Samsung Plugs

[ivor4781]

I'm intrigued to know how a Samsung (poss: other) mobile mains charging plug being no bigger than a standard 13 amp plug and indeed somewhat lighter cuts the voltage down from 240/250 to around 3 volts normally there's a biggish transformer on these ?

[rccatnap]

Because the current required is only a few milliamps, if that even, a transformer is not required. All that's needed is a suitable resistor that cuts the voltage, but there will also be a solid-state integrated circuit to measure the resistance of the battery and cut off the supply when the battery is fully charged.

[rccatnap]

I forgot to mention that the circuit can also be designed to drop the charge voltage to a trickle-charge rather than switch off completely.

[ivor4781]

thanks for that rccatnmap
what made me wonder was the two large plug-ins for two free view boxes i have the ones that get very warm,i thought it would be the same for the mobe chargers

[Hymie]

While it is possible to generate a low voltage (directly from the mains) via a suitable resistor; this method will not provide the required isolation from the mains circuit.

All these direct plug-in USB charger devices employ a switch-mode power supply that is able to supply the required voltage and current using a surprisingly small transformer.

[beso]

Mobiles charge at a lot more than a "few milliamps". It is typically in a range of several hundred milliamps. Dropping voltage with a resistor would dissipate hundreds of Watts and be incredibly inefficient.

As Hymie says, they use an isolated switch mode converter. These operate at frequencies of several tens of kiloHertz allowing the transformer to be very small.

BTW. Some ultra low powered devices that do not require isolation use a capacitor to drop most of the the voltage before feeding a regulator. Using capacitive reactance to impede the current does not produce heat.

The full current is drawn from the mains but the current is out of phase so the actual energy drawn is quite small. Fortunately domestic electricity meters measure the energy rather than current.