A Power Switch

Hello, William! You wrote on Wed, 16 Nov 2005 14:58:06 -0500:

WW> I do not understand how a power switch in modern day TVs, radios and WW> other electronic devices turn on the power to the chassis. Could WW> someone explain how this works. I presume that somehow a relay is WW> activated but don't understand the circuitry for accomplishing it. (My WW> past experience has always involved mechanical anolog switches, so when WW> a set goes dead, I am helpless). Further, how do you test where the WW> power is lost when you can't turn on the device?

WW> Bill

Sam already did that, and there is a wealth of information available here

about the operation and faultfinding of modern swithmode Power supplies. But the reason you can't figure out how to power a unit on, is that it's done with software using semiconductors as switches. You simply have to read all about it, before you can properly begin to understand it, let alone find out why it doesn't work.For a beginner there are some important safety rules you really must understand before you poke your little pinkies in there.

With best regards, 3T39. E-mail: snipped-for-privacy@hotmail.com

They use a small standby power supply which produces usually 5v, sometimes other voltages and is turned on all the time and keeps the microcontroller running. When you press the power button the microcontroller closes a mechanical or solid state relay which turns on power to the device. Often if a fault is detected somewhere the power on sequence will be aborted.

In today's TV's and etc., the power switch is usually a set of contacts that is read by a port of a uPC. The uPC has an output that controls a logic drive, or simple relay driver to turn on a power supply, or operate another control of some type.

There are many varations of how this is done, depending upon the particular design.

For most devices today, the simple latching power switch is no longer used, unless for a specific purpose.

Jerry G.

Thanks to everyone for the input. I have no problem repairing SMPS's. It is the numerous devices without them that I have trouble with when they are powerless. The repairfaq.org is of no help whatsoever in this regard.

- Bill