reversal happens then no practical safety issue as its the L that gets switched off and in a PME earthed house, N will be close to the same potential as the CPC.

However, if the L-N reversal is BEFORE the protective device, then the N gets cut anbd you still have a Live L with a potential od 220 V AC RMS relative to the CPC......

He clearly doesn't understand what AC stands for, and/or what a rectifier does.

Being relevant to the thread about Free Agent losing posts, why haven't I got ARW's original "Full wave rectifier with a smoothing capacitor"?

Why does that mean AC Mains, as opposed to any AC rectification?

If it were DC mains or a battery he's far less likely to have mentioned a smoothing capacitor - hence my guess that he's talking about AC Mains. And I remember DC Mains !

"AC rectification" is a process, "AC mains" is an input to that process. So they are different things - like petrol is to cars.

In this case AC mains is rectified, delivering only pulses of DC voltage of one polarity. In the case of DC mains/batteries, the process of rectification is virtually invisible, - DC in, DC out.

Half-wave rectification is accomplished using a single diode. The snag is that one of the two mains input wires passes through the process. Get that the wrong way around and what was thought to be the GROUND of the TV chassis is actually at LINE voltage. Been a service engineer - got the scars - still.

Bridge rectification is, effectively a switch being flipped 100 times a second. Thus the output is always pulses of the same polarity - and twice as many as for the half-wave. There's still the problem, though, that neither of the outputs is safe to touch! Bridge rectification has been known of for some time - but the cost of thermionic diodes was so high, once upon a time, that it generally didn't catch on. These days semiconductor bridges are so cheap - that's the way to go.

Isolating the Rectifier from the mains - i.e. making things safer - was generally accepted to require an isolation transformer ahead of the rectifier. At one time, these transformers were expensive. Now they aren't - so some simple PSUs nowadays contain one. Some geezer came up with the idea of having two identical output windings on the transformer. You could then connect these windings in a way that one was always delivering the mirror image of the other. This meant that when one winding was offering a positive pulse the other would be offering a negative - and vice versa. That cuts the number of diodes required from

Modern power supplies take all of those ideas and shake them all up. They start with a bridge rectifier, then a smoothing capacitor. The resulting ripply DC is chopped into a transformer at about 100kHz by a simple oscillator. The outputs of one or more of the post-transformer rectifiers (usually half-waye) is fed back to the chopper to stabilise the output voltages and this signal is the only thing that connects the pre-tranformer circuitry to the post-transformer circuitry - usually, nowadays, by means of an optical isolator, So the risk of electric shock is gone. Open up an old PC PSU and you'll clearly see the Berlin Wall nature of the isolation of one group of PCB tracks from the others !

E&OE !

PA

I'm trying to think of *any* domestic equipment that wouldn't work if the line and neutral are reversed?

The exception would be a half-wave rectifier when the 'cold' side of the rectified supply was earthed. This would be illegal, I think, but it would definitely not work if live and neutral reversed. And if, in normal use, the neutral was connected to the earthed chassis then bad things would happen if the mains was reversed. Of course, in that scenario any RCD on the supply might well be tripped in normal use, and the normal use would be even more illegal.

Such as? I also cannot think of any domestic equipment that wouldn't work.

That doesn't mean that domestic equipment doesn't reverse what seems common-sense by design. My gas boiler has all of its electronics referenced to the LINE potential.

Guess how I found that out !

PA

It would not spark?

On many boilers the flame detection is achieved by flame rectification between the ignition electrode and earth - I can see that failing if the wrong end of it is effectively at earth potential.

He was an A level physics teacher which probably made me think he might have had some understanding of the subject.

No, it was me that sparked when first diagnosing the controller PCB! A crazy system. From the neutral a 0u68 capacitor then two zeners in serial-opposition to the LINE. Two diodes picking off the clamped AC to offer +/- supplies for the electronics.

Needless to say, the capacitor (right on top of the heat-exchanger - natch) has dropped in value, twice, making the boiler dance a fandango once the regulator on the electronics plus supply runs out of input voltage.

Cowboys !

PA

And this boiler uses that sort of flame detection - but I was shocked, in both senses, when I first had to diagnose the control PCB. (See my other comment to Max).

If you want to understand something, you have the option of lashing something together in LTSpice. It's a kind of physics teacher too. LTSpice was offered by Linear Technology (makers of op amps and the like).

Paul