Main power switch location: primary, secondary, after rectifier...?

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In most power supply diagrams I have seen which include a transformer, the main power switch used to turn the device on and off is located between the mains supply and the transformer's primary coil. What are the reasons, if any, for prefering this arrangement over a switch on the transformer's secondary coil?

If I am using an AC wallwart to power a device, I obviously cannot get access to the primary coil to put a switch on it. In this scenario, what is the optimal place to put an on/off switch? At the secondary coil, immediately after the wallwart's connection to the device? Or should I switch the DC power after the rectifier and leave the connection between the low voltage AC and the rectifier always closed? If I should switch the DC after the rectifier, should this be done before or after the smoothing caps? I would really appreciate some detailed description of the pros and cons for all these options.
 
Safety. Safety. Safety.

In a sealed wallwart safety is provided by the sealed case. Where possible, AC switching can be better than DC switching as AC arcs tend to self-extinguish. Also, as a general rule, the switch should be as early as possible - so basically as soon as the supply enters the case. That minimises the live stuff in the case when switched off. Low voltage won't harm people(*), but people can still harm equipment if a probe slips during fault-tracing.

(*) an exception is very high current supplies such as vehicle batteries if there is any loose metal around, including rings
 
So the reason the primary coil is switched in non-wallwart designs is just to minimise the number of high voltage points when switched off for safety's sake? Is there any electrical difference between switching the primary or secondary coil, in terms of in-rush current or any other transient start up phenomena?
 
Easier (and lower loss) to switch 240Vac @ 1A rather then 24Vac @10A!

Heat goes as I^2R so for a given switch the 24V case produces 100 times the heat in the contacts.

Finally, high current DC at any kind of voltage is an absolute bugger to switch as the arcs do not self quench, which is why you see switches rated for 250V AC/24V DC, they generally mean it.

Oh yea, and think safety, safety and safety.

Regards, Dan.
 
I suppose efficiency could also be included here - the transformer does take up some power with no load, and switching the primary insures the device takes NO power when "off."

A lot of modern devices (desktop computers and remote-control TVs) have separate low-power power supplies that run all the time and power the electronics to detect either the front-panel pushbutton or an 'on' command from the remote, and then switch on or off the main power supply.

For a wallwart powered device I'd put the switch in series with the connector to the wallwart cable. This is surely the way small devices such as routers and cable modems are wired.
 
When properly desighned, a power transformer takes little quiescent current ( verry little often ), so that would not be a good enough reason to put the switch where it is, @DF96 presented verry well the main reason for this, witch is safety, witch it must outweigh anything else everytime, you can always change the power switch if it goes bad but you cannot undue a life injuring mistake. So to be short, when you switch the power off on any device you want no HV points left anywhere on it.
 
Thanks, everyone! I more than understand the safety issue here, and at no point meant to suggest that "mere safety" should take a back seat to other issues. I mostly just wanted to be sure that switching the secondary coil in an AC wallwart application (where no part of the amplifier is at high voltage no matter what) wasn't going to cause any unpleasant surprises, as I was sure I read something somewhere about switching the secondary coil causing some unpleasant start up behaviour.
 
I mostly just wanted to be sure that switching the secondary coil in an AC wallwart application (where no part of the amplifier is at high voltage no matter what) wasn't going to cause any unpleasant surprises, as I was sure I read something somewhere about switching the secondary coil causing some unpleasant start up behaviour.
If the secondary has filter capacitors, switching the filtered DC will produce higher dV/dt on the device being powered which in some cases might cause stability problems or high switching currents. Car Audio seems to manage OK though.
 
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