Letter: AC/DC dilemma

Sir: I'm afraid using an internal low-voltage DC mains system is still not a practical alternative to the problem of powering small electronic appliances which need or can use direct current (Letter, 17 August).

A DC supply is even more susceptible to glitches than AC (technically this is because of the difficulty of switching the power to inductive loads, like electric motors), and so the supply would need careful conditioning before it could be used in any sensitive apparatus. This is done in practice by a "DC to DC converter" - which converts to AC at high frequency, through an isolating transformer to get the required voltage and glitch protection, followed by a rectifier back to DC, then smoothing. Because of the high frequency, the transformer can be physically very small. The system will incorporate active feedback to stabilise the voltage against supply and demand variations. It's also very power-efficient and can supply several different voltages at the same time.

Since you've got to do all this anyway for anything but the smallest appliance, it makes no difference whether the "raw DC" at the input to the converter comes from a dedicated DC feed or very cheaply from a rectified 240-volt 50Hz AC supply. Given that the latter is already provided, you might as well stick with it.

DR TIM J DENNIS

Colchester,

Essex