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hot end pid issues

Posted by scarface_the_fox 
hot end pid issues
April 14, 2015 12:03AM
Hello everyone

i am currently having an issue with my ramps 1.4 board which only just started to happen where it doesnt keep temperature when the hotend is running in PID mode
in PID mode it reaches the proper temperature then it just drops down drastically from ~190ºc to ~150ºc but in bang bang mode it keeps temperature pretty spot on between 188 - 191

i have made sure that the power supply is capable of running everything as it has 3 12v rails all at 16A and a 25V rail at 16A for the heatbed
all the power wiring leading to the ramps board are all rated at 20A

also even in bang bang mode i need to have an external relay otherwise it doesn't keep temperature

i also tried to do PID auto tune but it just fails on the initial heatup with a timeout error

any help would be appreciated as i dont like running in bang bang mode as the relay clicking on and off is annoying

the build is As follows
printer base model : velleman K8200
control electronics : Ramps 1.4
hot end : Original k8200 hotend
power supply : G5 power mac PSU
Re: hot end pid issues
April 14, 2015 11:34AM
What is the duty cycle while the temperature drops down? If it is always fully on or nearly ("128" displayed in Marlin) and if the temperature only starts going down when you are actually printing, then it's probably just a lack of heating power to keep heating the incoming filament fast enough.

Since you say you need a relay for bang-bang mode to keep the heat, it could well be that you are just loosing to much voltage over the mosfet. What is the source-drain voltage over the mosfet (measured between Hotend minus and GND) when its heating at full power? If the mosfet is good, it should only be a few mV.
Re: hot end pid issues
April 14, 2015 07:42PM
Well i tested the voltage drop between hot end minus and ground and it was ~70.1 mv

also the output is constantly at 255 as its not reaching temperature anymore under PID control but still can with relay drive under bang bang mode
Re: hot end pid issues
April 14, 2015 08:59PM
Is your hotend/nozzle insulated?
Re: hot end pid issues
April 14, 2015 09:10PM
well its not insulated but have never had any issues before this printer was working in its current config minus the ramps board for ~ 6 months and with the ramps board about ~2 weeks
Re: hot end pid issues
April 15, 2015 10:17AM
If the output is constantly at 255, then the problem is definitely not the PID. Since your mosfet seems to be okay and and also the hotend resistor/heat cartridge (since it works via a relay), I propose that you have a good look at the cabling and the contacts. If these are good, check out the 5A polyfuse. Does it get warm? What is the voltage drop over it when heating? (Can be measured between Power In+ and Hotend Out+.)
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