For many of my amplifier builds, I have needed the ability to control and switch a mains connection for a transformer or SMPS with a low-current switch. I also frequently need a “stand-by” voltage for LEDs, fans and other circuitry outside of the main amplifier voltages. Some pre-made soft starts include both of these features, but they are mostly either fairly expensive or very low quality ebay-stuff (I have a few of those that work well, but I have also blown up a couple in the process of finding the good ones…).
One potential alternative is AMBs epsilon24 “switch driver” circuit. This was partly what I needed, but not quite. Firstly I wanted to use a latching switch (so you can easily see if the damn thing is on or off :D ) and secondly I wanted to integrate the mains switching part of the circuit as well, in order to eliminate as much clumsy mains wiring as possible. The end results (at least for now) are what you see here. I say “results” because actually I ended up with two versions of the board :D.
The first type is the one with the transformer. It includes the transformer (which is an EI-30 core type, so power is limited to around 3VA maximum) with a small rectifier and 78xx regulator to generate a regulated standby voltage – normally either 5V or 12V. The standby voltage is available for powering external circuitry and it also switches an on-board SSR (solid-state relay) to control a larger, mains-powered transformer or SMPS. The switch can be either a simple latching type and/or an input from a logic circuit such as a flip-flop or a microcontroller. There is also an onboard fuse for the load and the SSR is protected from transients by a snubber circuit and a varistor (MOV).
On the second board version I have replaced the transformer + the regulation stage with an integrated MeanWell IRM-series power module. I also added a second mains output, removed the fuse which should be placed off-board instead and thus managed to shrink the board by a few mm. in the process. Although a bit more expensive than the transformer, on paper there are several advantages of the integrated power module: More power (up to 10W instead of less than 3W), universal mains input, very low standby consumption (so easily compliant with the < 0.5W standby regulations), simpler and takes up less board space than transformer and regulator etc. However, the output voltage seems to fluctuate a bit more than I would like. If you’re just powering fans, LEDs or relays this is most likely not an issue, but the idea was that it could power logic circuitry as well and then I am suddenly not so sure it will work – we’ll see once I manage to do some more detailed testing.
So, even with two board versions that both work I am not sure it is really “mission accomplished” just yet: The transformer version might be a bit underpowered for what I would like (especially at 12V) and the SMPS version might not be stable enough for powering digital logic. If so, I might go for a rev. 3 of this idea and see if I can squeeze a bigger transformer on there to make just “one board to rule them all” :D
EDIT 07-09-2014: I have now done some more testing on the v2.0 board and it seems I was wrong about the output of the IRM module. At least now I can’t replicate the unstable output I saw last time any more. With both my normal multimeter and the more expensive Fluke, the output is now a rock solid 5.00VDC. I have also measured another IRM module (a 12V version) and that also shows a very accurate and completely stable output voltage. So, I guess it is “mission accomplished” after all and I I’ve shelved the plans to make another transformer version – at least for now :)