Hot end won't turn off

[VAXHeadroom]

I guess I've fried something, but my hot end control won't every turn off. Even after cycling the main power, I measure 12.3VDC at the Rambo board.

How did I manage that?
Well...
I was changing out the heating resistors for heat cans, and ended up with 2 heat cans that measure 4.2ohms. So instead of wiring them in parallel (the older resistors measured 8.4ohms each) I wired them in series resulting in 7.1ohms. OK - bad idea on my end, but now have I toasted drive circuit on the Rambo? And since I now had a HIGHER resistance than before why would that have occurred?
Any help appreciated.

[geneb]

I've heard that the MOSFETs on the RAMBo tend to fail "on", so it looks like you may need to send the board back to Ultimachine. (or take this as a sign the universe is telling you to upgrade to a DuetWifi and FSRs. )

g.

[VAXHeadroom]

I think what has happened is that the shell of the cylindrical heaters is connected to one of the leads of the resistor. This means that I've created a short-circuit between the two heaters. Additionally the new thermistor may have a lead touching the hot block and created a 'sneak path' for the control circuit. I haven't confirmed this yet - too many other project in front of the debugging task
I'll post what I find...

[VAXHeadroom]

Sneak path confirmed. Once I electrically isolated the thermistor wires from the hot-end, the system works correctly.
Except that now I've damaged the thermistor (handling - my fault).
They apparently are no longer available?!?
Is there an upgrade path for a different thermistor?

[Xenocrates]

Easiest upgrade is to an M3 threaded thermistor, such as this one. It's not exactly one I've personally used, but it is from the successor company to the folks who made the ones I've used. Alternatively, get an E3D hotend (or an SE300 or similar from Seeme), so that you can keep nozzles available and make use of heater cartridges instead of power resistors. I really, really like the E3D cartridge thermistors and PT100 sensors, as well as appreciating the ability to use silicone socks.

So, if you want a cheap fix, the M3 stud is easy, cheap, and robust. If you want to have long term support, pick a more modern hotend design. Or you can buy loose thermistors from folks like Filastruder still, if you don't want to upgrade.

[VAXHeadroom]

Thanks for the link, that one looks great. The B value is a bit different, so I think I'll have to update the firmware right?
Didn't really want to fully change out the hot end...

Easiest upgrade is to an M3 threaded thermistor, such as this one. It's not exactly one I've personally used, but it is from the successor company to the folks who made the ones I've used. Alternatively, get an E3D hotend (or an SE300 or similar from Seeme), so that you can keep nozzles available and make use of heater cartridges instead of power resistors. I really, really like the E3D cartridge thermistors and PT100 sensors, as well as appreciating the ability to use silicone socks.

So, if you want a cheap fix, the M3 stud is easy, cheap, and robust. If you want to have long term support, pick a more modern hotend design. Or you can buy loose thermistors from folks like Filastruder still, if you don't want to upgrade.

[geneb]

The thermistor hasn't been discontinued, it's just sold out.

They should have stock on them shortly.

g.

[VAXHeadroom]

The thermistor hasn't been discontinued, it's just sold out.

They should have stock on them shortly.

g.

OK Thanks. I looked around the internet and couldn't find them at either Mouser or Digikey. But those little tiny glass beads are not very robust system so the hard-mounted/threaded design has appeal for me

[geneb]

You could also buy the SE300 heater block. That takes the new cartridge thermistor and is a drop-in replacement for the older style HE280 heater block.

g.

[Xenocrates]

Except I'm pretty sure he's got an old style Seeme hotend with the PEEK section, given he mentioned changing out power resistors for a ceramic heater.

[geneb]

D'oh.

g.

[VAXHeadroom]

D'oh.

g.

Yeah, I really should have just bailed and bought an all-metal hot-end to put on it... Still will at some point in the future. For now I went with the M3 threaded thermistor suggested and used it as an excuse to buy a set of metric taps

[VAXHeadroom]

D'oh.

g.

Yeah, I really should have just bailed and bought an all-metal hot-end to put on it... Still will at some point in the future. For now I went with the M3 threaded thermistor suggested and used it as an excuse to buy a set of metric taps

I have the threaded thermistor installed, but I'm a little confused setting the thermistor values. The firmware (Generic #2 thermistor) has the following entries:
#define GENERIC_THERM2_T0 25
#define GENERIC_THERM2_R0 100000
#define GENERIC_THERM2_BETA 3950
#define GENERIC_THERM2_MIN_TEMP -20
#define GENERIC_THERM2_MAX_TEMP 300
#define GENERIC_THERM2_R1 0
#define GENERIC_THERM2_R2 4700

Most of the entries look right, but I can't figure out what to set the last value (THERM2_R2) to - any help?
thanks!
Emory

[VAXHeadroom]

D'oh.

g.

Yeah, I really should have just bailed and bought an all-metal hot-end to put on it... Still will at some point in the future. For now I went with the M3 threaded thermistor suggested and used it as an excuse to buy a set of metric taps

I have the threaded thermistor installed, but I'm a little confused setting the thermistor values. The firmware (Generic #2 thermistor) has the following entries:
#define GENERIC_THERM2_T0 25
#define GENERIC_THERM2_R0 100000
#define GENERIC_THERM2_BETA 3950
#define GENERIC_THERM2_MIN_TEMP -20
#define GENERIC_THERM2_MAX_TEMP 300
#define GENERIC_THERM2_R1 0
#define GENERIC_THERM2_R2 4700

Most of the entries look right, but I can't figure out what to set the last value (THERM2_R2) to - any help?
thanks!
Emory

OK...NVM the 4700 is the 4.7K in-line resistor...
Also: update hot end pic...[img]http://www.untiedmusic.com/emory/new_hot_end.jpg[/img]

[geneb]

Two cartridge heaters on that hot end is murderous overkill. Just sayin'.

g.

[VAXHeadroom]

Two cartridge heaters on that hot end is murderous overkill. Just sayin'.

g.

Well it pulls just about the same current as the two resistors did but should be way more resilient - I don't ever want to have to rebuild this again! Also that way I'm not changing anything about the thermal profile by having heaters on both sides. The temperature is WAY more stable and accurate than it was, but that's likely the higher quality thermistor. Except that now I have to re-cal all my ABS and PLA profiles to use a lower temperature. Before I was having to tell the system to use 230 to print PLA just to get it to flow right, so I know the temperature reading was off. Now ABS starts to flow at about 190 where it should. So all in all, I'm happy, but should have just ponied up for the new hot-end I'll get there eventually...