Re: Fire Hazard July 02, 2014 02:50PM |
Registered: 10 years ago Posts: 16 |
Re: Fire Hazard July 02, 2014 02:56PM |
Registered: 10 years ago Posts: 172 |
Re: Fire Hazard July 02, 2014 06:54PM |
Registered: 10 years ago Posts: 67 |
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thejollygrimreaper
i solved this problem many moons ago by using axial thermistors and insulating the leads with ptfe sleeve:
click here
using this method there is nearly no easy way to get it into a thermal runaway mode, as the thermistor cannot physically leave the heater-block,
Re: Fire Hazard July 02, 2014 07:37PM |
Admin Registered: 11 years ago Posts: 1,063 |
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Hazer
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thejollygrimreaper
i solved this problem many moons ago by using axial thermistors and insulating the leads with ptfe sleeve:
click here
using this method there is nearly no easy way to get it into a thermal runaway mode, as the thermistor cannot physically leave the heater-block,
Thermal runaway is not limited to loose thermistors. There have been failed-on MOSFETs and stuck processors.
Re: Fire Hazard July 02, 2014 09:19PM |
Registered: 11 years ago Posts: 210 |
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thejollygrimreaper
failed mosfets are a bit of a different story however they still can be protected against very easily however this comes back to the powersupply and actually having overload and short circuit protection as advertised, this is something that seems absent in most of these industrial power supplies that come in from china
what i have done to take care of this scenario in all the printers I've built is i use those 19v 180watt power bricks instead, so i have a situation where the overload protection will kick in if either mosfet fails for the bed or hotend the powersupply will shutdown until it is unplugged and plugged back in again, the downside is that these power supplies don't cope very well with the style of pwm that is used by default in marlin but it's ok thats i've solved too by switching to hardware pwm for which there is a pull-request for which gives you the choice of hardware or software pwm
Re: Fire Hazard July 02, 2014 09:58PM |
Registered: 9 years ago Posts: 34 |
Re: Fire Hazard July 02, 2014 10:04PM |
Registered: 11 years ago Posts: 210 |
Quote
Hazer
Quote
thejollygrimreaper
i solved this problem many moons ago by using axial thermistors and insulating the leads with ptfe sleeve:
click here
using this method there is nearly no easy way to get it into a thermal runaway mode, as the thermistor cannot physically leave the heater-block,
Thermal runaway is not limited to loose thermistors. There have been failed-on MOSFETs and stuck processors.
Re: Fire Hazard July 02, 2014 10:47PM |
Admin Registered: 11 years ago Posts: 1,063 |
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greenman100
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thejollygrimreaper
failed mosfets are a bit of a different story however they still can be protected against very easily however this comes back to the powersupply and actually having overload and short circuit protection as advertised, this is something that seems absent in most of these industrial power supplies that come in from china
what i have done to take care of this scenario in all the printers I've built is i use those 19v 180watt power bricks instead, so i have a situation where the overload protection will kick in if either mosfet fails for the bed or hotend the powersupply will shutdown until it is unplugged and plugged back in again, the downside is that these power supplies don't cope very well with the style of pwm that is used by default in marlin but it's ok thats i've solved too by switching to hardware pwm for which there is a pull-request for which gives you the choice of hardware or software pwm
No power supply can protect against a failed hot end MOSFET, your statement is false. Please don't spread potentially dangerous misinformation like this.
Re: Fire Hazard July 02, 2014 11:12PM |
Registered: 11 years ago Posts: 210 |
Re: Fire Hazard July 03, 2014 12:03AM |
Admin Registered: 11 years ago Posts: 1,063 |
Re: Fire Hazard July 03, 2014 12:08AM |
Registered: 11 years ago Posts: 210 |
Re: Fire Hazard July 03, 2014 02:40AM |
Admin Registered: 12 years ago Posts: 2,569 |
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Gonetotx
I am somewhat surprised at how common this fire hazard issue has been.
Re: Fire Hazard July 03, 2014 03:50AM |
Registered: 11 years ago Posts: 172 |
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DeuxVis
I would start delivering the kits with a smoke detector if I were a kit vendor... It's probably better not to wait for someone getting harmed !
Re: Fire Hazard July 03, 2014 04:03AM |
Registered: 12 years ago Posts: 300 |
Re: Fire Hazard July 03, 2014 06:11AM |
Registered: 10 years ago Posts: 474 |
Re: Fire Hazard July 03, 2014 07:56AM |
Registered: 10 years ago Posts: 553 |
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greenman100
So your "safety" system is predicated on the bed heater being on, and at a temperature high enough to draw an average of 60w. Its also a recipe for disaster if you or anyone else flashes firmware with incorrect MAX_PWM values.
