|Re: Real world hard numbers|
Message #18 Posted by Rodger Rosenbaum on 20 Mar 2008, 6:40 a.m.,
in response to message #15 by Randy
If I added to my previous post and said the power factor was 1.0, would you still disagree the dissipation was 1.8 watts?
Is the power factor in fact 1.0? How did you measure it? Why would you add that to your previous post if you didn't know? Do you think that designnut is just guessing that the current is almost all reactive?
Of course, if the power factor is 1.0, then the power dissipation would be 1.26 watts (not 1.8) watts.
But it is extremely unlikely that an unloaded adapter would present a 1.0 power factor load to the line, because what they contain is a small transformer, and sometimes a rectifier/regulator. The no-load power consumption is almost all hysteresis and eddy current loss in the transformer core (especially if there is no rectifier/bleeder or regulator).
You can't determine true power consumed (and power factor) by just measuring the current consumption of a load and multiplying by the applied voltage; this only gives the apparent power (see http://en.wikipedia.org/wiki/Power_factor). You need a true wattmeter such as the Kill-a-Watt, a handy low-cost device that is readily available; you can find multiple vendors on the web. But the Kill-a-Watt won't suffice to make suitable measurements of these kind of adapters because the power they consume when unloaded is usually less than 1 watt and the Kill-a-Watt doesn't have the necessary resolution.
However, having been in the business of designing transformers for a while, I have an electrodynamometer wattmeter specifically designed to measure the true power consumed by very low power factor loads. In other words, it's designed to accurately measure core loss of transformers. The power consumed by an unloaded adapter will be almost all core loss, even if the adapter has an internal rectifier and capacitor (but no bleeder resistor or regulator).
I measured (unloaded) the adapter that came with my HP-71, which has no internal rectifier, and found the following:
Current drawn from a 120 volt line is 20 mA.
True power consumed is .30 watts.
The product of 20 mA and 120 volts is 2.4 and it might be thought that this means the adapter is consuming 2.4 watts, but in fact it is only consuming about 12% of that amount (a power factor of .125).
I also measured (unloaded) the adapter that came with my HP-45 and got the following results:
Current drawn from the 120 volt line is 28.6 mA.
True power consumed is .7 watt.
The HP calculator adapters are quite low idle power consumption because they are only called on to supply a very small power in use and have a very small transformer.
In contrast, the wall wart that came with a Radio Shack battery charger draws 22 mA when unloaded, but consumes a true power of 1.2 watts. Its rated load is .5 amp at 12 volts, much more than the calculator adapters. It has a larger transformer and hence greater core loss. In a properly designed transformer, the core loss should be just about proportional to the volume (and therefore, weight) of iron.