Friday, January 15, 2010

AC Adapter Check

This is just something I thought about doing after observing a voltage greater than 6v with the AC Adapter back in an earlier exercise. It also showed me something that I didn't know about my breadboard (but mentioned in the book, thankfully, or I might have returned my breadboard thinking it faulty).

First, notice in the picture that I'm getting a voltage of zero with the probes even though I'm applying 12 volts. This is because the voltage columns on the top half and bottom half of my breadboard are not one piece from top to bottom. I'll have to cut four small jumper wires to connect them (two for positive voltage side columns and two for negative voltage side columns).

Once I figured that out, it was time to test my AC Adapter. My adapter has a small switch allowing me to select voltages of 3, 4.5, 6, 7.5, 9, and 12. I took voltage readings using two small wires stuck into the breadboard... as you can see from the photo, the actual voltages displayed by the multimeter do not match exactly. For the 6v, 9v, and 12v, the actual readings are almost all .25 higher... one-quarter volt higher than expected. I don't know yet if this will be an issue with later circuits but I imagine it'll involve using higher resistance values if a specific voltage is required... we'll see.

I suggest you do this same thing with your variable AC Adapter... make sure that the voltages you are reading are not too out of range or you may need to return it for a different one. If anyone has any knowledge on the risks of using an AC Adapter that has too much variance in its actual voltage supplied, please let us know.


  1. I assume that you did the Voltage test open circuit. If you put a resistor across the outputs the Voltage will go down. This will also happen if you do it with a battery. The adapter will probably put out the rated Voltage when the current maximum in the manual is reached.

    You have discovered that real DC supplies are more complicated than the simple theory in beginning texts. Usually a better DC supply model is introduced at the end of the DC theory semester, at least were I went to school.

    You are on track. It would not do any harm to verify the Voltage source with a meter when you take reading for your calculations.

  2. My adapter (stolen from a dead cordless phone) is labeled 9V, 300mA, but the meter reads around 16 volts!

    I put at 2.2K resistor across it, and that dropped it down, but only by about half a volt.

  3. Brian: That is because you need to have a load on the power supply in order to see the real voltage. A load meaning something actually using power from the supply such as an array of LEDs or even the relay switch pulling the contacts to a closed state.

    Also a good reason why to have a power regulator between electronics and the supply to prevent such a high voltage in the beginning before the loud pulls it down.

  4. "Wall wart" power adapters are cheap and nasty things. As noted, their output voltage will decrease if you load them. I used them in my book because for my experiments, they are good enough. When you get to chips, though, you will find that I quickly introduce a voltage regulator, since there is no way a Shack adapter will give you 5.0 volts precisely. And I use the obligatory capacitors to suppress the ripple that you get from AC adapters.

  5. One of the most frustrating puzzles I had early on was the same you mention here with the rails of your board not going the full length of the board. For the longest time I couldn't figure it out. It still gets me occasionally.

    I've found a lot of the components that I use have tolerances in their voltage requirements. For instance, an LED strip I'm working with allows for up to 12.5v, even though it states that it runs on 12v.

  6. BTW, the part number for the AC adapter is incorrect. I found it at as Model: 273-316

    Keep up the good work!

  7. Does anyone know where I can get a similar adapter from another shop? I'm from Canada but a number of the sites that sell components also ship here i.e. mouser, allelectronics, 123active, etc.