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.

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.


--
Cheers,

John.

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