# Electronic Circuits and Projects Forum

1. ## Power Supplies - Which are low standby?

Hello, just wondering, I want to buy an 'adapter', such as from 120VAC to 9VDC 2A. I want to have one that has the lowest standby power from the wall.

So for example if the load on the output is zero, then I would expect the adapter to be loading the 120V side with little to nothing. However I understand in most adapters the 120VAC goes immediately into a transformer, so that's quite a load already. I seen some adapters marked an "input: 120VAC, 11W" so I assume that's even with no load on the output.

I seen also some switching power supply adapters, but I'm not sure what the standby is like on those, are they less? Any recommendations?

Thanks.

2. Originally Posted by rnorman3
I seen some adapters marked an "input: 120VAC, 11W" so I assume that's even with no load on the output.
Thanks.
I would not assume that.

3. Well I guess you could start this way. Ac/DC adapters like this are commonly called wall warts. So plug a wall wart into the wall (for want of a better place to plug it into) and wait a while. Unloaded does it get warm? Yes, so some power is doing nothing. No getting around it, you plug an unloaded transformer into the wall and it gets warm so it cost something. Even a switching PSU unloaded will get warm so it cost something. If it makes heat from electricity it cost.

A 9VDC wall wart rated for 2 amps will draw 9 * 2 = 18 watts. Oh but wait, that would be 18 watts plus the loss. How efficient is the wall wart?

When you say:
"input: 120VAC, 11W"
I would assume under full load conditions,

While I have never delved into this much it works as follows. I pay about 7 cents (USD) per KWH for electricity. Running a max load of 2 amps at 120 VAC is 240 watts. So under full load a wall wart delivering 25 watts cost me about how much?

Ron

4. I am looking at a "EPS060100" power supply that outputs 6 watts and under no load it draws 0.5 watts.

5. OK, so would a switching PS use less power in standby than a non-switching?
Thanks

6. yes! yes!

7. Originally Posted by rnorman3
Hello, just wondering, I want to buy an 'adapter', such as from 120VAC to 9VDC 2A. I want to have one that has the lowest standby power from the wall.

So for example if the load on the output is zero, then I would expect the adapter to be loading the 120V side with little to nothing. However I understand in most adapters the 120VAC goes immediately into a transformer, so that's quite a load already. I seen some adapters marked an "input: 120VAC, 11W" so I assume that's even with no load on the output.
Not correct, that is the spec with rated load on the output.

Originally Posted by rnorman3

I seen also some switching power supply adapters, but I'm not sure what the standby is like on those, are they less? Any recommendations?

Thanks.
Depends, going into standby is a high level feature not found on most cheap power supplies.

8. Originally Posted by rnorman3
OK, so would a switching PS use less power in standby than a non-switching?
Thanks
It totally depends on the design. The switcher might have an offline bias rail to run on that has to stay alive all the time. It varies.

9. Recently most switching adapters have a very low standby power. It is almost imposable to sale one that uses more than 1 watt.
(green power)

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