Problem with Wall adapters - Page 1
Forum section: electronics / beginners

author: ElectroElvis
author_rank: Regular Contributor
timestamp: November 01, 2012, 10:16:47 pm
content: Hello all,I know I havent been around lately but once again I need little help.Currently I am working on to little project with small portable amplifiers and I have problems connecting evrything to wall adapter via 5V or 12V voltage regulator. I have 3 Wall adaptors:1. Trust 12V 500mA2. Hitron 15V 800mA3. Netgear 12V 1AWhen I test adaptor 1 and 2 I am reading only max 2V without being or with being connected to somthing.Only Netgear's adaptor works but since it is too powerfull it is not any good use for me.Is there something that I am missing with adaptor 1 and 2? I know that both worked normaly before powering their gadgets.Elvis

author: IanB
author_rank: Super Contributor
timestamp: November 01, 2012, 10:33:09 pm
content: Quote from: ElectroElvis on November 01, 2012, 10:16:47 pmWhat is your concern here? Your household mains outlets can supply something like 15 A. Does this mean they are too powerful to run a cellphone charger that only needs 0.1 A?

author: ElectroElvis
author_rank: Regular Contributor
timestamp: November 01, 2012, 10:52:59 pm
content: I want to know why other two doesn't work, since I cannot open them due to safety build. And to answer question: To step down input I would need to transform it to heat which I dont need and specially no aditional parts:)Thank you for your reply,Elvis

author: Monkeh
author_rank: Super Contributor
timestamp: November 01, 2012, 11:00:16 pm
content: Quote from: ElectroElvis on November 01, 2012, 10:52:59 pmIt's the same voltage as #1 and lower than #2, what's the problem?

author: Psi
author_rank: Super Contributor
timestamp: November 02, 2012, 01:31:13 am
content: Are you sure all the adapters are DCIf the first two are AC you might see ~2V on a DC meter

author: ptricks
author_rank: Frequent Contributor
timestamp: November 02, 2012, 11:38:31 am
content: Too much current isn't an issue as long as the voltage is the same.Some devices expect a load before they start delivering power, usually these are switch mode supplies. Also true for power supplies used on things like laptops, cell phones where the adapters are 'smart' and will cut the power if they detect something not right.

author: Chryseus
author_rank: Regular Contributor
timestamp: November 02, 2012, 02:15:24 pm
content: 2 and 3 are exactly the same power (12VA) so I don't know what you mean by too powerful.If 1 and 2 are switchmode (you should be able to tell by weight / size and input rating) what ptricks is saying is correct and you need a minimum load.You might also want to read the label to make sure they output DC not AC.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: November 02, 2012, 04:26:33 pm
content: Here's a pretty good reference if I do say so myself:AC-DC Power Supplies - Using Wall WartsThe first consideration is what type of wall wart it is. There are three general possibilities:1. It's a step-down transformer with an AC output.2. It's a transformer-based linear DC supply. The important thing to know about most of these is that there is no voltage regulator in the device. At the rated load, the voltage will be close to the nameplate, but at a light load, the voltage can be significantly higher.3. It's a regulated switching power supply. This is the usual case for cell phone chargers and many current-generation supplies for electronics devices. These will supply a constant voltage over the supply range; sometimes a minimum load is required for operation.A fourth possibility is beganing to appear, the constant-current switching supply for driving LEDs. If the rating is something like this, it's a constant-current supply:OUTPUT300 mA1.2 - 7.4 VThe third type is the most useful for powering projects.Once the type of supply is known, the second issue is the supply rating. Suppose we need a supply for a microcontroller board that doesn't have its own regulator. It's a 5 volt system, so we'll need a 5 volt supply. That part is easy.But what current rating do we need? Electronics circuits draw only the current that they NEED. A supply must be rated to provide AT LEAST this much current, but a larger rating won't cause the board to draw more current.Back to the microcontroller board. Lets say it has 8 LEDs that could draw 10 mA each, a PIC18F-series micro, and not much else on the board. The LEDs are the biggest current draw, 80 mA if they're all on. A power budget of 100 mA is about right. So a supply rated at 5v at 100 mA or greater will do the job. Operation and current draw will be the same whether the supply is rated for 100 mA, 500 mA or even 2000 mA.Do verify polarity of wall warts before using! Red is not always positive and black is not always negative.

author: ElectroElvis
author_rank: Regular Contributor
timestamp: November 02, 2012, 07:25:10 pm
content: Quote from: Jon Chandler on November 02, 2012, 04:26:33 pmThank you very much my friend for lovely explanation of what I have asked.Elvis

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: November 03, 2012, 05:45:05 am
content: Quote from: ElectroElvis on November 02, 2012, 07:25:10 pmFrom this response, I'm not sure if my post was of any help to the original poster...One detail I did not mention but which is in the linked article is how to tell the first two types of supplies from a regulated switcher. For either of the first two, remember that the largest component will be a heavy transformer, and the transformer shape is approximately a cube with equal-length sides. The enclosure must include some room for the rectifiers and filter caps (if it's a DC supply) but the package is approximately an even-sided cube.On the other hand, switching supplies aren't limited by the need for a large, heavy transformer. These supplies tend to be more compact, not a cube shape and put out much more current for the size.So, if a supply is roughly cube-shaped and heavy considering the output current, it's probably got a transformer. So how can you tell if it's an AC(just transformer) or linear DC supply? Perhaps measuring AC and DC output voltages will provide a clue!