[cctalk] Re: Rainbow H7842 PSU Fault

On Tue, May 30, 2023 at 11:33 AM Hans-Ulrich Hölscher via cctalk &lt;cctalk(a)classiccmp.org&gt; wrote: The normally-given reason is that an SMPSU approximates to being a constant power load and draws more current if the input voltage is reduced. This can cause the chopper transistor to fail from having to handle too high a current. That said, I have run many, many SMPSUs with a light bulb in series with the mains input and had no problems at all. I run them with a minimal load (say 1A on the 5V line, so 5W power going there) and put, say, a 100W filament bulb in series with the mains. The voltage drop across the latter is <20V (I've measured it), given the local mains here is around 240V, that means the input to the SMPSU is well within the acceptable voltage range. And while the seres bulb probably won't save the chopper transistor if things go seriously wrong on the primary side of the supply, it probably will save the bridge rectifier, expensive fast-acting fuse and PCB tracks. Not to mention my nerves. -tony