Re: Power Supplies, Saving Money, and switching vs. Linear?

[nobody@xxxxxxxxxxxx (Dave Houston)]

Actually, the comparison to a 125W bulb isn't really that subjective. Watts
are watts and if the transformer is wasting 125W it should be putting out
the same total amount of heat as the 125W bulb. It makes for a fairly good
sanity check. I find even a 25W bulb too hot to touch.

Your wattmeter should give you an accurate reading if you break the circuit.

I doubt you'll save much with a switching supply. The difference will be the
heat wasted by the transformer and linear regulator. Switching supplies also
waste some energy as heat (mine get about as warm as a 2.5W night light).
Once you have an accurate measure of the linear supply wattage you can make
a better judgement. You'll likely find that there are other devices that are
using the bulk of the power.

I'm assuming that PG&E is charging you for true power (W) and not apparent
power (VA). Most residential utility meters measure true power.

"Andrew(N)" <andrew.ward@xxxxxxxxx> wrote:

>Dave Houston wrote:
>
>> How did you determine the cost? How are you measuring the power? 12V and .5A
>> is 6W. I doubt that your transformer is really wasting 126W as heat. Does it
>> get as hot as a 125W light bulb?
>
>The $56/month was by determined by using a WattsUpPRO wattmeter. and
>multiplying KWH * Running time * Highest tier on my PG&E bill (corrected
>for a monthly # of hours, of course). This was for the whole wiring closet.
>
>I did not break into the specific AC circuit of that Altronics Power
>supply with the watt meter. I have a Fluke T5 current probe. It's not
>perfect, but gives an approximation.  It's not as hot as a 125'er, but
>that's pretty subjective.  The linear regulator also generates some heat.
>
>>
>> I suspect a wattmeter (e.g. Kill A Watt) will show a watt reading on the
>> primary that comes much closer to the calculated value for the secondary.
>>
>> I don't have any large linear supplies but a 12VAC/500mA transformer I have
>> on an ESM1 shows 50mA on the primary (6VA) but only 2W using the wattmeter
>> mode. In Power Factor mode the reading is 0.36 which is in pretty close
>> agreement with the VA and W readings. The transformer presents an inductive
>> load so the voltage and current are not in phase.
>
>OK... So I should probably break into the circuit and use the WattsSPpro
>meter to get a good reading.
>
>Still wondering if the switcher would save money power-wise.
>
>Thanks Dave - As Always --- Everyone appreciates you.
>
>-Andrew
>
>>
>> "Andrew(N)" <andrew.ward@xxxxxxxxx> wrote:
>>
>>
>>>My electronics/wiring closet is costing $56/month to run. Ouch. Too
>>>much! So one-by-one I am changing things to be more power-conscious.
>>>
>>>Starting with a big linear 12V power supply. Altronics 12V, 4A type
>>>linear power supply with battery backup: standard for alarms (a few
>>>years back). I currently draw about 500 ma continuously, (Not sure what
>>>my peaks are, but bells and sirens are powered separately.
>>>
>>>This draws around 1.1 A on the AC line, so 132 Watts continuously. Big
>>>transformer, always warm. Seems wasteful.
>>>
>>>IF I SWITCH TO A NEWER, SWITCHING POWER SUPPLY, say the Elk P412 (or
>>>any other you recommend) WOULD I DRAW MEASURABLY LESS AC CURRENT?
>>>
>>>I am powering various motion detectors,  InfraRed repeaters (Niles), a
>>>small, 7 Watt audio amplifier and a few other devices.  Would the 100
>>>MV ripple on the Elk power supply bring any problematic noise along
>>>with it?
>>>
>>>Thanks for your suggestions
>>>
>>>-Andrew
>>
>>