UNO R3 USB power and 9V battery work to power UNO but not wall plugin

I have a Uno R3 connected to a 8 relay board. The power to relay coils is not connected so all I am doing in test mode is fire the LED lights connected to the relay coil controls. The sketch works great when connected to the USB/computer as it does when I power the unit with a 9v (no load 7.78 VDC) battery connected to the barrel plug. I have tried 6 different 9 VDC and 12VDC wall plugin on the UNO and when I connect them all the LEDs on the relay board (that are connected to the UNO) go on and say on as well as the "L" yellow UNO LED and UNO green LED "on" LEDs. The power supplies are all between 500 ma and 1.6 A rated. All are Class 2 supplies. The 12VDC supplies measure ~15VDC with no load. When connected to the UNO the voltage across the barrel pins are closer to 14 VDC. The 9V supply measure ~14.7VDC with no load. When connected to the UNO the voltage across the barrel pins is 11.71 VDC. Any ideas what is going on? I have seen many posts about various problems with external supply and none of them seem to address the problem. I have what should be the right supplies with enough current capacity. The fact that 9 VDC battery works makes me thing that there is a noise problem, but will so many different supplies?? In all cases only one power source is connected to the UNO and the UNO is not set up to power the coils on the relay board.

I ran the example sketch Fade and modified it to keep the power the LED plugged into pin 9 constant. The voltage between pin 9 and Vin was 4.4 VDC with the UNO plugged into the USB port. When did the same thing with the UNO connected to the 9 VDC power supply the voltage reading was 11.7 VDC. Not good. When I tried this with the 9 VDC battery (AKA 7.78 VDC) the voltage between pin and Vin was 1.78 VDC. Any idea what is going on? It looks like the Arduino can not control a power supply but it can control a pure DC supply. Suggestion on a power supply. I though about using NiCd 9 VDC battery in parallel with a 9VDC supply to act as a huge filter capacitor. Any one have any suggestion on a power supply or why I am running into a problem.

Hi, check the wall plugs, with your digital multimeter in AC mode measure their output volts with no load, if over .5Vac then the wallplugs are not smooth enough in filtering their rectified AC.
You will need to add more filtering.

Tom..... :slight_smile:

I looked at the output from the wall warts with my DMV set to AC. Positive to positive and negative to negative a 1.5 VDC NiCd battery was reading 2.6 VDC, reversing the VDVM leads gave me ~.1 V, with the DVM set to DC it was reading 1.38 VDC. The power supplies, 12VDC, positive to positive and negative to negative gave me around 47 volts, reversing the DVM leads gave me about .2 V. Same with the 9 VDC supply except I was getting about 31 V positive to positive and negative to negative, , reversing the DVM leads gave me about .2 V. The lowest scale AC on the DVM is 200 VAC so I am not sure how good that .2 V reading is. However, the fact that it is reading something may indicate as you suggested that I need more filtering. I don't understand why the DVM seems to reading 2X the voltage from the DC supply (battery of power supply) when in AC mode. Increasing the filtering does seem like the next move. Any other ideas or should I assume all DC wall supplies made over the last 15 years are junk?

Quick update. I started looking up information on wall warts. Turn out there are unregulated and regulated wall warts. I am sure the ones I have looked at are unregulated. Which for one thing says that the voltage is totally dependent on the load and the input voltage. Basically, for this the Arduino they are junk. I am going to read a little more but it looks like I need a regulated supply.

Hi, I'd be doubting the AC readings you are getting from the wall warts, especially 47V on the 12Vdc unit, sounds like your AC range on the DMM is not good at all.

I cannot see why on AC your reading change when you swap over the leads unless your DMM just rectifies the AC to get a reading so your results unfortunately are not conclusive.

But on previous evidence, I'd be providing more DC filtering.
RE. Investigating Wall Units, good idea..
Tom, thanks for the try mate..... :slight_smile:

I've had no such trouble with wall warts as you seem to be having.

I could understand if one has a problem, but you say you tested several different 9V and 12V wall warts and they all had the problem ? This seems implausible to me.

Since there seems to be some doubt about what you are getting out of these devices, I'd suggest finding someone with an oscilloscope to check what voltage and ripple or noise you are getting when you connect up your system.

I have to agree my DVM on AC measuring DC from a wall Wart has a problem. Time for a new meter. I have an oscilloscope and I was just about to fire it up and check out the supplies when I found a solution. All the supplies I have had a problem with are unregulated linear transformer based) supplies. From what I have read the voltage output from unregulated supplies of this type is all over the place and based on load and input voltage. The fact that I am see very high voltages on my UNO output pins makes me thing the problem is that I am trying to use unregulated supplies.
SOLUTION: I have a USB hub that is powered by a regulated 5 VDC 2A supply (from the size it must be a switching power supply). When I use this supply with the UNO through the UNO USB port the UNO works fine. I have to believe the type and quality of the supplies I have tired are the problem. A 5V .7A USB chargers for cell phones works. Mine is a USB charger for a Samsung phone. You can get 5V 1A USB phone charger on ebay between $1.80 and $4.00 free shipping. I think computer USB are rated at .5A, if that is the case these simple charger should work for 90% of the applications to run the UNO. To all those who may run into power supply problems a regulated supply may be your solution. Thanks for every ones input.