wall wart (not mart) efficiency ???

People talk a lot about swapping linear v.regs out for switchers, because switchers are so efficient. But I'll bet 95% of arduinos are powered by wall-warts. Anyone know what wall-wart efficiency typically is? I'd bet less than 50%.

EDIT: subject line, lol.

They might give you enough information to figure that out. Do they specify voltage and current input and voltage and current output? You can convert both sides to Watts.

Hmmm… I read the title " wall mart efficiency ???" and I thought you were wondering about or commenting on how good or bad Walmart stores are.

I have opened a few “wall warts” and found “just the basics”… Transformer, diode bridge, and a 1000uF capacitor… that was it. While I agree that many current day devices are actually switching power supplies… there are no real standards. Some have horrible or no regulation… while others are quite professionally built.

I have seen some that “use the right parts values” but still get it wrong… IE; a recent dissection showed me a perfectly legit design… but with a badly specified 100 uH coil that could not even slightly handle the printed current rating on the outside.

You pay you money, you take your chances.

You can't tell from the ratings on the power supply, those are maximum ratings.

In the case of SMPS, efficiency isn't constant from 10-90% load. On the low end, given little draw, "switchers" can blow chunks, incl. many of the little 78xx-footprint devices.

Efficiency does not tell the whole story. A low power linear regulator that draws very little power for itself is not much better than a 7805 under high current draw, with both the efficiency is slightly worse than Vout/Vin. So under higher current draw, a SMPS is going to be a lot better. However, an SMPS may draw more current for itself under very light loads.

So the question then is more complex than efficiency. If a given circuit spends more time drawing very little current, a low power linear regulator may be a better choice. If it spends more time drawing a lot of or a medium amount of current, an SMPS may be called for.

Well I've never calculated efficiency but: 1. I don't keep or use anything in it with a transformer.. I only use SMPS devices even to power other SMPS devices... and 2. I've never noticed an SMPS wall mart get hot or even warm yet. This is my only observation about efficiency... "Regular" wall marts with transformers inside do even when unplugged from the device they were intended to power. Some get quite warm under load, power wasted in the transformer that didn't get to the load. I think I'd consider that a sign of failure and get rid of it. The New Breed of smart phones have much larger batteries than their earlier counterparts and this is a rich field for 5V 1A battery chargers to be repurposed as 5V supplies for Arduino Projects. I live in a very large apartment in Southern California and those are frequently contributed to the trash bins mainly with mangled cords and connectors. I've repurposed several because of the relatively stable 1A power out and the price is hard to beat too.

Doc

I agree with Docedison's observations. Years of experience with big, heavy wall warts, they stay warm even when no power is being drawn. I have some 12V SMPS wall warts I've drawn 2A from without them getting noticeably warm.

Given the answers, I guess I'd summarize the question of wall mart wart efficiency as, it's kind of a waste to use a switching regulator over a linear v.reg if you're gonna power it using one of the older style [non-switcher] wall-warts, due to their abysmal efficiency. Under 50% probably is a good guesstimate.

Well that’s why I toss them. Most deliver as much thermal power as they deliver electrical power.

Doc

Thank god you finally fixed the title, it was killing me seeing it on the forum.

Hey, I did that over a full day ago, ;-).

Does this not entirely depend on what you wish to power?

cjdelphi: Does this not entirely depend on what you wish to power?

That really has a lot to do with the original question, but I get the impression no one really knows.

It's easy enough to calculate the power output from a wall-wart, but probably no one has tried to calculate the actual input power. For that, you would have to measure the AC current into the device, do some complex math based upon phase shifts [in inductors], and compute the RMS power using calculus - integrate (V*I)^2 (each being phase-shifted AC waveforms] over a cycle, and do a little more math. Something like that, it's been so long since college, I can barely remember what to do, LOL.

Why would you need any complicated math? It’s simple enough:-

(VA Power Out / VA Power In) * 100 = % Efficiency

To measure these you could simply hack an extension cord so that you can get a multimeter and ammeter into the input. Measure the same on the output and you have the figures you need.

You could then load up the output to anything from 0-100% and see calculate a table and graph the results of efficiency over secondary load.

I wouldn't get worked up about efficiency of a power supply that is supplying milliamps, like ones that supply Arduino do, unless they are VERY INEFFICIENT. You can find somewhere else to save a watt-hour or two way more easily than to try to save it on your Arduino. Maybe turn off some lights that you are not using, for example.

tack: Why would you need any complicated math? It's simple enough:-

(VA Power Out / VA Power In) * 100 = % Efficiency

To measure these you could simply hack an extension cord so that you can get a multimeter and ammeter into the input. Measure the same on the output and you have the figures you need.

You could then load up the output to anything from 0-100% and see calculate a table and graph the results of efficiency over secondary load.

The problem is measuring VA Power In, since it's an AC quantity on the power mains. You have to break the 120/240VAC connection to measure current, unless having some equipment I don't have. Plus my DMM does not measure AC current.

Also, it's been so long since college, I forget. Can you simply multiply Vrms * Irms, ie what the DMM/meter would give you, and get the correct value for AC power, or do you have to take the relative phase-shifts into account? The latter, I think. ???

JoeN: I wouldn't get worked up about efficiency of a power supply that is supplying milliamps, like ones that supply Arduino do, unless they are VERY INEFFICIENT. You can find somewhere else to save a watt-hour or two way more easily than to try to save it on your Arduino. Maybe turn off some lights that you are not using, for example.

The real point of the thread was that many people are touting use of switching supplies over linear v.regs on their arduino boards, but then probably powering from wall-warts with low efficiency.

It isn’t as simple as measuring current and voltage at the input. Most wall wart switching supplies do not draw current in a smooth sine wave. And any phase shift means it isn’t all real power.

But I think we’re seriously overthinking this.

The quick answer... is if it gets even warm there's a problem, find it. I substitute a known good supply and monitor it's output with both my trusty dusty Tek 2213 and the somewhat newer Fluke 179. Both measurements are done on a bare breadboard with a 10 uF. cap and a 10 ohm 5 watt resistor. I look for variance in the output voltage and for any change in the noise as seen by the scope. 10 ohms @ 5V is one half an ampere or the maximum load computer USB port is good for and the supply that I am testing should be as clean and show the same regulation. For low noise applications I use a separate linear regulator for power. So far I've been lucky... I haven't seen any failures after this simple test. I also check the losses by running this combination and noting whether the 'wart' gets warm after 20 or 30 minutes... If it does I toss it. I get about 1 in 5 that pass. I might note that I use discarded cell phone chargers for this purpose. I use 7.5 V SPMS supplies to power any of my Arduino's because the regulator loss is least at that point and my boards although warm don't get unreasonably hot.

Doc