Heater bed can't keep up

I have been having an issue with my heater bed keeping up with a print anytime I have any kind of fan going. I was able to block the air (from the extruder fan) from hitting the bed and that works. When I install a fan for the nozzle though, the air from that cools the bed and it shuts down the print. Does anyone have some tests I can run or have a seperate psu idea for the bed? It's for a folgertech prusa i3.

Check in configuration h file on the heat bed seccion that you got full 255 power. It will also be wise to run an auto tune for the bed so the PID values are for your bed and not a generic one.

Was at 255 and trying new auto tune now. Hopefully that will help.

I cant even get the auto tune to finish. It times out because it takes forever to reach target temp. Its almost like its not getting enough juice.

I got the auto tune to work and input the new values. We will see if the new values work and then I will update.

The new auto tune values didn't work. It gets to target heat just fine, but the second I turn the nozzle fan on, it loses heat and can't get back to target. It remains about 5 degrees below target. Any other ideas?

Does the LED in the heated bed stays on all the time as its fails? Its blinking or off?

Disconnect the bed from the ramps and plug it straight to the power supply and while watching on temperature turn on the fan, if same effect you can discard board issues and reduce it to wiring or under power issues. Next connect a fan and heat bed to power supply and power your board only with USB cable so the power goes only to bed and fan but you can see temperature on your computer, if problem percist check the wiring and solder points if all ok then problem is underpowered supply.

If you have a multimeter check bed ohms against manufacturers data, if your bed is a dual 12-24 volts and is set for 24 and you are supplying12 it might fail in same fashion.

Now, understand that with the 2 tests I mentioned the bed will continue to heat up so once you reach bed temperature (70 degrees) you unplug one of your wires to the bed (black is the safest to use) and once lower temperature is reach you plug it back and see if it can catch back on temperature.

One last thing, a low temperature bed won't stop a print but a low hotend temperature will. If your printing stops it wasn't because of bed but because of hotend, if your fan is cooling the heater block and you are using low extruding temperatures, it could stop the print due to min temp.

What temperature are you trying to maintain? Some heated bed designs are OK for maintaining 55-65C (for printing PLA) but struggle to maintain 100-110C for printing ABS unless you increase the supply voltage.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I had the same issue with my purse i3.
I tried to get the bedtime up to 110, and maintain at 90 during print.
It is something related to the amount of power that the heated bed can/will use.
I have a work around for this: Cover the bottom of the plate with cardboard to prevent air from taking away the heat, and cover the top with a rag or cardboard, when it is heated up, remove top cover and start the print.
The first layer at 110 is relatively okay(it wil drop!!), the rest of the layers at 90 should be okay as well, might drop to 87, but it will maintain the heat better by the covered bottom.

I hope this helps you as good as it helped me, but make sure you check the cardboard from time to time, i do not have any scorch marks, but i cannot guarantee the cardboard will hold in your case as well, just be careful and watch what happens in the beginning.

I am trying to maintain for abs. I can get the bed to heat up to target just fine, but any (and I mean any) kind of airflow will cause the bed to lose temp. I was hearing that some people put their bed on a seperate (xbox) power supply. I just don't get how the wiring goes so that you still have control through the interface.

You need a higher voltage to the heated bed, or a better bed heater. If you are using a LED-style PSU, it probably has a potentiometer at the low voltage end of the terminal block that you can adjust to increase the output voltage. But you need to heed a couple of warnings:

1. Cheap printer electronics such as Arduino Mega/RAMPS may not tolerate much more than 12V input to the board as a whole, especially if you are running additional loads from the 5V output, e.g. LCD display. Without a LCD display or other 5V loads, Arduino Mega is OK up to about 15V.

2. The PSU has a rated power, therefore if you increase the output voltage, the maximum current you can safely draw from it decreases in proportion.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].