Why do some 12V adapters work and others not?

Hi all,

I understand from a number of messages and sites that 12V DC adapters with positive center will work for Arduinos. So far, so good. What puzzles me is that at least half of the 12 V wall warts that I plug into an Arduino do nothing. Nothing useful, that is. The LCD attached to my Arduino may light up, and flicker, but no text appears. Or nothing happens at all. Or on the other hand they just work beautifully. When I use a multimeter over such wall warts, they typically report values between 12 and 16 Volts; their rating is between 250mA and 1.5 A and so on. Apart from their aversion to Arduinos, they seem to be in sound working order.

So just using any 12 VDC adapter is not enough. What more should I look for to select sound, working adaptors for my projects? No, I don''t want advice to buy this or that approved wall wart: part of the fun is to use salvaged stuff...

Paai

Are the connectors making good contact? Is it the right connector? There are a huge number of different barrel jacks used on power adapters, and in some cases, you can physically fit the wrong kind of barrel connector in, but it won't make reliable contact.

The problem with wallwarts is that the ratings printed on them are often optimistic or even simply wrong.
One that says 12VDC @ 250mA might produce exactly that. Another with the same rating might drop to well below 12VDC if you try to draw more than about 150mA.
Only way to know for sure is to measure them under a known load that they should be able to handle.

Pete

I usually go for at least 500 Ma or above depending on the project but always stay below 12 volts as there is enough evidence around to suggest that it can overheat some regulators on some boards.

7.5 to 9 V seems to be a good range.

Also what type of supply's as some dont work well under load.

I am now regularly planning an MP1584 into my projects that brings down the voltage to VCC + ~2V. Actually I had PCBs made where that and an SMD-LDO +caps fits under the Wemos D1 Mini. The rest of the 10*10cm board is a perfboard. And it is not even more expensive. 1,25€ for such a noard in a pack of 20.

Mmm... for some wall warts I think that the opening of the central pin may be too large. See: http://electronics.stackexchange.com/questions/59595/how-can-i-tell-the-size-of-a-barrel-power-connector Anybody knows the size of the central pin of an Arduino? My calipers cannot get into the hole.

In any case, my barrel connectors have 1.6 or 2.0 holes in them.

However, in some adapters with 1.6 holes the arduino works for the first twenty, thirty seconds or so, then the backlighted LCD screen flickers and dies. This typically happens when the wall wart is rated for 1 A or more. My projects typically involve an LCD screen, a four-digid LED display, an RTClock and a relay, all powered easily by an USB outlet.

So all in all, most of the 12V adapters that I collected over the last few years do not work. If they work, they tend to have lower ratings, like 250-500 mA.

I must be missing something...

Paai

paai:
So just using any 12 VDC adapter is not enough.

It certainly looks that way.

Assuming that the arduino starts up when the power adapter is used, then the main thing to consider is maximum power output specification..... or maximum output current specification.

paai:
Mmm... for some wall warts I think that the opening of the central pin may be too large. See: http://electronics.stackexchange.com/questions/59595/how-can-i-tell-the-size-of-a-barrel-power-connector Anybody knows the size of the central pin of an Arduino? My calipers cannot get into the hole.

Pretty sure it's a 5.5mm x 2.1 mm barrel socket. That's an extremely common one in consumer electronics. The high-current 12V supplies I've seen (like for an external hard drive) use a different size though.

Do you have any 9V wall warts to try? Those might be more likely to fit.

Your driving an LCD and maybe some other stuff. Don't forget that as Vin voltage increases, there is less and less current available from Arduino's regulator.

Using an unregulated 12V wall wart that outputs 16V when lightly loaded will be on the verge of overheating Arduino's regulator.

Using a 7.5V (or 9V) regulated adapter would ensure 100's of mA is available.

Using USB 5V power (bypassing the regulator) ensures up to 500mA is available.

I needed a 12 volt power supply recently. It said in the kit I'd made that it wouldn't work with a power-supply above 12 volts.... 3 of mine didn't work. So I pulled out my multimeter (that's what they're called in Denmark) ... 2 of them measured 16 volts, and the last one 17 volts....
I haven't played with this stuff very long, but that blew my mind... It wasn't just a bit off... that's not just "a bit"; that's crazy...

Always use a REGULATED supply for sensitive electronics.
They output the same voltage (+/- 0.25volt) unloaded and loaded.

Old/heavy/transformer based supplies can be regulated or unregulated.
Output voltage of an unregulated supply can be 40-50% more unloaded.
A 12volt supply could be over the voltage limit of an Arduino board.
Always measure, or just don't use them.

Modern switching supplies are all regulated.
You can recognise them by weight (light), or by looking at the power label.
If it's a universal voltage (~85volt to ~250volt) supply, then they are always regulated.

Avoid 12volt supplies if you can. 7.5volt or 9volt is much better.
It doesn't stress (heat up) the 5volt regulator as much if you draw current from the 5volt or 3.3volt pins.
Leo..

