Does anyone know if there is available a simple "soft switch" which I can put between the transformer of my preamp and the mains? I think it would be a good idea to have something like that. Opinions? Thanks.
Why, because I think it would be a level of protection against damaging components on a preamp board.... No?
Well, switching a power supply ON should NEVER damage the power supply. If it does, you need a better power supply, not a better or “soft” switch.
Regards, Gerrit
Regards, Gerrit
The usual light/small transformers of preamps have pretty moderate inrush current. I was just measuring one and a 7VA toroid tolerates a 50 mA T fuse for many switching actions. In fact it did not blow at all.
Now SMPS are another matter. These usually/often require heavier fuses because of their higher inrush current. A 20W SMPS module I measured this week required a 630 mA fuse.
That was seen from the 230V 50 Hz mains side of things. From the loads side there is also not much to worry about unless very cheap SMPS are involved. Also when caps on the preamps board are charged voltage will sag. Usually voltage regulators are used and these normally have overcurrent protection.
In short: you probably are overthinking the consequences. Just when switching DC things start to get more complex.
Now SMPS are another matter. These usually/often require heavier fuses because of their higher inrush current. A 20W SMPS module I measured this week required a 630 mA fuse.
That was seen from the 230V 50 Hz mains side of things. From the loads side there is also not much to worry about unless very cheap SMPS are involved. Also when caps on the preamps board are charged voltage will sag. Usually voltage regulators are used and these normally have overcurrent protection.
In short: you probably are overthinking the consequences. Just when switching DC things start to get more complex.
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The simplest soft start circuit I know of is just one NTC meant for inrush current limiting in series with the load. It has a relatively high resistance until it gets hot, then the resistance drops. The trick is to find an NTC that can handle the voltage and that, after heating up, has low enough resistance at the expected load current.
Isn't that like a death capacitor? ~7mA is a lot of shock. Especially if someone cuts corners when working on the unit and assumes off means off...
I'd suggest an RC snubber across the load is a much safer way to do this, and you won't be wasting 2W of electricity when its turned off. They charge you for the apparent power. Note the R in an RC snubber needs to be fusible, and use a class-Y capacitor so it fails open, not short.
Its probably easier to just use a beefier switch really...
Caps or more typically proper R/C snubbers are actually common in lots of commercial gear. The caps must be suitable for this type of use which is as Marks suggests, a Class Y type suitable for your line voltage. The resistor in the snubber could typically be around 47 to 100 ohm and the cap a 0.01 to 0.047uF, again Class Y.
And here a 0.01uF cap on its own. This is a very large high current supply in a 2 x 150 wrms amp.
And here a 0.01uF cap on its own. This is a very large high current supply in a 2 x 150 wrms amp.
Its certainly getting 'up there', especially with 240 volt mains.
Old, and I think it was Mitsubishi CTV's used to use a (gulp 😱) 0.47uF cap before the switch and across L and N. I've fallen victim to those on many occasions and the belt you get from the prongs on the plug was (literally) a real shocker.
Old, and I think it was Mitsubishi CTV's used to use a (gulp 😱) 0.47uF cap before the switch and across L and N. I've fallen victim to those on many occasions and the belt you get from the prongs on the plug was (literally) a real shocker.