Why do some devices turn on with the mains before the "On" button is pressed?

[sacentre]

I've noticed this behaviour with a number of cheap Asian consumer products I own eg Draco HDT-2 7700 DTB-T set top box, HAYEAR digital microscope camera etc. The Draco runs directly off 220VAC, the Hayear takes a DV12v supply. Both devices have power buttons on their front panels but they power up as soon as they're turned on at the mains, bypassing the panel button. The button will turn the device off. Why is this so common? Is is just a bit of corner cutting on the power supply circuit design?

 

[Bluejets]

Maybe it's only for turning off...did you read the instructions?

 

[Martaine2005]

May be because most items having a standby circuit need constant power. So when plugged in, there is always power to the unit even when switched off. Hence the standby circuit waiting for a command. When I plug my TV in, it comes on and then goes off waiting for a command from the remote.


Martin

 

[Alec_t]

Many domestic appliances these days never switch off fully: they only go into a deep sleep from which they can be woken with a button press or screen touch.

 

[sacentre]

May be because most items having a standby circuit need constant power. So when plugged in, there is always power to the unit even when switched off. Hence the standby circuit waiting for a command. When I plug my TV in, it comes on and then goes off waiting for a command from the remote.


Martin

As far as my experience goes "standby" mode is pretty much universal. Every TV, PC Monitor, VHS/DVD recorder, tape deck, audio amplifier, PC sound system etc I have ever owned works the same way: you plug them into the mains, a standby light comes on then the device waits for you to press the power button on the device itself or on the remote control to wake them up so you can start using them.

The devices I'm talking about don't do that. They wake up to full on without waiting for the button to be pressed. They can be turned off and on again with the button on the device or remote subesequently however, but if I kill the mains by unplugging or switching off at the socket switch, they wake up immediately once mains power is restored.

My guess is they were designed that way ie the power button is only used to turn the device OFF after initial mains plug in. Thereafter, the button works as BOTH on and off. I was just curious what the rational for it could be (if not cost cutting).

 

[sacentre]

Many domestic appliances these days never switch off fully: they only go into a deep sleep from which they can be woken with a button press or screen touch.

Except that the devices I'm talking about don't wait to be woken up with the button.

 

[crutschow]

My thought is that it's just a bad/lazy design.
They didn't bother to insure that the device would not power up when first plugged in.

 

[sacentre]

My thought is that it's just a bad/lazy design.
They didn't bother to insure that the device would not power up when first plugged in.

I can't think of a more likely explanation.

 

[crutschow]

That fault is especially annoying if the main's power goes out, since you then have to remember to go around and turn them all off after the power returns.

 

[Martaine2005]

That fault is especially annoying if the main's power goes out, since you then have to remember to go around and turn them all off after the power returns.

Especially if you’re away on holiday!
But any smart person would unplug those types.


Martin

 

[sacentre]

That fault is especially annoying if the main's power goes out, since you then have to remember to go around and turn them all off after the power returns.

Exactly!

 

[crutschow]

But any smart person would unplug those types.

But that is only practical for things you don't use often.
Perhaps easier to plug into a power strip with a switch.

 

[Martaine2005]

But that is only practical for things you don't use often.
Perhaps easier to plug into a power strip with a switch.

Yes totally.
I used ‘unplug’ to mean turn off, disconnect, switch off etc in whatever manner. Because each unit might be in a different room.


Martin

 

[Bluejets]

a number of cheap Asian consumer products

So don't buy them....you seem to be aware of it and yet persist in buying them, then complain of the outcome.
Who knows how the designers of these things think....
good bet it's to do with being cheap though.....

 

[John Canon]

The sudden application of power will cause a small surge to the various capacitors in the device. The inrush will then cause solid state devices to receive false triggers unless there is good circuit design to prevent these false triggers

The 2 cheap Asian consumer products you own obviously do not have that good circuit design. Your complaint is noted.

 

[sacentre]

Thank you John. That was helpful. I was wondering if adding a suitable cap somewhere might fix it.

I guess my use of the word "cheap" turned out to be a little problematic but was in no way meant to sound as if I was complaining about the devices (which seems to have ruffled Bluejets' feathers). This was completely unintentional. Maybe I should have said something like "low-cost and otherwise perfectly satisfactory for the price". In point of fact, the DVB-T box was distributed FOC here (Singapore) by the govt to help the transition from analogue TV but I would happily have bought one anyway with or without the power issue.