Hot end temperature issues

Not sure where to post this, so I went with general..

I have a newly build Prusa i3 with a RAMPS 1.4 and an E3Dv6 hot end that I'm having temperature issues with.
When I set the temperature to a specific value, it always stops at ~10 degrees below what I set. It fluctuates quite a bit (maybe +-5 degrees) but never reaches the desired temperature.
I figured this might be a PID tuning issue, so I tried running a PID autotune with M303 E0 S200 C8. However, the result of this was that the temperature of the hot end slowly fell until the autotune timed out.

What might cause this kind of behavior?

Did you use the PID values come with E3D V6 Hotend?

// T3P3 E3D V6 with Semitec 12V heater cartridge
    #define  DEFAULT_Kp 21.28
    #define  DEFAULT_Ki 2.37
    #define  DEFAULT_Kd 47.76

I made custom PID values once but had exact symptom like you after replacement of the thermistor. After few hours of PID tuning, I got back to the E3D setting and it solved the problem. According to PID tuning (http://rigidtalk.com/wiki/index.php?title=PID_tuning) guide, M303 gives PID values that works for a specific temperature.
To find PID values that covers from 190C to 230C, you need to tweak the PID values manually.

I figured this might be a PID tuning issue, so I tried running a PID autotune with M303 E0 S200 C8. However, the result of this was that the temperature of the hot end slowly fell until the autotune timed out.

Reducing the cycle number might help PID tuning.

Can't say that meets what I get on my printer.
I do the pid tuning at 210° and from 170 to 260 it is spot on with no fluctuations.
However, if the heating is insufficient or the power settings are not at max the autotune will not work.
I had hotends sent back after checking the thermistor and heater.
One was labeled as 12V / 40W but had a resistance of 8Ohm - making it not even a 20W model.
Can't be bothered with those problems anymore and make my own hotends now.

I will test those settings tomorrow.
However, I'm starting to think the heater cartridge is broken, like Downunder35m suggests.
When I have the heater plugged in and set the temperature to something higher than the hot end is right now, the power on the hot end connectors isn't 12v, it fluctuates between 0.5 and 3 or something like that.
Plugging in the heated bed or nothing at all gives a steady 12v, so I assume the heater is broken. I've ordered a new one and will test with that one.
Thanks for the suggestions.

PID changes made no difference, neither did the new heater cartridge.
After messing around a bit, I changed the firmware to 2 hot ends and plugged the hot end into the ports for the second hot end. Works fine, the hot end heats up without problems and keeps the temperature.
I guess the RAMPS is defective.

The bed is likely running bang-bang control - IE on or off, and the extruder pidtemp, which uses pwm to control it's heater, and won't necessarily read 0 or 12, like the bed . . .

- Tim

Quote
tadawson
The bed is likely running bang-bang control - IE on or off, and the extruder pidtemp, which uses pwm to control it's heater, and won't necessarily read 0 or 12, like the bed . . .

Yeah, you're right. That makes sense.
Now I get 0v on it consistently though (even when I've switched to using bang-bang on the hot end), so the board is definitely broken. But thanks for clearing up why I was getting other readings before.

If you can solder, it may be as easy as spending a buck and replacing the MOSFET.

- Tim