I've looking to introduce a soft-start circuit into a power supply (230VAC to a 625va 2 x 25V Toroidal Transformer wired to give 50VAC @ 12A, rectified to 70VDC @ 12A). It has raised some questions for me that relate to what actually happens in components. While I understand the function, there is a gap between what they do, how they do it
and what impact that has that I'm not too sure about. I'm a lawyer not an electronics guru and I suck at science
While there seems to be some disagreement over whether the resistance element needs to be isolated after being bypassed, I still plan to do it as (i) I have the bits lying around and (ii) I want to do it. Whether the circuit uses simple resistors or a thermistor is still undecided, though for the example I'm using resistors. While the PSU is not likely to by cycled on and off in short periods of time during normal usage, there may well be situations where I want to restart it before a thermistor has had time to cool back down.
My initial thought was Version 1 below - a double pole relay that is NC to the resistors and NO to the bypass. Neat solution with a single component, however I'm concerned that there will be a delay due to the contact switching time that would result in no power at all passing through either pole for a very brief time. However, as the point of this is to get some juice into the circuit and limit in-rush current when first powered - and by the time the relay is triggered the transformer/capacitors should have achieved some level of charge - would this actually have much of an effect?
Failing that, Version 2 uses two separate relays with the isolating relay triggered once the bypass relay has switched. RL-2 is NO, RL-3 is NC. The question here relates to what happens while the second relay coil is energising. Does this provide a load but not stop the current until it is fully energised (so there is still current flowing to the transformer) or does it completely prevent current flowing until it is fully energised? In other words, could it also create a situation where there is no power flowing to the transformer for a brief time? If so, like with Version 1, does this actually create a problem given the purpose of the circuit?
I believe that a diode across the base of the second relay in version 2 (RL-3) would eliminate the problem if there is one, but would it be needed?
Versions 1 & 2 - knocked up quickly to demonstrate using the first components in the list, not meant to be a technically accurate circuit