# Understanding 110/220V Transformers and Designing Linear 24V 3A DC Power Supply

• likephysics
In summary, switch-mode designs, such as those used in laptop power supplies, allow for a wider input voltage range without significant power loss. This is achieved through the use of a transformer and kilohertz frequencies, resulting in smaller and lighter power supplies. Linear designs, on the other hand, would require a large transformer and would have lower efficiency.
likephysics
I can't figure out how those 110/220v transformer work. Like the ones in cell phone chargers.
Say for example, if the primary voltage is 110v, the secondary will be 24v. But if the primary is connected to 220v, the secondary should be 48v.
Are the stages after the secondary designed for up to 48v?
These are the ones without 110v/220v select switch like in the back of a desktop PC SMPS.

I am trying to design a linear 24V, 3A DC power supply. I couldn't find a small enough transformer. Everything I found was big(toroidal - 10cm dia and 5cm height) and heavy(1Kg).
But the transformers in DC adapters like laptop have small form factor.

I think they're designed with a switchmode supply after the transformer (which can deal with a large range of input voltages) or are pure solid state designs (without big transformers). In either case, I don't think you can go linear, unless you're willing to deal with a lot of power dissipation (a 7805 can be connected to up to 30V input, but it gets awfully toasty if you start drawing even modest amounts of current from it).

Yes, such things as laptop power supplies which have large power outputs for their size normally use switch-mode techniques.

Switched-mode design makes it practical to design for a wide input voltage range without the very large power loss which would be inherent in a linear design. If you think about it, a 110V to 220V input linear design could have no more than 50% efficiency at 220V, actually less when you allow for transformer and rectifier losses.

It is usual to rectify the line input directly without a transformer. The necessary isolation from the mains is normally achieved using a transformer fed by the switcher. Now for the clever bit - that isolating transformer works at kilohertz frequencies, much higher than the 50Hz or 60Hz line frequency. This allows it to be far smaller and lighter.

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## What is the purpose of a transformer?

A transformer is an electrical device that is used to convert electrical energy from one voltage level to another. It is commonly used to step up or step down the voltage in an electrical circuit, depending on the application.

## What is the difference between 110V and 220V?

The main difference between 110V and 220V is the voltage level. 110V is a lower voltage and is commonly used in North America, while 220V is a higher voltage and is used in most other parts of the world. This difference in voltage levels also affects the design and use of electrical devices and equipment.

## How does a transformer work?

A transformer works on the principle of electromagnetic induction. It consists of two coils of wire, a primary coil and a secondary coil, that are wrapped around a core made of iron or other magnetic material. When an alternating current (AC) flows through the primary coil, it creates a changing magnetic field which induces a voltage in the secondary coil, thereby transforming the voltage level.

## What is the role of a transformer in designing a linear 24V 3A DC power supply?

In the design of a linear 24V 3A DC power supply, a transformer is used to step down the voltage from the AC power source to the desired level of 24V. This is necessary as most electronic devices and equipment require a lower voltage level for operation. The transformer is also used to isolate the circuit from the AC power source and provide safety.

## What factors should be considered when selecting a transformer for a 24V 3A DC power supply?

When selecting a transformer for a 24V 3A DC power supply, some important factors to consider include the input and output voltage levels, power rating, efficiency, and size. It is also important to ensure that the transformer is capable of handling the required load and that it meets safety and regulatory standards.

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