External SMPS - Why no power switch?

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john12ax7

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More gear these days uses external line lump supplies,  but they almost universally omit an on / off switch on the unit.  Curious as to why?

Certainly you can't switch mains voltage,  but why not switch the incoming + / - 15 V?

Any reason the smps supply itself wouldn't like this? If not then all that's left is cost,  but that's a strange way to save a few bucks on a high end unit.
 
I'm thinking the main use for these is laptop computers that when they're "off" the power supply is charging or keeping the battery fully charged, so they're pretty much meant to be on all the time that they're plugged in. Being switchers, they're reasonably efficient (they get pretty warm with full load, but much less so when you unplug the load), so they don't use "very much" electricity when the load is turned off.

I guess you could add a line cord switch like some lamps (such as lava lamps) have.
 
I meant more so for pro audio gear,  not power charging.  For example the 8 space api lunchbox,  or dangerous dbox don't have on / off switch.
 
john12ax7 said:
More gear these days uses external line lump supplies,  but they almost universally omit an on / off switch on the unit.
Dunno about universally. Until you mentioned those two, I couldn't even think of one!
But yeah, no reason why the power supply wouldn't like it. It looks like simple cost cutting. Many units (like the lunchbox) would need a three (or more) pole switch, which is expensive, or a simpler switch and some MOSFETs, which is equally expensive.
 
john12ax7 said:
More gear these days uses external line lump supplies,  but they almost universally omit an on / off switch on the unit.  Curious as to why?

Certainly you can't switch mains voltage,  but why not switch the incoming + / - 15 V?

Any reason the smps supply itself wouldn't like this? If not then all that's left is cost,  but that's a strange way to save a few bucks on a high end unit.
SMPS are not happy with no load; good reason for not switching the output.
In-line or wall-wart are often hidden in a corner, so switching them is not convenient.
User pays the electricity bill, so designer doesn't care. In addition most SS gear can run 24/7.
Very often, SMPS are bought from a vendor; as aresult of the chain of margins, adding a $1 switch ends up upping the street price by $10-15.
 
Isn't rack gear almost always connected to some kind of power filter or sequencer or similar which has a power switch? So you switch the entire rack or groups of gear on together. Having a "master power switch" kinda makes sense to me actually. That's what I do. I have a Monster PRO 2500 which has a sequencer so that my monitor speakers are switched on after the USB audio interface. I have one other rack and that just uses a cheap power strip. I just reach behind and switch the entire rack on. I've built a few channel strips and they share the power supply so I have one switch for that. Otherwise, you would spend 10 minutes just flipping power switches on.
 
abbey road d enfer said:
SMPS are not happy with no load; good reason for not switching the output.

Yes, but to what degree? What I've noticed is typically current minimums need to be met to not negatively impact performance. I suppose a long term term unloaded output could reach some permanent damage as it tries to adjust unsuccessfully. I haven't seen that happen,  so am curious as to how much of a concern it actually is.
 
squarewave said:
Isn't rack gear almost always connected to some kind of power filter or sequencer or similar which has a power switch? So you switch the entire rack or groups of gear on together. Having a "master power switch" kinda makes sense to me actually. That's what I do.

I have something similar as well. With a lunchbox though you can't hot swap, so you need to power down all your gear just to change a module.
 
john12ax7 said:
Yes, but to what degree? What I've noticed is typically current minimums need to be met to not negatively impact performance. I suppose a long term term unloaded output could reach some permanent damage as it tries to adjust unsuccessfully. I haven't seen that happen,  so am curious as to how much of a concern it actually is.
Switching supplies modulate on time vs off time to regulate output. With zero load the on time is zero... can't switch between off and off... :eek:  Most manage with some internal load, but too much cost efficiency.

JR
 
john12ax7 said:
Yes, but to what degree? What I've noticed is typically current minimums need to be met to not negatively impact performance. I suppose a long term term unloaded output could reach some permanent damage as it tries to adjust unsuccessfully. I haven't seen that happen,  so am curious as to how much of a concern it actually is.
Under no-load condition, most SMPS dissipate more than under normal load; the heat build-up impairs long-term reliability.
 
my last gig, we made  a few units that did not have a power switch.  The idea for us was two fold. 1. the rack gear is usually connected  to a power strip or some other form of a bulk on off for the gear or rack so it would be unnecessary or in the case of a active direct box we made, wanted something that people couldn't turn off by accident while onstage..., so to eliminate  any confusion or issues we didn't have one.  I did mod a few where the owners wanted a power switch. No big deal...
 
abbey road d enfer said:
Under no-load condition, most SMPS dissipate more than under normal load; the heat build-up impairs long-term reliability.
I am... not convinced this is true.
 
Not all SMPS will opperate without a load , a laptop psu needs to be able to do this ,because it gets plugged out of the pc while still energised by mains . A  desktop computer psu probably wont ,as it gets its on signal from the motherboard its always connected to before it wakes up .
I modded up an xbox switcher psu before  ,worked away great  for ages until one day I forgot about making sure it was loaded before switching it on .I asked someone in the know and they explained for a switcher to remain stable off load an extra bit of circuitry is needed ,if the situation of no load is never suppose to arrise , the designer wont include it as it saves on effiency .

I think it  is something you could quite easily retrofit to an existing supply to cover the no load scenario ,but I didnt invesigate into it any further .The grey Xbox psu is good for about 10A at 12 volts,  has decent enough filtering and screening and comes in a neat tidy plastic package , the xboxes themelves have long since cooked there tripes and died ,you can get them very cheap or free  from recycling . An extra pin on your supply cable connector ,to energise the supply only when connected to something would be easiest cause the xbox supply itself is potted in white compound it doesnt lend itself to fixing or modding .
 

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