Posted: 1/12/2014 5:16:36 AM EDT
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  |
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Depending on the type of load that may not be a functional 'Soft start' circuit.
Running load current through a relay coil is a good way to burn out the coil, if the load will even function weith that m,uch impedance on ots source. It is normally done electronically. A transistor that allows the voltage and current to be controlled as the load is applied. |
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Quoted:
Depending on the type of load that may not be a functional 'Soft start' circuit. Running load current through a relay coil is a good way to burn out the coil, if the load will even function weith that m,uch impedance on ots source. It is normally done electronically. A transistor that allows the voltage and current to be controlled as the load is applied. I'm no expert but I agree with this. If you want to use relays version 2 is an awkward way to do it and you'll probably burn the relay out. It's correct that version 1 will not allow any current to flow for a brief period while switching. Whether or not this is an issue is a complex question. Its going to depend on the switching time of the relay and the amount of charge being held by the capacitors when switching takes place... and how much voltage ripple you can tolerate on your output. Also, I question whether you really need a double-pole relay for version 1. There's nothing wrong with doing that, but a single-pole relay that bypasses the resistors would be good enough. Almost no current is going to pass through the resistors when they're bypassed anyway. |
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Quoted: Depending on the type of load that may not be a functional 'Soft start' circuit. Running load current through a relay coil is a good way to burn out the coil, if the load will even function weith that m,uch impedance on ots source. It is normally done electronically. A transistor that allows the voltage and current to be controlled as the load is applied. Quoted: Depending on the type of load that may not be a functional 'Soft start' circuit. Running load current through a relay coil is a good way to burn out the coil, if the load will even function weith that m,uch impedance on ots source. It is normally done electronically. A transistor that allows the voltage and current to be controlled as the load is applied. Never thought of that. That does rather put version 2 out of the running. Quoted: It's correct that version 1 will not allow any current to flow for a brief period while switching. Whether or not this is an issue is a complex question. Its going to depend on the switching time of the relay and the amount of charge being held by the capacitors when switching takes place... and how much voltage ripple you can tolerate on your output. Now that I couldn't rightly answer. In the absence of knowing enough to decide, it might be more wise to just work on the basis that it wouldn't be appropriate for the situation. Quoted: Also, I question whether you really need a double-pole relay for version 1. There's nothing wrong with doing that, but a single-pole relay that bypasses the resistors would be good enough. Almost no current is going to pass through the resistors when they're bypassed anyway. This seems to be one of the questions that keeps coming up with different answers - does it need to be bypassed at all? I happen to have a double-pole relay in the bits box, which is why I was looking at it. It also appeals to my selective OCD Thank you both for the replies, I appreciate the input. I think it seems likely that I will forgo the isolation and just use the relay to bypass the resistors. I can always come back and revise things later using specifically-bought components rather than relying on the spare bits box! |
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This is probably easier.
I have one of the Astron power supplies mentioned in the article and I added the inrush current limiter. As advertised, instead of the horrifying "twang" when I turn the supplu on, now I get blessed silence (and there doesn't seem to be a noticeable delay in the supply ramping-up). The inrush current limiter is added in series with the transformer's primary windings (110VAC side) and, when cold, has a high resistance. When you apply current to the transformer, the high resistance limits current for a short while (< 1 sec) then heats up (almost but not quite instantaneously) and its resistance drops to a low level, no longer limiting the current. |