5vSB Thermistors Problem

Hi everyone..

I have a problem that is bothering me for quite some time now and i wasn't been able to find any solution to this day. I've made my P3STEEL few weeks ago and modify my old PSU Tagan 380W which is considered to be a very stable power source. Everything went smoothly up to the point I've connected bed an hotend thermistors to my RAMPS 1.4. It came out that thermistor readings are very unstable oscillating from 10 to 30C in room temperature. This fluctuations made my printer almost unusable. I've checked the 5vSB line and its supplying rock solid 5v. After brief investigation I've come to conclusion that it seams to be something wrong with my RAMPS board but this is the moment when its stars to get weird. I've noticed that when 5vSB is disconnected and MEGA is powered from USB readings are stable and everything is working fine and I've manage to make few quite successful prints. I thought that maybe 5v from USB is regulated so ive connected 5vSB and neutral through USB cable but thermistor readings was again very unstable. I have no clue what can be wrong with 5v supplied from 5vSB. Only difference ive noticed is that 5v from my laptop USB port is supplying 4,6v and 5vSB from PSU solid 5v. I would be very grateful for some kind of advice to deal with that problem. Also English is not my native language so sorry for any mistakes.

I had the same thing trying to use the 5V from my ATX PSU. Others have had better luck. I think it probably depends on the particular model of power supply. On mine, the 5V line is very badly regulated and when the heat bed turns on and off the 5V will pick up a lot of noise. Did you take the D1 diode out of your ramps board? If not, the 12V supply to the ramps should feed the mega2560 in the same way as if you connected it to the power jack.

If German is your native tongue feel free to use in a PM.
I noticed in recent weeks that the postings on the web in regards to 5V problems with Ramps are growing, especially for those with added displays.
So I will make a new thread and explain the problem there, please check in a bit

try adding a 10Ohm 10W resistor to the 5V line. some PSU's need a load to regulate properly.

That is actually a good one and I second that!
I use two old break lights from my car gives a nice optical indicator that all is working.

Thanks for reply..

5v line is already balanced (first thing Ive done trying to deal with that problem) using one 50w halogen light and during print (full load on hotend and bad) when electronics is powered through usb cable 12v and 5v line is stable. I know that there are ways to power ramps/arduino in a different way but i would like to know what may be the reason of that 5vSB problem. What is the magic behind stable 4.6v from laptop USB that cousing no problem and stable 5v from 5vSB that is ruining thermistor reading? I have to add that the problem also occurs when PSU is turned off through marlin software and only 5vSB is active.

Hmm, are you able to look at the 5Vsb line on an oscilloscope? A multi-meter doesn't update fast enough to catch small transients and noise. I am betting it is noisy, maybe poor quality PSU or failing 5Vsb.