Hardly a good solution, and arguably less safe than using 12v power with 12v rated parts.
Re: Fire Hazard July 03, 2014 07:59AM |
Registered: 11 years ago Posts: 210 |
Re: Fire Hazard July 03, 2014 08:11AM |
Registered: 11 years ago Posts: 172 |
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cnc dick
With the arrival of kits things are getting dangerous a lot of people are buying these and assembling them and not really understanding it's an experimental machine. It's not like a toaster or microwave that is UL or CE approved they are not really ready for the average person.
Re: Fire Hazard July 03, 2014 08:14AM |
Registered: 12 years ago Posts: 300 |
Re: Fire Hazard July 03, 2014 08:22AM |
Registered: 10 years ago Posts: 67 |
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thejollygrimreaper
using my setup as an example:
on my hotend i have a 40watt 12v heater cartridge and a 12v 120watt silicone bed , and i run a 19v 180watt power supply , obviously i have my max_pwm value set as to not instantly overload the powersupply but to a point where my bed and hotend can function as desired
scenario #1 : if the hotend mosfet fails, the hotend heater cartridge will want to suddenly draw around 100 watts on top on that there is the current draw from the stepper motors and the bed which still drive the powersupply into overload and shut it down.
scenario #2 : if the bed mosfet fails it will draw around 300watts well above what the powersupply can do so it'll overload and shutdown not to mention the load from the steppers and the hotend adds on top
Re: Fire Hazard July 03, 2014 08:39AM |
Admin Registered: 11 years ago Posts: 1,063 |
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Hazer
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thejollygrimreaper
using my setup as an example:
on my hotend i have a 40watt 12v heater cartridge and a 12v 120watt silicone bed , and i run a 19v 180watt power supply , obviously i have my max_pwm value set as to not instantly overload the powersupply but to a point where my bed and hotend can function as desired
scenario #1 : if the hotend mosfet fails, the hotend heater cartridge will want to suddenly draw around 100 watts on top on that there is the current draw from the stepper motors and the bed which still drive the powersupply into overload and shut it down.
scenario #2 : if the bed mosfet fails it will draw around 300watts well above what the powersupply can do so it'll overload and shutdown not to mention the load from the steppers and the hotend adds on top
This is false. When a MOSFET fails, it simply becomes a short. It does not force more wattage into the loading element (the heaters in this case). So if your 40W cartridge is on all the time, it draws 40 watts (or 3.33Amps) constantly, heating your hotend up to dangerous temperatures. Same goes for your bed heater. So at full on, your PSU which is rated for 190W will easily power your 120W bed and 40W cartridge for a few hours until it burns.
Re: Fire Hazard July 03, 2014 09:00AM |
Registered: 10 years ago Posts: 67 |
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thejollygrimreaper
Quote
Hazer
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thejollygrimreaper
using my setup as an example:
on my hotend i have a 40watt 12v heater cartridge and a 12v 120watt silicone bed , and i run a 19v 180watt power supply , obviously i have my max_pwm value set as to not instantly overload the powersupply but to a point where my bed and hotend can function as desired
scenario #1 : if the hotend mosfet fails, the hotend heater cartridge will want to suddenly draw around 100 watts on top on that there is the current draw from the stepper motors and the bed which still drive the powersupply into overload and shut it down.
scenario #2 : if the bed mosfet fails it will draw around 300watts well above what the powersupply can do so it'll overload and shutdown not to mention the load from the steppers and the hotend adds on top
This is false. When a MOSFET fails, it simply becomes a short. It does not force more wattage into the loading element (the heaters in this case). So if your 40W cartridge is on all the time, it draws 40 watts (or 3.33Amps) constantly, heating your hotend up to dangerous temperatures. Same goes for your bed heater. So at full on, your PSU which is rated for 190W will easily power your 120W bed and 40W cartridge for a few hours until it burns.
no, the bed is rated at 120 watts on a 12v supply ,however on my printer it is given a 19v supply so if a mosfet fails there is no pwm current control anymore and the bed will draw 300watts which will overload the powersupply and shut it down
the hotend heater cartridge under the same conditions will recieve 19v and will want to draw 100watts not 40watts, about 1/2 an hour ago i did a quick and dirty test by setting the duty cycle on my hotend to 100% without the bed turn on and it still sent my powersupply into its overload protection and shut it down so i kind of wonder what the steppers motors draw in total
most of this stuff can be verified using a ohm law calculator
Re: Fire Hazard July 03, 2014 09:09AM |
Admin Registered: 11 years ago Posts: 1,063 |
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Hazer
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thejollygrimreaper
Quote
Hazer
Quote
thejollygrimreaper
using my setup as an example:
on my hotend i have a 40watt 12v heater cartridge and a 12v 120watt silicone bed , and i run a 19v 180watt power supply , obviously i have my max_pwm value set as to not instantly overload the powersupply but to a point where my bed and hotend can function as desired
scenario #1 : if the hotend mosfet fails, the hotend heater cartridge will want to suddenly draw around 100 watts on top on that there is the current draw from the stepper motors and the bed which still drive the powersupply into overload and shut it down.
