@nop_head : Since both things are controlled in firmware, a single firmware fault could cause both the heater to be 100% on and the fan to be off.
Yes that is what I said above. It needs to be wired direct to the PSU.
There are also dual thermistor setups where the firmware detects if there is a significant difference between the two thermistors inserted into the same hotend.
@Whosa_whatsis werenāt you one of the folk playing with dual thermistor firmware?
@nop_head that does not help if the PSU voltage were to change due to a fault. You are removing any over voltage protection if you just connect it to the PSU direction.
Thermal fuse would be great if one existed at 300-350C
In the unlikely event that the PSU voltage goes up then the firmware would regulate the correct temperature. So it would take a double fault to cause a problem. E.g. shorted MOSFET and faulty PSU, or firmware crash and bad PSU.
Normal safety approval only needs to cover single fault conditions.
PSU over voltage could cause a firmware lockup/MOSFET stuck on.
Or the PSU voltage could dip making it impossible to heat up the hot end.
The simplest solution is not always the right solution for consumer electronics. (hobbyist tools maybe)
FYI protection should be in series, So one fault causes the system to fail. It should be impossible to continue with a failure until it has been repaired.
Consumer electronics safety approvals only need to cover single fault conditions.
An ATX PSU has over voltage protection so for it to produce too much voltage and not shut down is a double fault.
Also typical reprap electronics can handle more voltage than it could ever produce on the 12V rail even under a fault condition.
I wonder how much of an impact humidity and ambient temperature would have on the āterminal temperatureā of an always-on heater. Youād also have to take into account elevation.
This means you would need to define an upper limit on ambient operating temperature and operational elevation in order to be able to consider it safe. Unfortunately the printer would work just fine outside of these ranges, it would just not be safe should it fail.
I still prefer the thermal fuse method. (although the dual thermistor approach where you have one thermistor monitored by an independent micro controller (like an ATTiny) that can shut down the PSU also sounds adequate, and as an added bonus it can be reset more easily)
Current draw, or define a max duty cycle for element heating directly in the firmware that would limit the highest temperature a hotend could reach if the thermistor failed for any reason.
IE say a hot end can take 800 C. Deternine the on/off duty cycle that produces that temp, and put it in the firmware. Then when you calculate how long to provide current, or what amount, you NEVER exceed that maximum in code. So even if the thermistor fails, the hot end doesnāt run away.
Then you also add a watchdog that monitors it. You will likely never need to run it that hot, so if the duty cycle or current stays at that level for more than a certain length of time, you assume there is a fault and shut it all down.
@Daniel_Joyce That does not work with a software fault or a short in the transistor.
This is an interesting dilemma for me. Originally E3D hotends were intended as enthusiast/pro additions for your printer. For the kind of prints where you babysit your machine and watch the polycarbonate print at 300C, or other materials at even higher temperatures. When I introduced them that certainly was true. They were intended for those people that wanted to get silly hot and print weird materials for particular purposes, not as general purpose everyday parts.
More and more I am now seeing that our hotends are being installed by default for normal run of the mill printing duty by average Joe. Which is certainly positive and something Iād like to encourage. However it leaves me in a bit of a quandry, I donāt want to eschew our original purpose and user base, but I certainly donāt want people to cause damage either.
Iām lucky to be in a position where the heater cartridge manufacturers will produce me whatever wattage I please, so I have compete freedom to dictate that.
Iām leaning towards having the default wattage be 25w and then offering an upgrade as an option to 40w.
I would get 25w and 40w for the same price. If I simply had an option on the page when buying a lot of people would just choose 40w, so Iād have to artificially inflate the price of 40w and perhaps decrease the other of the 25w default unit in order to coerce buyers into choosing the safer option while still offering the 40w option.
Manipulating prices makes me a little uneasy, especially charging enthusiasts more for the sake of it and becoming a nanny manufacturer, but perhaps in this case it is worth it, for everyoneās sake .
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Oh and @Joe_Spanier , just get in touch with us via the contact form on our site and weāll get you sorted out with new heater blocks.
@Sanjay_Mortimer1 while I understand your desire to not babysit, and not increase the price, why not do this instead? Since most people donāt need the 40W head, only sell the 25W. Offer the 40W heater as a separate item only. People will then either end up with a safe daily use head, or have a spare heater for when they need to swap (say to verify that the heater hasnāt burnt out). Iāll make people think before buying. It also gives you a very obvious vehicle to provide additional safety warnings.
āBeen there done thatā probably a year ago now- the e3d guys got me going again!
The problem is when the thermistor sits a tiny distance away from the block, so it still responds to the heater but shows a much lower temp.
With a mess of kapton tape this can happen too easily. With then new e3d block or axial thermistors that shouldnāt happen.
I have also melted a JHead with a resistor heater due to a software bug, so I would like to see another failsafe.
@Sanjay_Mortimer1 I agree with @Jerry_Rodberg
@Camerin_hahn that is why have multiple failsafes. A fuse in the hot end power feed, software watchdog of hot end power draw or duty cycle, etc.
Iād stick in a second hot end surface thermocouple and a tiny arduino clone whose sole purpose is if it gets too hot cut the power to the entire printer.
how about one of these looking at the hot end?