Hi 
I am using an Arduino Pro Mini 5v. I would like to power it with a 12v 1a adapter because I need 12v for a water pump (it's not connected yet). When I connect the power to raw and gnd the arduino led blinks once. I can measure 12v (11.98v) coming in, but when i measure on the vcc pins on the board only 0.3v is measured. I expected 5v. I need 5v to control the relay for the water pump.
Have I burned off the regulator already or what is wrong with the brand new boards?
I tried on two boards with same results.
Thanks in advance
Hi, @Farbror
Welcome to the forum.
Have you got the 12V connected the correct way around?
Is the relay connected?
Tom.. 
Is anything at all connected to the Arduino while this is happening?
It sounds like you're powering your relay using the micro tiny 5V regulator on the pro mini. That's not going to work. You need a separate regulator for the relay. That regulator is pretty much only for powering the uP. With 12V coming it's already pushed just to power the uP, and the tiny (looks like SOT23-5) regulator can't dissipate enough heat.
If it is a SOT23-5 package, it can dissipate about 700 mW, if you're drawing 100mA, with the 12V input, you're pretty much at your limit already. You possibly have some additional power consumption from LED's and whatever else you have powered by the 5V source. This is only a guess, but two regulators don't burn out without reason. Apologies if I guessed wrong, it would be clearer with a schematic or pictures...
What program is in your Pro Mini?
I always first try the Arduino Example "blink without delay" just to see if the processor is working.
I'm thinking perhaps your 5V measurement isn't making good contact because the LED does blink.
BTW do not attempt to power any external devices from the 5V pin on your Pro Mini.
Thank you all for your messages!
I got no code on board number two and the relay is not connected yet. Nothing is connected. I only added power on raw and gnd and measuring the output power on vcc and gnd. I got 12v+ to raw and - to gnd, it should be correct
As @wolframore is saying, I shouldn't power the relay with vcc from the board so I might need a 12v to 5v regulation and use that for powering the arduino and maybe that will solve it. I just don't understand why I can't get 5v from vcc, I guess the regulator can't handle it?
Is the red power LED lit?
If you have NOTHING connected to the Nano, it should be fine to give you 5V.
Can you please post some images of your power supply, Nano and how you connected them up.
Thanks.. Tom...
@TomGeorge
I have attached an image of the power supply and the board where to two caples from the power supply is solded on. The two arrows is where I expected to measure 5v but only 0.3v is measured.
Are your wires good? Measure the voltage on GND and RAW on the bottom of the board, or at the inputs to the regulator.
If it's connected, you should also be able to measure the RAW voltage across that input capacitor. This might help:
???
Hi,
Thanks for the pictures.
Have you soldered the two wires to the Arduino?
Tom...
It blinks one time when I turn on the power.
Yes they are.
When I turn on the power the led blinks one time. I can measure 11.98v on the cables.
on the solder pads on the bottom of the board? I'm trying to assure myself that you don't have a failed solder joint.
The LED should stay lit.
Either the regulator has gone bad or there is a short circuit on the board somewhere between 5V and ground.
Or intermittent solder joint or wire. We're getting nowhere.
When you are facing a brick wall, walk around it. Does the board power up correctly when you apply 5V to the VCC pin?
I don't believe I have, then I should have made the same mistake on two different boards and two times on the last board.