Effects of static electricity on 3D printer Hot Ends

[Dirk]

3 hours ago, DanPin said:

temperature control after a long print is finished to see what that tells you.

I have an EB2209-RP2040 on my toolhead. The only problem I am having is, due to the cramped-up design from BTT, where they put the thermistor pin and 120ohm pin under the CW2 edge. So trying to push everything in there, I damaged the pins. I wrote to them and they said: ours fits well. But I use simply a mod that does not bump into the CW2 edge. 

 

Anyway, what I wonder is, what should be watched? I do regularly 'long' prints. I could take a look if it is not too difficult.

 

3 hours ago, DanPin said:

Also, why don't we hear about more thermistor problems in general,

this was also what I questioned. But the arguments from people having a lot of static after a print and experienced people recommending grounding, made me question this aspect a less.

[ChicagoKeri]

7 hours ago, DanPin said:

This is a interesting thread.  What troubles me is that in the orig. post Keri states that the longer prints complete without a problem.

But when she starts the next print, the thermistor is bad.

If it was a static problem from plastic flowing through the extruder, the thermistor should have blown sometime DURING the print.

It seems like the problem happens between the last print and the start of a new print.

If you ever go back to the EBB2209, you should check temperature control after a long print is finished to see what that tells you.

The solution may still be adding a ground path, but the source may well  be elsewhere.

Also, why don't we hear about more thermistor problems in general, maybe something is unique to her printer.

Hi!  Thanks for the observations.

I rather suspect static is the cause and have come up with this theory as to what was happening.  

0- The Thermistor circuit supplies 3.3volts though a 4.7k ohm resistor, through the variable resistance of the Thermistor, then to system Ground. This causes the voltage at the point between the  4.7k resistor and the Thermistor to vary according to Temperature, which is read by the Toolboard ECU.  The Thermistor and its insulation are only designed to withstand this low voltage and current.

1- Static buildup during extended prints flash through the insulation of the Thermistor and the discharge is partially directed through the Thermistor element itself, damaging it.  This causes a brief, large excursion of Temperature reading but quickly stabilizes to something relatively close to correct.  The system compensates by increasing or decreasing power to the Heater until the target temperature is indicated, however the actual temperature is probably now different.  As this cycle continues, the inaccuracy grows but the system is still capable of keeping the indicated temperature without straying into Thermal Runaway type faults.

2- The print ends, the heaters shut off and the system cools down to ambient temperature. The damaged thermistor element shrinks as it cools, damaging it further.

3- Upon the next print, the damaged thermistor again expands as it heats up, possibly damaging it further and further increasing the error between indicated and actual Temperature.  Typically, the system would notice the increased power needed to maintain the target temperature before the system fully stabilized and would then go into Thermal Runaway shutdown early into the next print.  

4- Upon installation of the MellowFly SB2040, this condition ceased and no further Thermistors were destroyed.   An analysis of the Thermistor Input circuitry reveals a couple of differences to a similar circuit.  The difference I think that has made the difference is the MellowFly has a 100nf capacitor directly across the Thermistor while the BTT board does not.  

My suspicion is that the 100nf capacitor across the Thermistor on the MellowFly board shunts the static discharge to the thermistor (-) pin which is connected straight to 24v Negative (system Ground).  This keeps the voltage across the Thermistor element down to a survivable level during the discharge but does not stop the discharge.  The discharge will still have a detrimental effect upon the Thermistor insulation and possibly upon the MellowFly board as well, though no noticeable fault has yet occurred.

As static charges very often achieve many thousands of volts before discharging, I feel it wise to bleed them off as they are being developed at the source. 

Why only my machine?  I very much doubt this has only affected me though I might be the first to make a fuss about it.   If prints are kept shorter in duration than many hours, the change might not build up enough to cause a problem. If the humidity is different or the material of the reverse bowden or filament is different, again the charge might not build up. If one is fortunate enough that any charge flashed though only to the Thermistor Negative wire, that would likely not damage the Thermistor.

