Soft Start Inrush Current Limiter- PCB design

[Muzzer]

The issue is actually about the transformer cores periodically saturating when connected to the mains, something that happens on a fairly random basis as I explained. The tendency for them to saturate depends on how much magnetising flux has been designed in and how close that is to the saturation flux limit.

Some transformers run very close to saturation and are more prone to the effect than others. To increase the margin requires higher inductance which in turn requires more steel and copper and higher material cost. There's a commercial decision involved, which is one reason we see variation between different examples.

You may notice that apparently similar transformers from different suppliers behave differently - this may be seen as different steady state (unloaded) temperatures, for instance. I'm guessing your toroidal audio transformer doesn't run a high magnetising current / flux, which is why you don't see tripping breakers. I have 2 yellow site transformers that are both rated at 3000VA and are of similar dimensions, yet one sits at around 50C and the other barely warms up (perhaps 30C) when left powered but unloaded.

[the great waldo]

Hi TStar

" as I understand it" Doesn't quite instill me with confidence ? But i'll check it out.
Cheerds
Andrew

[Muzzer]

Yes, TStar is right. The relay simply shorts out the resistor after the programmed timeout. It's so simply you barely need a schematic.

[Neale]

I've been musing on this one, still wondering why relatively low-power audio amplifiers need something like this when, in my limited experience, toroidal transformers in the 500-650VA range for stepper supplies seem to work happily without anything needed. Is it that inrush currents per se are not the issue with audio amplifiers; is it actually to avoid the switch-on thump which is annoying and potentially damaging to loudspeakers? I would guess that even a relatively high-power (domestic) audio amp is unlikely to be rated at much more than 300VA and unlikely to trip breakers or anything like that. Much larger transformers are likely to trip breakers at switch-on so might need special measures - although a suitable curve MCB might help?

Muzzer - happy with your explanation of saturation effects. Vague memories of B-H hysteresis curves seen through closed eyelids while the world's most boring lecturer copied his yellowing notes to the white board... However, I have no feel for the magnitude of these effects so bow to your more specialised knowledge. Clearly there are several factors at play including smoothing capacitance, transformer primary resistance, and so on, but I wasn't aware of core saturation effects. What I can say, and maybe this is directly relevant is that this last weekend I installed a 500VA toroidal transformer in my newly-acquired CNC lathe, with each 50V secondary directly connected to a stepper driver rated at 60V AC/80V DC. I assume that the driver contains whatever smoothing capacitance the designer thought it needed. While testing since then I have probably switched that thing on and off dozens of times a day and nothing has gone pop yet! I expect to spend more time making sure E-stop mechanisms are in place than I shall about inrush limiting, but that's my personal view. We all plough our own furrow!

[TStar]

From what I've read, around the 500VA mark is where a soft start begins to be useful, and anything below 300VA really doesn't benefit at all. I think at 500VA it would be unlikely to trip a fuse unless you power up under full load, which would (hopefully) never happen with a CNC machine. The soft start is just a bit kinder to the drivers, hopefully extending their lives a bit, and also kinder to the supply and other appliances connected to it.

I intend to run a power line filter upstream of this module, I've heard the TDK Lambda RSEN-2030L is good? As far as I can tell the -L version does not use Y-caps which I think is the right approach in our case

Of course yes, things like E-stops are more important, I'm just fiddling round the edges at the moment.

Anyone using differential signalling on their E-stops by the way? That's a module I've been thinking about building at some point...

https://product.tdk.com/system/files...log/rsen_e.pdf