EE Majors/Techs: Power Supply/Consumption?

[Robert Coats]

Take a typical AC/DC power supply, like for a cellphone or other low-voltage DC device...these are rated at how much power (watts) and/or current (amps) they can deliver. I have some questions:

1. Plugged into a standard 120VAC outlet, how much power is consumed/generated with No Load on the output?

2. If there was a theoretically-optimum battery on the other end, is there a sweet voltage that maximizes charging efficiency?

3. I will guess the max output is instantly reached when connected to a fully discharged device, and then is reduced as the battery(s) approach full charge? Once fully charged, why does the power supply stay so hot?

4. How much (percentage?) energy (consumed vs. transferred) is lost to heat? If the power supply pulls X watts per hour, how much of that is actually loaded onto the battery?

5. Is there any tech out there than can economically capture/re-use the otherwise lost heat/energy?

Apologizes in advance...I used to work in product planning, and the one question the R&D guys would always ask, 'How many more can you sell if we make that work?' (sigh)

[hcoles]

Policies to reduce no load power
Energy Star, the EU code of conduct on standby and other mandatory and regulatory standards are encouraging manufacturers to make further reductions in no-load energy demands.

For example: under the Energy Star V2.0 (level V) voluntary standard introduced in November 2008, the no-load consumption of a typical 4.25 W charger has to be less than 0.3 W. The EU code of conduct version 4, introduced in January 2009, has a limit of 0.25 W for mobile handheld battery-driven applications, reducing to 0.15 W in January 2011.

[70SATMan]

Robert, because the charger itself is still a load on the ‘transformer primary’ side (AC to DC rectifier).

[masraum]

I'm not an EE, so this is just my gut thoughts which are probably not worth any more than you're paying for them.


1. Plugged into a standard 120VAC outlet, how much power is consumed/generated with No Load on the output?
I assume it's pretty minimal

2. If there was a theoretically-optimum battery on the other end, is there a sweet voltage that maximizes charging efficiency?
I assume the Voltage is pretty steady and is already in the sweet spot. I think that there is some intelligence around the amperage used. I know that when I plug my phone in at night it says it's using optimized charging and should be fully charged by X:XX time. I think it's probably for battery health. I think slow and low is the best way to charge most batteries, but fast is in place for convenience.

3. I will guess the max output is instantly reached when connected to a fully discharged device, and then is reduced as the battery(s) approach full charge? Once fully charged, why does the power supply stay so hot?
I think a lot of chargers or maybe the devices being charged have intelligence built in to vary the demand. I'm sure that there are times when the charge goes to full load, but I don't think those times are 100% of the time.

4. How much (percentage?) energy (consumed vs. transferred) is lost to heat? If the power supply pulls X watts per hour, how much of that is actually loaded onto the battery?
That's a good question. I have no idea. Heat seems to be a big energy loss in many, many systems. I feel like it's not as bad as it used to be. I feel like power supplies used to run a lot hotter than they seem to run these days.

5. Is there any tech out there than can economically capture/re-use the otherwise lost heat/energy?
I think you're SOL on heat reclamation/conversion other than resting cold feet on a warm power supply.

[Arizona_928]

Peltier element (thermoelectric module)… ^^^

Still not worth it imho

[masraum]

Originally Posted by Arizona_928

Peltier element (thermoelectric module)… ^^^

Still not worth it imho

I had a Peltier cooler for a CPU once years ago. Can you reverse the process such that applying heat causes it to produce electricity?

I assume the problem is cost vs efficiency. You'd need to insulate the area being heated to try to retain as much of the heat as possible to then make it possible to perform as much conversion as possible.

[1990C4S]

What Steve posted is pretty much spot on.

'Heat' is only wasted if you're not heating your house. It is a tiny tiny amount, not worth reclaiming, and not worth adding load sensing to cut the transformer power consumption.

[aschen]

Swich mode power supplys very efficient and low power devices to begin with. Would be like trying to capture condensation on your beer glass to help with the water bill.

There are several cheap devices that you can use to experiment and prove to your self. I have several usb power bricks with current and voltage out meters, hardly any more expensive than normal ones. Phones do some wierd things with ramping up current and charge modes. There are also similar devices for the ac plugs called kill a watts they will show you idle current and power of things plugged into them

[GH85Carrera]

I have all my "wall warts" plugged into a power strip. If they are not being used to actually charge something, I turn off the power strip. I likely save a nickle oor two per year.

I have several tool battery charges in the garage for various electric devices. All of them are plugged into power strips and turned off for the 95% of the time I am not needing them.

[MBAtarga]

Glen - I do the same with my chargers. Turn on/off the power strip which they are plugged.