 

[ChicagoKeri]

6 hours ago, Dirk said:

I have an EB2209-RP2040 on my toolhead. The only problem I am having is, due to the cramped-up design from BTT, where they put the thermistor pin and 120ohm pin under the CW2 edge. So trying to push everything in there, I damaged the pins. I wrote to them and they said: ours fits well. But I use simply a mod that does not bump into the CW2 edge. 

 

Anyway, what I wonder is, what should be watched? I do regularly 'long' prints. I could take a look if it is not too difficult.

this was also what I questioned. But the arguments from people having a lot of static after a print and experienced people recommending grounding, made me question this aspect a less.

Hi!

The board I was having Thermistor destruction issues with was two separate BigTreeTech SBB2209 CAN Toolhead boards. I had bought the second one thinking that there was possibly a fault with the first one but changing the board made no difference.  Installing a MellowFly SB2040 CAN Toolhead board caused the Thermistors to stop failing.

 

What I would keep an eye out for is rapid temperature fluctuations while printing.  I thought I had noticed this but failed to get  screenshot of it back when thermistors were failing.   Nowadays, with the Hotend Earthed, the temperature reading is quite stable with no significant fluctuation in the graph.

In fact, here is my current print at 3 hours, 40 minutes.... with a Target temperature of 260C, Extruder temperature varies only between 259.1 - 260.3. Not bad considering there are extrusion stops, starts and retractions which necessitate a continual adjustment of Heater power.

Before, it would leave noticeable squiggles in the graph. (Extruder is Red line)

[Dirk]

Big Tree Tech calls them the EBB SB2240 / 2209 The first versions which had 1.5mm jst's, have been made bigger in the EBB SB2209 (RP2040) version.

And the Mellow calls its version the Mellow Fly SB2040. v1, v2, max, supermax, plus and pro versions. 

All we are doing is memorizing these chips that are named by numbers

But anyway, I got your message. But I was wondering if I could do some measurements @DanPin suggested after the print... 

I will be honest, my current curve looks the same, also after prints of several hours. just a straight line. But the squiggly temperature curve I have also had when I had problems with the pins that broke off during installation as I showed in the previous post.

 

[DanPin]

 

17 hours ago, ChicagoKeri said:

My suspicion is that the 100nf capacitor across the Thermistor on the MellowFly board shunts the static discharge to the thermistor (-) pin which is connected straight to 24v Negative (system Ground).  This keeps the voltage across the Thermistor element down to a survivable level during the discharge but does not stop the discharge.  The discharge will still have a detrimental effect upon the Thermistor insulation and possibly upon the MellowFly board as well, though no noticeable fault has yet occurred

This is a screenshot during a 38hr print (currently at 26hr).The temp is very solid. This is PLA + filament, EBB2240 CAN board.

For sure swapping toolhead boards does seem to correct the problem. But it still bothers me that:

1. The thermistor is made from a semi-conductor material - not an ideal conductor
2. One lead is at ground potential (by a direct connection)
3. Both the heater and Thermistor have a direct connection to ground. (discharge should follow one of these paths)
4. How come the MCU input pin is never damaged (it should be more sensitive to a voltage spike than the thermistor
5. Any static charge should find the lowest resistance path to ground.
6. Can the path through the thermistor itself be a lower resistance path than through a ground wire (maybe wire insulation is a better insulator than the potting compound)

But hey....if I was Keri, and I did not lose any more thermistors, I would probably quit  looking for any more answers.... and just move on to see if it happens down the road.

[sntlewis]

Jumping in as I'm currently running an EBB36 (on a heavily modded Hypercube) and will be going canbus on my Voron (soon to start the build).  There is a thread on the BTT github that has some simularities...  TMC2240 undervoltage error · Issue #28 · bigtreetech/EBB · GitHub

I'm more on the software end of things than the hardware, but on my 5X10' router there can be nasty bites due to ESD on long runs with high static materials.

[sleepster217]

On 2/14/2024 at 12:16 PM, ChicagoKeri said:

In my various researching of CANbus, I might have found a solution to a problem I was having.

