Ability to run on 120V/220V?

Thread Starter

brightnight1

Joined Jan 13, 2018
56
I have a device that runs on a typical US outlet at 120V and on the power supply I can can flip a switch to run it off 220V. I assume the switch moves back and forth between two different transformers?

If I wanted to buy or design a power supply for a higher power system to run off both 120V and 240V without having a human interact so that I could plug it into a 120V outlet or a 220V outlet without thinking about it, would I need something that automatically selects which transformer to use so I don't have to do it manually, or are there power supplies that have that wide of a voltage input range? How is it typically done? I'll probably be pulling 10-25 Amps. I realize I haven't added many details such as output voltages, etc, but I'm just focused on the general design and idea of something that can run off 120/220V. Thanks in advance!
 

Hymie

Joined Mar 30, 2018
846
Products that have some switch between 120/240V input voltage normally do not use two transformers, but switch between two 120V primary windings either in parallel (120V operation) or series (240V operation).
 

MaxHeadRoom

Joined Jul 18, 2013
21,201
You can also get a single transformer with either 120v or 240v primary with a 0-120v-240v secondary to accommodate the two types of outlets.
You could also do it with two identical 120 to 120 sec transformers.
Max.
 

ElectricSpidey

Joined Dec 2, 2017
1,193
A quick and easy way to do it would be to place a 220 volt relay on the input and use it to switch between windings.

The normally closed contacts would attach the input to the 120 volt windings and the 220 volt windings would be on the normally open contacts.

This would work properly only when connecting to the power, as long as the relay didn’t engage at 120 volts, but a warning…do not switch from 220 to 120 without disconnecting the power first, because the relay may not dropout and apply 220 to the 120 winding.

A very careful relay selection would be in order here.

Somebody else may have a better solution, using voltage sensing circuitry, but I don’t have the time to work something out.
 

MaxHeadRoom

Joined Jul 18, 2013
21,201
I was assuming the OP needed a permanent/instant selection which 2 types of outlets fed from the transformer would give.
I also assumed the voltage did not need to be switched between different outlets.?
Or simply run a 3 wire and earth GND from the panel.
Max.
 

crutschow

Joined Mar 14, 2008
25,256
The normally closed contacts would attach the input to the 120 volt windings and the 220 volt windings would be on the normally open contacts.
The possible problem with that is, when 220V is first connected, it is momentarily applied to the 120V winding until the relay operates.
That might draw too much momentary current and blow the fuse.

One alternative is to use a voltage sensing circuit that will pull in a relay when 120Vac is sensed, but not when 220V is sensed, and have the NC contacts connect to the 220V winding.

Alternately a current surge-limiter thermistor could likely also work to limit the current during that time.

Speaking of fuses, the 120V and 220V winding connections need separate fuses with the 120V fuse rated for twice the current of the 220V fuse.
 

MisterBill2

Joined Jan 23, 2018
6,718
The power supplies that are of the universal type almost never start out with a transformer. The cange from the 220 volts input to the 120 volts input is caused by an electronic means of altering the configuration of the input rectifiers. It changes from a simple rectifier with the 220 volt input to a voltage doubler when the 120 volt input is sensed. I was not watching closely enough when the explanation of exactly how it works was presented, so can't explain it any better than that. Sorry.
 

dendad

Joined Feb 20, 2016
3,591
Some info of what the device is would help.
Most power supply bricks and plugpacks are universal input. They have the switch mode supply designed to run off a wide input range.
What is it you want to power? What is the voltage and current of your device?
 

AlbertHall

Joined Jun 4, 2014
10,230

MaxHeadRoom

Joined Jul 18, 2013
21,201
I am wondering what is the reason or need for 120v and 240v outlets and the purpose of both?
For me, there is a lot of information missing?
If building something for each type of jurisdictions, 120,240 etc, why the need for automatic detection?
Something doesn't make sense?
Max.
 

MisterBill2

Joined Jan 23, 2018
6,718
Some info of what the device is would help.
Most power supply bricks and plugpacks are universal input. They have the switch mode supply designed to run off a wide input range.
What is it you want to power? What is the voltage and current of your device?
The circuit switches to the correct voltage by measuring with a simple circuit. But indeed the question is what voltage and what current.
 

MrChips

Joined Oct 2, 2009
21,647
TS said 10-25A.

That is huge!
Minimum 10A @ 120V = 1200VA.
Maximum 25A @ 240V = 6000VA

Let's assume TS means 10A @ 240V = 2400VA
25A @ 120V = 3000VA

Even 3000VA is a lot.
I cannot imagine an automatic switch-over or universal switcher at that amount of power.
TS will have to live with a manual switch over and a BIG sign to remind the user.
 

MisterBill2

Joined Jan 23, 2018
6,718
TS said 10-25A.

That is huge!
Minimum 10A @ 120V = 1200VA.
Maximum 25A @ 240V = 6000VA

Let's assume TS means 10A @ 240V = 2400VA
25A @ 120V = 3000VA

Even 3000VA is a lot.
I cannot imagine an automatic switch-over or universal switcher at that amount of power.
TS will have to live with a manual switch over and a BIG sign to remind the user.
That 10 to 25 amps would be the output of the supply, not the line voltage input. I don't think that the output voltage was ever mentioned. The obvious solution is a default to the 220 volt input mode with a switchover if the input is lower.
 

Thread Starter

brightnight1

Joined Jan 13, 2018
56
I basically wanted to be able to run an air conditioner, battery charger, and a few other devices from wall power in the US or Europe without having to change anything about my system so someone doesn't have to be aware of what they're doing, they can just plug it in.
 
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