On Thu, 19 Sep 2019 10:18:56 +0100, Martin Brown wrote:

On 18/09/2019 20:41, Commander Kinsey wrote:
On Wed, 18 Sep 2019 20:32:06 +0100, Dave Liquorice
wrote:

On Tue, 17 Sep 2019 21:02:22 +0100, Commander Kinsey wrote:

Switched mode PSU's tolerate brownouts remarkably well.

Most computer PSUs I've used will shut off after a 1 second powercut.

Powercut != brownout. Most computer power supplies will happily run
on any thing from 100 V to 250 V AC or DC.

Perhaps so, but it's not usually just one or the other. It most likely
drops a random amount unevenly, possibly getting under the 100V for part
of that time.

The most common situation round here is that one of the three phases
goes open circuit and neutral floats up to a voltage mid way between the
two remaining live phases. So 240 vac drops to around 200v. LED lights
and PC's don't even notice this but filament bulbs glow dim orange.

You must have some weird supplies there. I thought they were all similar to what I have. There's a substation across the road from me which takes 3 phase 11kV in. It outputs 3 phase 240V. The neutral is grounded at the substation with a big stick. Each house (they alternate between phase 1, 2, 3 as you go along the street) gets a phase and the neutral, supplied to the house with an armoured cable. A 100 amp live carrying a phase, surrounded by the armour which is neutral (and ground). My neutral can never be anything but zero volts, because of the big stick. Mind you, I guess if the supply to the substation lost one of the three 11kV phases, some weird **** might happen to th voltage on my 240V phase. But I'd expect breakers or something to detect that and shut it off before someone gets unusual voltages.

This abilty of SMPSU's to run on almost anything confused the heck
out of me when the supply dropped to about 125 V at 0400 one morning
An ice storm brought the distribution lines down in half a dozen or
so places and snapped a few poles. Some bits of kit were on and quite
happy, others off.

How come the mini SMPSUs in my LED lighting make the LEDs go noticeably
dimmer when the voltage drops a bit? (Like 10-20V) Yet they are rated
85-265V. Or are they actually just dimming for a fraction of a second
before compensating, and my eye detects this as dimming? I thought the
way SMPSUs worked, the output wouldn't vary one bit?

Decent ones don't. The cheap and nasty ones might. Most will work OK
with anything above about 100v just drawing more current to compensate.

Perhaps I'm just seeing a very temporary dimming, the human eye isn't that accurate. If they can take 85V, then they can't dim much by dropping from 240 to 220. It's just I thought it would be impossible for a SMPSU to do that. In its simplest form, it's charging a bulk capacitor to 340V DC, or less if your input AC is lower. Then that's passed in a controlled manner by an IC and transistor to the lower voltage output capacitor (in my case 70VDC for the LEDs). Surely if the bulk capacitor lowered in voltage due to a brownout, the duty cycle for transferring power to the output capacitor would just increase to compensate?