Switched mode PSU current draw

[nicebutdim]

Hi, bit of a technical question here as I can't find the information online. Does a psu draw less current from the mains when you place less load on the output? Or will it still draw the same, no matter what it is powering?

 

[Gizzy]

Voltage varies little with load according to Wiki....

http://en.wikipedia.org/wiki/Switched-mode_power_supply

No mention of what the mains current does but if the voltage is constant, then the current must vary if the load does.

V = IR Ohms law still applies I reckon....

 

[Neil Robinson]

I think power supplies are reasonably efficient.

Simplistically if they were the impossible 100% efficient then if you doubled the load on the output the input power would double, halve the output power and the input power will halve.
Again simplistically the power may be considered as the voltage multiplied by the current so, for example, a power supply with 24V output and 240V input would give 10 amps output for 1 amp input.

 

[nicebutdim]

I see what you mean, Gary. The companies that manufacture the psu(s) don't mention the mains draw, except for 60W power dissipation. To give a better idea of what I mean, say a 5A 12v psu drains 1A from the mains when 50 leds are connected, will it drain 500mA when 25 leds connected?

 

[nicebutdim]

I think power supplies are reasonably efficient.

Simplistically if they were the impossible 100% efficient then if you doubled the load on the output the input power would double, halve the output power and the input power will halve.
Again simplistically the power may be considered as the voltage multiplied by the current so, for example, a power supply with 24V output and 240V input would give 10 amps output for 1 amp input.

Thanks Neil, that is what I was hoping for. Now, next question, would a pv switch (dusk till dawn 240v switch) be ok to switch a psu as I know you have to be careful with inductive loads and the like?

 

[Neil Robinson]

Naturally that would depend on the rating of the switch. If it's cheap enough with a reasonable current rating I suggest you try it and see. If it proves to be unreliable buy another to switch a relay that in turn switches the power supply.