Hot end temperature variation when bed is also on

Hey all,

I noticed a few weeks ago that my hot end temperature would sometimes vary wildly during my prints (target ~230, jumping between 210 and 250 sometimes in between a single reading) but it didn't always happen. It even caused some failed prints because it turned itself off at excessive temperatures. Well, it seems that I can't get a steady temperature now.

I figured that it must be my thermistor but I traded it with my bed one (which was having no issues) and that didn't work. I then turned on just the hot end and it heated up great - no variation whatsoever. The second I turn on my bed, though, the temperature varies like crazy. Ah ha! I'm not the best with electronics...but it must have something to do with the fact that they are both on.

It can't be that the temperature is actually varying that much, although when I turn the bed off and it goes back to normal the hot end has normally heated up quite a bit.

I have a prusa i2 with an arduino mega 2560 and ramps 1.4. 650 W power supply. A while ago I had two wires cross which blew one of the orange diodes so I took them out, soldered in some wires, then put fuses on the wires coming in from my power supply. Could something in that configuration be causing this? I'm so confused . Thank you for any input that you might have!

I'm not sure what you mean about the orange diodes. You mean the flat rectangular things near the power connection? Those are resettable fuses, not diodes. If you blew a fuse, there's no telling what else may have been damaged. Was it the larger of the 2? If so, that's the hot bed circuit, and perhaps the damage might be limited to the MOSFET controlling the bed. It's really hard to say without having the card in hand.

Sorry! I meant to say fuse.

Ah I thought maybe that caused more extensive damage. I was hoping not though...Would you recommend buying a new ramps 1.4 then probably?

It's what I would do, but I'm sure there are others who would do a but more diagnosis first.

What type of fuse did you replace the blown PTC fuse with? Another PTC? Slow or fast burning fuse like an automotive or glass-tube? If it was a PTC fuse they may be doing it's job and cutting out power and resuming after it's cooled a bit. Another thing is what sized power supply are you using? If it's an ATX powersupply, what is it's nameplate current rating for the 12v rail?

Hey cdru,

I used automotive fuses. I used a 5A and 15A, because I couldn't find an 11A automotive fuse, per someone's recommendation. Could that be causing trouble?

I have this power supply:
[www.newegg.com]

It's an ATX, and I believe that the current rating for the 12 v rail is 54 amps.

You're power supply should be fine. Even if the manufacturer was a little generous with it's rating there's still plenty margin of safety. A 15 amp automotive fuse should not be causing the issues you're experiencing although you've likely at or exceeding the current ratings of the terminals.

I'd double check ALL your wiring to make sure everything is right where it needs to be and is secure. Also double check solder joints and connectors to make sure they are solid and not showing any signs of overheating. In the event that something was fried when the wires touched, I'm not what would be causing the symptoms you're exhibiting. Just for grins and giggles, what happens when you disconnect the heated bed but still try to use it like it was connected? That would rule out a high-current overload issue and possibly indicate something wrong somewhere on the ramps board.

What wires were shorted?

I've double and triple checked my wiring over the past few days, and disconnected and reconnected everything. Unfortunately to no avail.

Good thinking! I tried exactly what you said and it had no trouble heating up and holding a constant temperature.

So, I'm going to order a new ramps 1.4 board and try it out. I'll let you know if it does the job. Thank you for all of your input and help!!

Quote
austinqu
Good thinking! I tried exactly what you said and it had no trouble heating up and holding a constant temperature.

So, I'm going to order a new ramps 1.4 board and try it out.

What is your resistance of your headed bed measured at the wires that are inserted into the RAMPS board? If it didn't have problems with the heated bed unplugged, I'd look possibly at the bed. I'm wondering if there is a short or almost no resistance on the heated bed causing voltage control issues with the power supply.

I would be looking at the heated bed and it's thermistor.

Specifically, check for any resistance at all between the heated bed power wiring and the heated bed thermistor wiring, even in the high mega-ohm ranges.

If there's some leakage resistance between them, then voltage fed into the heated bed power circuit will leak into the heated bed thermistor circuit, throwing off the readings. If the voltage gets high enough, this can actually fry your Arduino analogue inputs (the ones used for the thermistors).

I would also make sure that the wires to the thermistor are twisted together to reduce voltages being induced from the heated bed wiring. You can also do this with the heated bed power wiring as well.

Note: This can happen if the thermistor is glued to the bed, and somehow the glue is touching a PCB track and one of the pins on the thermistor. Many high temp glues conduct electricity, and it's very easy with many thermistors to get glue on at least one leg. Even a few mega-ohms is enough to cause issues with the thermistor circuit, especially if 12V is involved.

PS: The same issue is true of the hot end, and is why you need to be careful with anything even remotely conductive around the thermistor circuits.

Edited 1 time(s). Last edit at 09/11/2014 07:15PM by Cefiar.

It sounds to me that you haven't separated the analog and digital grounds. The ground side of the thermistors and any associated smoothing caps should be connected to a dedicated ground pin on the Arduino. I don't think you have that, instead I suspect there is a common ground wire connecting both the hot end thermistor and either the source terminal of the bed heater mosfet or the power supply.

---

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].