View Full Version : DC Power Adaptors

10-05-2005, 10:21 PM
after gettin new speakers, which uses an AC to DC adaptor..i started wondering about this..

my speakers are constantly plugged into the mains, with it's switch on
only the speakers itself is off

the speakers runs off DC..so it use an AC to DC adaptor
this adaptor stays plugged in for 24/7

for this adaptor does it actually still uses up power when my speakers are turned off?

as far as i know..these adaptors are just little transformers...and by leaving them plugged in, wouldn't they still let induction occur thus using up power?

i'm just a bit curious about this.. because i've also notice the adaptor.. or any ac to dc adaptor seem to heat up a little when plugged in..even when the device drawing the power is not turned on
and now it's getting cold and heaters are being turned on...
i really don't want my powerbill to grow any larger than it needs...

Billy T
11-05-2005, 08:32 AM
Yes, they do still draw power, though at a reduced level from that when loaded. The actual power losses depend on the quality of the transformer design. I'll run some tests on a variety of adapters and post the results.


Billy 8-{)

Billy T
11-05-2005, 08:58 AM

Over a series of adapters from small to fairly large (2 amps at 13.5 volts) the power loss on no load ranged from 2.5 watts up to 7 watts. In some cases with very low power adapters there was very little difference between on load and off load current.

It would take 143 hours for the 7 watt example to use 1 kiliwatt of power. Of course the number of adapters in use needs to be factored into the equation, so say 5 adapters at an average of 3.5 watts would use a kilowatt every 48 hours, 15 kW/month, or around 180 kW/year.

This is a pretty rough and ready calculation but it is near enough for all practical purposes.

I currently have 9 adapters in use in my office, but most go off at night via a master switch.


Billy 8-{)

11-05-2005, 09:02 AM
My tests showed consumption between 0.8 watts to 3.5 watts on a variety of adapters running without their load connected. Even when the load is "off", the devices can still draw current. A typical example is a battery charger for NiMh AA cells, with no cells in the charger cradle there is still 1 watt extra of loading on the adapter unless it is completely unplugged from the charger.

At 3.5 watts, that is about $4 or $5 per year, it all adds up quite significantly.

11-05-2005, 02:46 PM
thanx heaps~
wow... i didn't even thought someone would even go and all these measurements for me..

looks like i should start unplugging some adaptors then..

um..also..how exactly did u take these measurements anyway?
i dont' mind taking some values for mine..

Billy T
11-05-2005, 05:25 PM
um..also..how exactly did u take these measurements anyway? i dont' mind taking some values for mine..

I don't know about GF, but I used a very short "phase break-out" extension lead and a Fluke 80i-110s ac/dc clamp on a Fluke 43B Power Meter. This reads the load/no-load current in the phase lead with reasonable accuracy and the rest was simple mathematics.

It could be done much more accurately with a series AC current meter but that can be dangerous on live AC feeds.


Billy 8-{)

11-05-2005, 06:01 PM
I use a series AC energy meter, that measures the current and the voltage and reports in watts. For more accurate results I also have a larger, unchanging resistive load running as well due to the possible measurement inaccuracies at the low level of load of a power adapter.

I look at the change in load with the adapter in and out of the circuit.