Universal Supply Range

Thread Starter


Joined Nov 16, 2006
Need a bit of advice, when you buy a power supply or other equipment
that will allow 110Vac to 240Vac how do the manufacturers ensure that
they have the correct fuse rating for each voltage range?

I have a unit where the power supply will autorange so therefore i can sell
the unit as a unit that will take 110-240Vac 50-60Hz, however trying to size a thermal breaker (thermal breaker mounted in rocker switch) it becomes impossible to size a thermal breaker that will operate with the
same time/current characteristics because at 110vAC i try for eg 4Amps and at 220Vac i draw 2Amps?

Any advice on how i can still slap a 110Vac to 240Vac sticker on my unit?



Joined Oct 31, 2006

Well the manusfacturer ensures usually the Fuse rating base on the
Power Supply rating either in 100 VAC amd 240 VAC , which the explanation
of this is that usually the Power Supply ensures the maximum current
rating which is during minimum input voltage and also ensures safety
during maximum input voltages.

Thread Starter


Joined Nov 16, 2006
hey thanks
if the power supply has no fuse?
Do you see any problem in me rating the thermal breaker for the 110Vac current, bearing in mind at 220Vac it will be able to draw roughly twice the power through the switch and hence power supply before the switch would trip?


Joined May 16, 2005
I think you have it backwards...

The load runs off the secondary, and therefore a fixed voltage (i.e. 24Vac) and draws a reasonably fixed current (i.e. 20A).

Two primary windings are then connected in either parallel (for 120Vac operation drawing 4A input) or in series (for 240Vac operation drawing 2A). A switch is sometimes used to select between input voltages.

If you put a 2A breaker in series with each primary winding, they can be switched right along with windings.


Joined Jun 22, 2006
Many of the modern day power supplies rectifies the wall outlet voltage immediately to DC using high voltage diodes or other rectifying devices. This high voltage DC is then converted to other lower DC voltages via switching supply methodologies. This eliminates very large transformers and the efficiency is very high. Have you notice those tiny adapters that come with laptops that could provide 100 VA and is warm to the touch?


Joined Apr 20, 2004
It is more conventional to get a power entry module that takes an IEC 320 power cord connector and has a multi position block that lets it operate on 115 or 240 VAC (it actually changes the input transformer leads). There should also be built-in fuses for both voltages.