Wall Warts

Oh, the ubiquitous wall wart!

Wall warts, AKA wall power adaptors, are everywhere yet there seems to be a lot of misunderstanding about them. People are often surprised when a 12V wallwart measures 16V on the multimeter. Thus, this blog.

The first thing to realized about WWs is that they usually are UNREGULATED power supplies. There are regulated wallwarts but they are definitely less common and we won't look at them today. Some of them have nice big filter capacitors and decent transformers. Others, well, let's just say they don't. The device a wall wart plugs into is responsible for creating clean power. Generally, everything works fine but when it doesn't...

A big confusion point is about the labeled capacity of a WW. I have in my hands one that claims 7V at 700 mA. Why then does it read 11.4V when I measure it without a load and 10.7V with an 18 mA load? It goes back to the fact that it's an unregulated supply. Ohms Law, baby! In theory, if I placed a 10 ohm load on it, It should read 7V with 700 mA current. Yes, we know that label is probably more marketing fantasy than engineering reality but let's roll with it anyway.

So, what does my little WW do under different loads? I pulled out my junkbox and found some power resistors to measure against. First, I measured their actual resistance. Then, I measured the voltage drop across the resistors. It's all pretty predictable based on Ohms Law but here are my results.

In graph form, it becomes a bit clearer

Unfortunately, I didn't have a 10 ohm, half watt resistor to use but it looks like it would have come pretty close to the rating of 7V at 700 mA. Remember, Ohm's Law says that R = V/I. So, my little $0.50 wallwart that I got from a thrift store looks like it actually delivers on the promise of the label.

Another confusing point about wallwarts is that even educated engineers often think "bigger is better". If given a choice between 12V, 300 mA and 12V, 1A, take the 1A one. The extra current means it won't cause problems. Right? The answer is a definite maybe. You really need to look at what your circuit draws. and what kind of power supply you have. Going back to the measurements I did, you can see that my 7V/700mA WW delivers approximately 10V with a 100 mA draw. Feeding that through a 5V linear power regulator, we will see 5V drop. That's 1/2 Watts of power being put out as heat. How will the poor little regulator handle it? Let's look at a common regulator, the LP2985-50. It's in a tiny little SOT-23 package. The datasheet says it has a thermal resistance of 208 C/Watt. This means that the device temperature rises 208 degrees C for each Watt dissipated. In our case a half watt would increase it's temperature by 104C. Adding that to the ambient temperature of, say 25C, we get 129C. Not killing the chip since it can go to 150C but hotter than boiling water. And if the regulator is not exposed to free air, it will get even hotter. Needless to say, not the best plan. So, maybe that little WW isn't such a good idea for powering a fairly loaded Arduino system.

Finally, I pulled out 4 more random wallwarts from my junk box and tested them. Here are the results. I highlighted the places where I exceeded the spec'd current. As you would expect, the voltage drooped below the spec. Nothing exploded but I'd expect the WW to get pretty hot over time. Definitely NOT recommended.

As you can see, three of the four deliver at least as they claim. The 12V/300mA WW appears to not be delivering as promised. It hits 12V at 250 mA. I would guess it delivers about 11.4V at 300mA. Not the end of the world and it's usable for small projects.

Let's go back to our calculation of temperature rise. Looking at the 12V/300 mA WW, it will probably deliver around 13.75V at 100 mA. That's 8.75V of drop through the 5V regulator for 875 mW of heat or a 182C rise. Adding ambient temperature of 25C and we are getting close to the point of melting solder! And touching the voltage regulator would give you a vivid demonstration of power dissipation. Yikes! All that from a puny 100 mA draw. Kids, don't try this at home. And, when you look at the 12V/800mA WW, add another 1.5V for over 10V of drop. That would definitely turn your sweet little arduino into a blue smoke dispenser. Bigger definitely is not better.

The lessons learned from from this exercise:

• The voltage spec on a wall wart is for the specified current.
• The lower the current you pull, the higher the voltage.
• Higher current wall warts will have higher voltage.
• Linear regulators turn the voltage drop into heat.
• Keeping your voltage drop lower is good.

So, I think you can see why I really like my 7/700 $0.50 thrift store wall wart!