Soft Start Inrush Current Limiter- PCB design

[TStar]

Hi Muzzer
Any chance of a quick schematic for timer relay and a metal clad resistor wiring ?
Thanks in advance.
Cheers
Andrew

I'm not Muzzer but as I understand it, just wire the resistor across the terminals of the timer on the input side ie in parallel. When the timer relay is open the current flows through the resistor, expending inrush current as heat. Then after a short period the timer relay closes, the current will choose the path of least resistance and flow through your main circuit instead. Just have to make sure your resistor is appropriately sized.

It's actually the same thing as my module just using commercial units.

[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