On 08/04/2021 08:45, Peter Able wrote:
On 08/04/2021 09:30, Jeff Layman wrote:
On 07/04/2021 23:46, John Rumm wrote:
On 07/04/2021 21:42, ARW wrote:

Am I correct in saying that it does not matter if there is a LN reversal
on the input?

Correct...

It will always have a correct +/- DC voltage.

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.

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.


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.

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.

--

Jeff