It seems that when I built my Voron 2.4 with a CANbus toolhead, a BTT SB2209, I kept destroying Hotend Thermistors.  Typically, I would complete a 4 or 5 hour long print, and upon the next print I would get thermal runaway shutdown and an erratic Thermistor reading.  Replacing the Thermistor would return the printer to service but again, after a long print, the new one would fail.  My collection of failed thermistors included a couple of E3D 3mm, generic 3mm, an E3D Revo (white wires), a Revo Knockoff, (Red Lizard?) and even a Creality 3mm (worth a shot). Halfway through this pile of busted Thermistors, I swapped out the SB2209 to no effect.   Finally, I installed a MellowFly SB2040 which has not yet eaten a Thermistor.

But why?  BTT might not be the best ever but the boards themselves didn't fail.   The Thermistor connector, P13, has only 2 wires, a well-protected sense wire to the MCU and a ground.  The sense wire has nearly 5k of resistance between it and the MCU with protection in the form of a diode and a capacitor to ground and a diode to +3.3v.  Nothing here should be capable of killing even the most fragile of Thermistors. 

So, today I was looking into CANbus alternatives and on Duet3D's website I found a reference to grounding the metal parts of the Hotend to reduce static buildup!  Ah-Hah!

Supposition: pulling plastic through a reverse bowden and pushing it through an extruder might be building up a static charge on the completely isolated metal parts of the Hotend, which is bolted into a Stealthburner made of plastic.  The only connections to the Hotend are the plastic filament path, the Heater wiring and the Thermistor wiring, which likely has weaker insulation than the heater.  Over time, the charge could build up enough to flash through the insulation of the Thermistor and ground itself to either the Ground at P13-1 or though the protection circuitry of P13-2. If the charge goes through the Thermistor element on its way to ground, Poof!

So, What to do about this? 

Well, the direct answer is to take Duet3D's advice and ground the metal parts of the hotend with a 10k - 10MΩ resistor.  This could be done at a heatsink bolt and a mounting bolt of the SB2209.  This would be a bit messy.

So here's my plan:

Print the Stealthburner parts in conductive ABS that is meant to be static dissipative.  In my case, this would not only provide a path to the CAN Toolboard mounting bolts but also a path to the metal Mellow TAP which in turn is bolted to a metal X Carriage, which is bolted to the Metal Linear Rail Carriage. 

Another option is paint the relevant parts with conductive spray paint, such as graphite containing paint. 

Fortunately, I have access to a High voltage insulation breakdown tester capable of 3,000 volts.  I'm going to zap a thermistor or two and see what they're good for.  Also, see if the conductive ABS I just ordered conducts at elevated voltages.

Any thoughts or ideas?

This sounds interesting I am going to follow this post as said way above my pay grade

[chrisalbertson]

Far simpler is maybe to slip a small bare wire in with the hearer cartridge and bond it to the grounded heater lead.  I measured and found the metal can the heater is in, floats.  If it were boned to the ground, that would fix the problem. 

Also there is ground on the tool head PCB, a 4 inch length of #30 wire would directly connect the hotend mount screw to the PCB

[DanPin]

9 hours ago, chrisalbertson said:

Far simpler is maybe to slip a small bare wire in with the hearer cartridge and bond it to the grounded heater lead.  I measured and found the metal can the heater is in, floats.  If it were boned to the ground, that would fix the problem. 

Also there is ground on the tool head PCB, a 4 inch length of #30 wire would directly connect the hotend mount screw to the PCB

That is exactly what I thought should be tried. Thanks for explaining it. I would wrap the #30 wire around the heater wire and make the connection at the PCB

[chrisalbertson]

16 hours ago, DanPin said:

That is exactly what I thought should be tried. Thanks for explaining it. I would wrap the #30 wire around the heater wire and make the connection at the PCB

Usually, static drain grounds have a resister in them to list the current but I don't see the need in this case.  What you are doing with the wire is simulating an all-metal tool head.

[sleepster217]

21 hours ago, chrisalbertson said:

Usually, static drain grounds have a resister in them to list the current but I don't see the need in this case.  What you are doing with the wire is simulating an all-metal tool head.

