On Saturday, October 10, 2020 at 5:03:02 AM UTC-4, T i m wrote:
On Fri, 9 Oct 2020 16:21:38 -0700 (PDT), "
wrote:

snip

Use the Schottky diode.

Yeah, I will, I was just wondering why / if they were used in that
role? ;-)
Sometimes a fast/soft recovery diode will work, but I've run into problems where the drive IC or transistor will run hot or fail or the secondary voltage will be lower if the Schottky diode is subbed from original spec.
Ok.
Also, sometimes those diodes check very leaky even when they're not - depending on your meter and the polarity of the leads, even when not using the diode scale.
I'll check it again but I believe I measured the resistance both ways
and it looked short ... and the PSU wasn't working (but warming) and
everything else I could check tested out ok.
You can put in a fast recovery to see if the circuit works, but I wouldn't run it more than a few seconds.
Understood. I'll wait for the right things to arrive and keep my
fingers crossed it was that. ;-)

Assuming the spec I looked up was correct, and it was a 2A device, and
the PSU itself was rated as 400mA, I wouldn't have thought straight
current would have taken it out so would there me a recommended 'safe'
startup procedure that might put the least shock load on the diode?


400ma is the maximum continuous duty design output, but during the first few cycles, it's essentially feeding a short circuit until the filter caps charge. Most modern smps circuits ramp up the drive in a controlled manner using a slow or soft start strategy to mitigate that. Still, sometimes diodes just short, and I've seen more random Schottky failures than other types..



eg, Should you connected charger clip to the battery first, then
switch on at the wall (and likely what it says in the instructions)
... or switch on first (allowing the PSU to get the output cap charged
up) and then connect it to the battery?

Wall wart / PSU lv Lead charging clip / LED status Battery.


I wouldn't worry about the start up procedure unless there are specific instructions to the contrary. Consumer electronics are generally designed to be plug and play regardless of the sequence.