Why AC to DC are transformer based, mostly?

There are non-transformer AC->DC adaptors, but I dont see them much around...

Why every mobile/etc. uses charger with a transformer inside? just a couple of resistors +diodes would not be enough?

Resistors wud be a lossy option and mite generate heat.

safety

There is also the safety issue. Safety laws require that current from connecting any output to earth ground should be under several mA depending upon the class of the item.

Simple fuse would not solve that?

No. The fuse should be rated 30 mA or less to protect the user from dangerous AC current through the body. 30 mA * 230 V is only 6.9 W which makes the AC adapter unuseable for high power loads such as a laptop.

In addition, the fuse must blow very fast to protect.

Most countries requires galvanic isolation between the primary 230 V AC input and secondary voltage outputs. This is achieved by the transformer.

There is another reason

Traditional regulators first convert AC to DC. Then, it lower down the DC value to a voltage suitable for the equipment.

For example, the output of a transformer, if, after the diode/capacitor stage, is 10V DC, and you need to lower this down to 5V DC, this mean that half of the power will be lost during that conversion. So, if the device pump 1 amp @ 5V, it generate 5Watts. This mean that 5Watts would also be lost during the conversion from 10V to 5V (lost in the form of extra heat dumped by a heat sink).

If you were to take 120V or 230V AC, and convert this to DC immediately, without transmformer, then, the loss would be considerable when lowering it to 5V. This mean, in the end, that you would be paying your electricity bill mostly for heating your room instead of powering your device.

The newer power supply use a switching technology. This is a far more complex technology so that's why you don't see it often. It cost more, but it's much more efficient. The concept is that instead of 'dumping' the extra power, during the voltage lowering, in the form of heat, it use a different process. The process is hard to explain without a schematic to domonstrate, but it use the electro-magnetic property of an inductor coil to regulate the voltage. When the transistor is 'active', the voltage raise at the output, and at the same time, an electro-magnetic field is created around the inductor coil. When the transistor is 'inactive', the extra power that is stored in the form of electromagnetic field slowly collapse and create current through the inductor coil. Thus, even if the input DC voltage is applied only periodically to the regulator (usually with a cycle in the 40KHz to 1~2MHz range), there is constant current flowing through the regulator. Much less energy is lost because the extra power isn't loss through heat, but is constantly stored in the form of an electromagnetic field, and then constently recycled.

There are some issues to be respected:
1) safety issue: for most of the uses, a galvanic insulation between mains and user is mandatory, so a transformer is needed;
2) Power efficency; the more "green" behaviour in people is asking for high efficency in power supplies to avoid waste of power; the transformer is the ideal choice because its efficency is usually higher than 90 %
3) overvoltage protection : usually a transofrmer ( only the 50 hz ones ) is "strongher than the common electronci circuits and for this reason is preferred
4) project safety : if most of your circuit is interrested by the mains line, is higher the possibility for an incident during maintenance, tuning, repairing, etc
5) Components availability : if you look at the industrial / commercial market, you must try to use the technology available and the components available on the market

Hope it is enough

Mandi

It is mainly safety issue. The safety testing bodies will simulate the effect that will cause human electric shock after some components burned out.

VSWR is scorrect: it is an issue of goverment regulations requiring galvanic isolation. You can read more about circuits that directly convert without an isolation transformer if you do a search for "offline switchers".

andru123 said:

There are non-transformer AC->DC adaptors, but I dont see them much around...

Why every mobile/etc. uses charger with a transformer inside? just a couple of resistors +diodes would not be enough?

Click to expand...

I dont get it. You want AC to DC adapter/converter not to contain a step-down transformer but rather other components instead?

I mean it is possible but using a transformer is far more easily. I can not think a converter charger for your cell to be without a step-down transformer. Am trying to visualize one if I can make such one.

Using resistors and capacitors? The power dissipation across the resistor connected in series between the mains input and the low voltage output will be very high. Try calculate using this: P=VI
You'll find that most of the power supplied is dissipated by the resistors.

Hello

They use tranfo becouse the freqwency is 50MHz.The only trafo for hendeling this freqwency if iron trafo.sorry for my bad english

Hey dudde...

How on earth can you convert AC to DC without a transformer (for Linear PS's)....come man...you need why would you want to waste power...and without t/f's imagine the cost of the diodes and their sizes then is the Resistors imagine their wattage and so many other probs...so the best way to get around is to use a step down and solve the prob...
Imagine the benefits...isolation being the main..and also less loss due to low dissapation...so its profits every where...

Because the resistors will generate heat which will have adverse rudution effect on the output power.

There are many ways of reducing the voltage. I like the idea of a series capacitor with the mains. I also like the use of voltage multipliers to give you the DC you want. It's all based on the same things. One thing about the series capacitor to reduce mains is that because the filter capacitor is high, you would need a high value series capacitor in order to get voltage at the filter. This results in a pack of capacitors because of the need to increase the capacitance and the need to withstand the high voltage. This is a viable solution.

Yes, thats the main trouble with capacitors, the energy density. Capacitance depends on a surface area whereas inductance depends a volume, so its easier to pack a transformer into a given volume than a capacitor.

It is safty isolate and energy transform

1. first for safety reason because the transformer introduce isolation from mains power supply.
2. the resistor or cap. in series does not introduce voltage regulation becauce the drop voltage will depends on current so if mobile works through charging voltage will be fluctuates

You could implement a very simple AC-DC system with a handful of diodes and a capacitor or two. It would convert XVolts RMS AC to X Volts DC. Most applications for AC-DC converters are from line voltage to a device that needs substantially less than line voltage.

If I try to use a resistor to lower my voltage I generate lots of wasted heat, not to mention spending a small fortune on the high power resistors.

If I use voltage regulating IC's I also find myself with a power and heat problem. A transformer allows for a very cheap easy way to get the AC voltage down to nearly what I require. I can then rectify the lowered AC voltage with 4 cheap diodes and a filtering capacitor network. For added robustness I follow with an IC voltage regulator and have a very clean, very stable DC source with little wasted power. The drawback to this nonswitched supply lies with component size. If my load needs much power I end up using huge transformers and capacitors. My efficiency also drops.

Thats where the switching supply comes into play. By switching the incoming AC signal very rapidly one can increase its frequency to a point that smaller more efficient components can perform the conversion. Entire books on the subject exist so you can really dig deep if you'd like.

AC to DC convertion can be done using resitors(potential devider->rectifier->regulator).
But they consume more power and hence generate more heat.
Also they are fatal sine there is no isolation between mains and the derived power supply.

Added after 41 seconds:

AC to DC convertion can be done using resitors(potential devider->rectifier->regulator).
But they consume more power and hence generate more heat.
Also they are fatal sine there is no isolation between mains and the derived power supply.