I plug in a 12v 800ma dc into the arduino barrel Jack but there seems to be a lot of noise. I am running servos of a breadboard Jack with pots. When I plug the USB into the uno, the servos work fine. When I unplug USB, and plug in the dc into the barrel Jack, the servos jitter a little bit. Ideas?
Bstanko6:
I plug in a 12v 800ma dc into the arduino barrel Jack but there seems to be a lot of noise. I am running servos of a breadboard Jack with pots. When I plug the USB into the uno, the servos work fine. When I unplug USB, and plug in the dc into the barrel Jack, the servos jitter a little bit. Ideas?
Yes, servos should utilize there own dedicated +5vdc supply rated at one amp per servo being used (two servos, two amp supply) for reliable operation. You need to add a common ground between this independent supply and a arduino ground pin. Servos draw too much current to be powered by the arduino 5V pin regardless of using USB or barrel jack.
Lefty
Hi, do you know if the 12V is regulated or not, also is the DC filtered.
If you have a meter, measure the 12V output without a load and then with the project connencted.
Do this twice, the first with the meter measuring DC Volts, the second time measuring AC Volts.
This will give us an idea if your supply has enough filtering and is or is not regulated.
Hope to help, Tom.... 
Tom, how would I test if the 12v Jack is used? In other word, if I plug it into something, how to measure?
Bstanko6:
Tom, how would I test if the 12v Jack is used? In other word, if I plug it into something, how to measure?
Typically this is done by wiring a voltage divider network and then measure the voltage from the Vin pin with a analog input pin. If you wished to add a short wire jumper on your board (assuming Uno or other board that uses auto-voltage selection circuit) one could wire the output signal of the voltage switch op amp to a digital input pin and that would tell one if they were running on USB or external DC jack.
Thank you
Hi, first do you have a meter to measure dc and ac voltage?
If you do, then try this.
Without the plug in the arduino measure the voltage at the 12V plug, you should be able to put the probes down the end of the 12V dc output plug.
Do this measurement twice, first with the meter set to Volts DC. secondly with the meter set to Volts AC.
Note down the readings and post them please.
Tom....
Tom, sorry for not getting back sooner.
I tested my 6v dc cord for my servos.
I got 6.3v on the dc side, and 13v on the ac side. Does that mean unregulated?
HI, you get 6Vdc from the end of the plug when the meter is set for DC volts, and get 13Vac when the meter is set for AC Volts, your DC plug pack is only supplying rectified AC.
It is DC but is pulsed, it is not filtered, you need to place a say 2200uF 25V electrolytic capacitor across the DC output of the plug pack.
This will filter the DC, but also raise the DC measured voltage, so after fitting the capacitor, measure the DC Volts, if its over 12Vdc then don't use it in this application until some calculations are done to see what will happen when you try the regulator.
Tom....
Tom, I placed the capacitor as you said. I tested as well. No change in either voltage, AC or DC. The cap is working, i turned on the servo and it seems to work. Still not sure if I solved my problem.