Really stupid electronics question... How the heck do "soft" switches work?

[Viper GTS]

I know this has to be simple, but I don't understand it & I haven't been able to find anything on Google.

How the heck do "soft" switches work? For example, the power switch for an ATX power supply. Also, turning "off" A/V equipment (such as a TV) with a remote control. I don't understand how the power is actually shut off from those portions of the device. Obviously nothing physically changes (or does it?), so how can a device decide to block power from a portion of its circuitry?

Can anyone explain this to me? Or even a suggestion on what to search for...

Viper GTS

 

[GasX]

google search terms: electron, power, switch, fluffy pillow

that should set you right...

 

[Demon-Xanth]

Analog switches and relays. With "soft off" things aren't completely off.

 

[Wingznut]

Originally posted by: Demon-Xanth
Analog switches and relays. With "soft off" things aren't completely off.

Correct. Semiconductors... The processor gets an input from the switch, and it shuts off a transistor, which shuts off the power.

 

[SagaLore]

Originally posted by: Viper GTS
I know this has to be simple, but I don't understand it & I haven't been able to find anything on Google.

How the heck do "soft" switches work? For example, the power switch for an ATX power supply. Also, turning "off" A/V equipment (such as a TV) with a remote control. I don't understand how the power is actually shut off from those portions of the device. Obviously nothing physically changes (or does it?), so how can a device decide to block power from a portion of its circuitry?

Can anyone explain this to me? Or even a suggestion on what to search for...

Viper GTS

I knew this once... can't remember what it's called. It has to do with MOSFET transistors.

 

[PottedMeat]

hmm, i think for A/V equipment one possibility is that incoming AC could go into a transformer with multiple outputs, for example a high power output (to power amplifiers/tubes) and a low power low voltage output that runs logic and control. The low power circuitry might control the on/off state of the high power circuit with a relay or mosfet or something. but the low voltage circuit would be on at all times to accept control inputs like a remote control IR or RF signal. the power consumed by the control circuit is negligible compared to that of the amp/tube circuit.

 

[Viper GTS]

Originally posted by: Demon-Xanth
Analog switches and relays. With "soft off" things aren't completely off.

I know that much, that's what was confusing me... What separates the portion of the circuitry that is on from the portion that is "off?"

If it is transistor based, how do they handle high levels of power (like a CRT)?

Viper GTS

 

[Demon-Xanth]

Most monitors actually have relays inside thier power supply sections. And FETs can handle very high amounts of current while dissipating low amounts of power (10A while dropping 0.25V is only 2.5W, and 0A is 0W) when used in an on-off only application. The "on" portion is only the bare minimum to control the device. Much like parts of your car are always "on" but not the whole car.

 

[NogginBoink]

With an ATX power supply, when the supply is "off" it's still generating a very little bit of current that it supplies to the motherboard. Effectively, the only circuit on the motherboard that gets power is the power switch. When you press the power button that little bit of current tells the power supply to turn the rest of itself on.

 

[Demon-Xanth]

Originally posted by: Mwilding
google search terms: electron, power, switch, fluffy pillow

that should set you right...

btw, 536 hits, adding quotes to fluffy pillow only results in one hit though.

 

[Woodchuck2000]

Originally posted by: NogginBoink
With an ATX power supply, when the supply is "off" it's still generating a very little bit of current that it supplies to the motherboard. Effectively, the only circuit on the motherboard that gets power is the power switch. When you press the power button that little bit of current tells the power supply to turn the rest of itself on.

The power provided whilst off is also enough to run bits of the network card/modem for WOL/WOR functionality. Devices with switched mode power supplies dissipate a surprising amount of power when 'off'...

They either use a relay or some kind of FET to control power to the rest of the system.

 

[Wingznut]

Originally posted by: Viper GTS

Originally posted by: Demon-Xanth
Analog switches and relays. With "soft off" things aren't completely off.

I know that much, that's what was confusing me... What separates the portion of the circuitry that is on from the portion that is "off?"

If it is transistor based, how do they handle high levels of power (like a CRT)?

Viper GTS

The transistors are basically just switches that turn on and off. They control mechanical relays (or power transistors) which control the higher levels of power.

 

[NeoPTLD]

Originally posted by: Viper GTS

Originally posted by: Demon-Xanth
Analog switches and relays. With "soft off" things aren't completely off.

I know that much, that's what was confusing me... What separates the portion of the circuitry that is on from the portion that is "off?"

If it is transistor based, how do they handle high levels of power (like a CRT)?

Viper GTS

It works kind of like an ignition switch. The power is always there to the key cylinder in the steering column. The control input from the key activates the drive relay that makes the connection between battery and starter.

The control always have power and when the remote orders the control to turn on the main circuit, it does so either with a mechanical relay like the starter relay or with an electronic switch.


I don't know if they're using transistors, but semiconductors can handle a lot of power. Traction motors in MAX trains are both controlled electronically by huge semiconductor chips so compared to these, CRT is nothing.