On 08/04/2021 09:59, Roger Hayter wrote:
On 8 Apr 2021 at 09:30:47 BST, "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.

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

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.

--

Jeff