Thank you all. I found this explanation of regulated vs unregulated also very helpful: http://www.apogeekits.com/power_adapter_selection.htm

Perhaps the general remarks on http://playground.arduino.cc/Learning/WhatAdapter should be reconsidered.

Paai

paai:
However, in some adapters with 1.6 holes the arduino works for the first twenty, thirty seconds or so, then the backlighted LCD screen flickers and dies.
...
I must be missing something...

Yes, you are missing something. The clue is the 20-30 seconds. That's about the right time for an overloaded component to heat up and die. In this case it's not actually dying but it's shutting off because it detected the overheat condition.

The particular component that is overheating is the voltage regulator on the UNO. The one which is converting 12V (or more) down to 5V. This is a "linear regulator" which means the excess power is dissipated as heat. Remember power is voltage times current. So it works just fine when running the UNO alone, for example 0.1A times the 7V drop means it's dissipating 0.7W (watts) of heat. Add a LCD backlight which might be drawing 0.5A of current and your power dissipation is now 4.2W.

So how much power can the regulator dissipate? You might think that 0.6A is well within the quoted maximum of 1A for this device but it's the power that's going to heat it up and (eventually) kill it. To work that out, you need to find the exact part number on the Arduino schematic and then look up a datasheet for it. The regulator part number is: ncp1117st50t3g and a quick google search will get the datasheet for you.

The useful item in the datasheet is the thermal resistance from the junction (the active semiconductor) to the ambient air. For the "ST" option case, this is 160°C/W. You can simply multiply by watts to get the temperature rise above ambient temperature and then add the ambient temperature to get the final temperature of the device. Multiply by 4.2W and add 20 degrees room temperature and you get 692°C. Obviously the regulator is not going to survive this temperature - it would be glowing red-hot.

The reason why it might work with some 12V supplies is their output voltage sags when asked to deliver 0.6A of current. A "good" one won't, which means it will cook the UNO regulator very quickly.

MorganS:
The reason why it might work with some 12V supplies is their output voltage sags when asked to deliver 0.6A of current. A "good" one won't, which means it will cook the UNO regulator very quickly.

I'm thinking that the arduino info should get updated every once in a while, such as.... instead of saying 7 to 12V recommended, they should change it to something like 8 to 10 Volt (with a power rating of such and such Watt etc).

7-12V works perfectly well for the Arduino alone. The problem is trying to run an LCD backlight. You expect that the current draw will be minimal and it isn't - LCDs need a lot of power for the light.

The combination of current and voltage and ambient temperature isn't something that can be written as a simple specification. You have to know how to calculate it. I bet I can still cook the regulator with a 10V input. There's no simple way to say what's safe and what isn't.

Take a power supply that does not work and slowly withdraw the pin. Does it spring back into life? If so then it is the center pin that is the wrong size.

Some larger holes ( 2.6 ) make a good contact on a 2.1 pin others do not. Get the right size plug, cut off the existing one and solder on a 2.1 mm pin socket on the flying lead.

@Morgan S: thank you for your lucid explanation of the behaviour of my Arduino with various adaptors.

Let us not forget that there are many, many people like me who are good programmers and who are smart enough to connect the various wires and components, but who have absolutely no training in electronics. (Ohms law? Yes, I remember learning about that guy fifty years ago, when you needed a slide rule to compute anything complicated. I never understood slide rules).

Also, the internet, marvellous as it is, contains a lot of information, some of which is useful, but other pages are misleading or just wrong. For instance, it is easy to find where you can by a 4x20 LCD screen, and whether it is an Euro cheap0er than the next one, but far more difficult to find the amount of power draw.

I hope that my question may help others. It certainly helped me.

Paai

paai:
Let us not forget that there are many, many people like me who are good programmers and who are smart enough to connect the various wires and components, but who have absolutely no training in electronics. (Ohms law? Yes, I remember learning about that guy fifty years ago, when you needed a slide rule to compute anything complicated. I never understood slide rules).

Trust me, we know. That’s why this forum exists for people like you to get help.

Also, the internet, marvellous as it is, contains a lot of information, some of which is useful, but other pages are misleading or just wrong. For instance, it is easy to find where you can by a 4x20 LCD screen, and whether it is an Euro cheap0er than the next one, but far more difficult to find the amount of power draw.

I hope that my question may help others. It certainly helped me.

Paai

Buy from a retailer or distributor that gives you an actual datasheet to download, and not just the cheapest of chinese ebay factory reject crap. I got one with my 20 x 4 serial LCD (think I got it from Mouser), and the supply current is right on page 5 of the datasheet: 229 mA typical.

NHD-0420D3Z-FL-GBW-V3-29857.pdf (576 KB)

paai:
Let us not forget that there are many, many people like me who are good programmers and who are smart enough to connect the various wires and components, but who have absolutely no training in electronics.

That's ok. There are (often) many ways to solve a problem. If the problem is no electronics training, then.... one way (these days) to solve this is to read electronics text books, and watch videos of online courses, and learn from people that can help you. Many members on this forum can help you, as you've just seen. Getting some basic foundations is good for getting into electronics. Everybody has to start somewhere.