Question for u is the greese or oil on the rail going to stop the planed earth from working as intended through the linear rails 

[DanPin]

On 3/10/2024 at 9:58 PM, sleepster217 said:

Question for u is the greese or oil on the rail going to stop the planed earth from working as intended through the linear rails 

The ground path will NOT be through the rails.  The CAN cable contains a negative wire so that can be picked up on the toolhead PCB.

HOWEVER...as I am typing this I realize that Earth ground is not tied to 24v negative (as far as I know).  But I suspect making this connection will still bleed off any static...IMO

[ChicagoKeri]

7 hours ago, DanPin said:

The ground path will NOT be through the rails.  The CAN cable contains a negative wire so that can be picked up on the toolhead PCB.

HOWEVER...as I am typing this I realize that Earth ground is not tied to 24v negative (as far as I know).  But I suspect making this connection will still bleed off any static...IMO

Hi!

There is much debate in other places as to whether one should tie 24v Negative to Earth Ground.   It apparently is done sometimes but the power supplies are not usually built that way.   I suspect that it is not necessary.

One thing of note is that the Power Supply will often have a small interference suppression capacitor, perhaps 20nf inside of it, tying 24v Negative to Earth ground. This will allow alternating current to pass through to ground but will also allow short duration pulses or spikes in voltage to be shunted to Earth ground.

So, in an ideal Power Supply,  if a static charge builds up gradually it might shift both 24v Negative and Positive up or down in relation to ground.

But in the real world, I suspect that the normally occurring ripple in the Power Supply output will pass through the capacitor and keep the 24v Negative very close to Ground potential, passing not only the ripple but any static charge along to Earth Ground.

[DanPin]

Keri,  Where are you at on your static problem?  

Are you staying with the Mellow Fly board, and calling it fixed??

Side question...does the mellow fly give you more room in the toolhead shroud for wires?

[ChicagoKeri]

7 hours ago, DanPin said:

Keri,  Where are you at on your static problem?  

Are you staying with the Mellow Fly board, and calling it fixed??

Side question...does the mellow fly give you more room in the toolhead shroud for wires?

Static is gone, as the Toolhead is now Earthed. Shortly, the entire Gantry will be Earthed also as I'm doing a little tidying up of the printer at the moment.

The Extruder temperature reading seems smoother than previously, but I had failed to get proper "before Earthing the Toolhead" screenshots or even "Before installing the Mellow" screenshots.   Am I likely to re-install the EBB SB2209 and see if the Thermistor Destruction Tendency or the Squiggly Temperature Graph returns?  No, Probably not.  So the confirmed and verified cause will likely remain a mystery.  I still feel that static was the cause, however.

The Mellow Fly SB2040 is a bit of a mixed bag so far as room.  It has fewer connectors and features than the BTT EBB 2209 but these connectors are bigger and the board is smaller. So, bigger connectors are more fiddly in the very constrained space of the Stealthburner electronics bay.  As the Mellow board has the same shape as the Hartk Breakout board, it can be used in Stealthburner variants that the larger EBB SB2209 cannot. 

The bigger connectors are much easier to crimp however! They progressed from  "Nearly impossible to get a decent crimp on any wire thicker than 26ga" all the way up to "Merely difficult to get a decent crimp even with specialized crimpers and good magnification"

[ChicagoKeri]

Some pictures of my Static Abatement Efforts.... so far.

 

Extruder parts sprayed with graphite paint, rendering them conductive so as to provide a circuit path to an Earthing wire:

 

Resistor connecting Earthing wire to Extruder Housing:

 

The other end of the Extruder Earthing Wire, earthed at rear of Electronics Bay:

 

Gantry Earthing by means of shielded CNC Cable for "B" Stepper:  Note conductive path is through metal "B" Stepper Motor Mount.  There will be a separate post on this topic.

 

Earthing of A/B Stepper Shielded CNC Cables at front of Electronics Bay:  Note that the CNC Cable is motion rated 22 gauge wiring that is shielded.  This is one size thicker than the standard 24 gauge. Difficult to see, but both shields are tied together at this end.  Also note the "A" stepper connector has been marked in Red.