scenario #2 : if the bed mosfet fails it will draw around 300watts well above what the powersupply can do so it'll overload and shutdown not to mention the load from the steppers and the hotend adds on top
This is false. When a MOSFET fails, it simply becomes a short. It does not force more wattage into the loading element (the heaters in this case). So if your 40W cartridge is on all the time, it draws 40 watts (or 3.33Amps) constantly, heating your hotend up to dangerous temperatures. Same goes for your bed heater. So at full on, your PSU which is rated for 190W will easily power your 120W bed and 40W cartridge for a few hours until it burns.
no, the bed is rated at 120 watts on a 12v supply ,however on my printer it is given a 19v supply so if a mosfet fails there is no pwm current control anymore and the bed will draw 300watts which will overload the powersupply and shut it down
the hotend heater cartridge under the same conditions will recieve 19v and will want to draw 100watts not 40watts, about 1/2 an hour ago i did a quick and dirty test by setting the duty cycle on my hotend to 100% without the bed turn on and it still sent my powersupply into its overload protection and shut it down so i kind of wonder what the steppers motors draw in total
most of this stuff can be verified using a ohm law calculator
Yup. Your right. Good point to explain that (especially since I missed the 19V myself).
So for you it is a good cover for thermal runaway of the heaters. For the rest of us, it doesnt help much. I have dual extruders, so your PSU would not help. Also I still use 12V. It also does not cover electrical based fire hazards (shorting, connectors, etc). But good for you
Re: Fire Hazard July 03, 2014 09:37AM |
Registered: 10 years ago Posts: 172 |
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And since a thermal fuses are not rated hgih enough to watch the hotend directly, and can only detect fire AFTER ignition in other dangerous parts of the printer, you can already assume the printer is burning away.
Re: Fire Hazard July 03, 2014 10:00AM |
Registered: 12 years ago Posts: 300 |
Re: Fire Hazard July 03, 2014 11:13AM |
Registered: 10 years ago Posts: 553 |
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umdpru
Your assumption here is just not grounded in engineering logic. The order of events due to a failure in which you have a runaway hotend will be:
1. Hot end begins heating up
2. Body of J-Head heats up to melting temperature.
3. Smoke
4. Fire
You can turn the machine off at #2 by making use of a thermal fuse. It DOES NOT need to mounted directly on the heater block to be effective. If you open up and look at most any heat generating appliance, you will find a thermal fuse NEAR the heating element and NEVER on it.
The temperatures at the junction between the heater block and the body can be monitored via a themral fuse. The temperatures here are much lower than the heater block temperatures and are within the operating range of a thermal fuse.
I tested my J-Head heated to ABS printing temperatures (235ºC) and probed the temperature at the juntion of the PEEK body and the heater block. See the attached pictures. As you can see, the maximum temperature at that point is only 178ish ºC. If the heater were to run away, that juntion would quickly increase in temperature to beyond the PEEK melting point (around 245ºC). I purchased these thermal fuses [www.amazon.com]. That fuse has a holding temperature of 200ºC. It will fuse and open before fire can develop at the hotend. Once it opens, the mains will be cut off and no further heating will occur and thus no fire.
YMMV if you're using a different hot end but the concept is the same. you mount the thermal fuse NEAR the heating element not on it and it will work. Major applicance manufacturers do this for the same reasons we need to - the thermal fuses can't stand the heat of the heaters.
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fma
Did you try to run such overheating condition, to check how it works?
Hotend manufacturers should include a support (simple hole) for these fuses, near the top of the hotend; using a 100°C fuse should do be OK.
Re: Fire Hazard July 03, 2014 12:41PM |
Registered: 10 years ago Posts: 1,381 |
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thejollygrimreaper
i have a 12v powersupply in transit and i plan to do a youtube video demonstrating this lack of short circuit protection and show exactly what happens in the process,
Re: Fire Hazard July 03, 2014 12:45PM |
Registered: 10 years ago Posts: 24 |
Re: Fire Hazard July 03, 2014 01:28PM |
Registered: 10 years ago Posts: 67 |
Re: Fire Hazard July 03, 2014 01:29PM |
Registered: 10 years ago Posts: 790 |