5 volts from the Mains

Published:2011/8/11 1:11:00 Author:Amy From:SeekIC

Paul Goossens

Sometimes we would like to hide certain equipment to prevent cluttering up our liv­ing room or any interior in general. A number of devices lend themselves to be built into a wall adapter. Think of a remote-control extender, for example.

All these devices need a power supply and we would prefer to use the mains so that no external connections are required for the power supply.

The power supply in this article is intended for exactly these situations, namely converting the mains voltage to a 5-V power supply voltage. The accompa­nying PCB fits exactly in a round wall socket enclosure.

A power supply is usually fitted with a transformer to reduce the voltage and also provide galvanic isolation between the device and the mains. In this power supply a transformer cannot be used because of lack of space. That is why we use a step-down regulator here. A problem with most step-down regulators is that they cannot be supplied directly from the mains. Hence, in this schematic (Figure 1) we first create a rough power supply voltage of around 40 V using passive components and subsequently present it to step-down con­verter IC1. The converter can operate from a maximum input voltage of 45 V.



The mains voltage is first fused by fuse Fl and then converted to full rectified sine wave by the bridge rectifier.

FET T2 is used as a switch, which is turned off when the voltage is greater than 40 V. The gate of T2 is driven via R3 and R4. As soon as the voltage exceeds the value of 40 V, transistor Tl will con­duct which causes the gate-drain voltage of T2 to be so small that T2 stops conduct­ing. Because of this, electrolytic capacitor CI cannot charge any further and the maximum voltage across CI is therefore limited to about 40 V.

This voltage is converted by IC1 and sur­rounding components to a 5-V power supply voltage. The maximum output cur­rent is 1 A.

The PCB (Figure 2) that has been designed for the power supply fits, as mentioned before, in a round wall socket enclosure. Note that resistor Rl and the link have to be soldered first, then comes R5, in the air above Rl because of the tight space.



Populating the remainder of the PCB should not present any difficulties. When mounting the PCB, do note that it is directly connected to the mains, so make sure no conductive parts can be touched when the circuit is in use. The mounting holes are separated less than 6 mm from the PCB traces so the board has to be secured with plastic screws in order to sat­isfy the safety requirements. Also, after fit­ting the board, a cover has to be mounted over the PCB so that it is impossible to touch the PCB when the wall socket is opened. As always, you cannot be too careful when dealing with mains voltages!

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