Hot end maximum wattage possible
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@elmoret said in Hot end maximum wattage possible:
@doctrucker said in Hot end maximum wattage possible:
Well, that's just not true, or there wouldn't be fuses on printed circuit boards. Or in plugs. You can use a fuse to protect a wire or a device. But don't take my word for it!Plugs often have wires coming out of them. Printed circuit boards have wires coming out of them, and their traces are un-insulated wires. Fuses are not quick enough to protect the likes of the mosfets on the board which you are proposing the fuse protects in the fail-short failure mode.
Why? Why wouldn't the fuse blow? No sense in fitting a fuse 2x that of the rated load.
A typical guaranteed quick/near instant blow is twice the fuses rated current. If you've used cabling whose rated current is above the fuse then the cabling will take the brief short to earth for long enough to blow the fuse. To avoid nusance trips you would have to size the fuse above the normal current to avoid nusance trips, which is going to mean double current is not twice your fuses rated current. Your margins for fuse selection are getting small, and the range of fuses is not infinate - hence why there is a 15A fuse on the duet 2 for bed heater not 18A.
This would be difficult to protect against with fusing as a fuses are often only guaranteed to quick fail at double rated current.
Which would be experienced in this case.
No it wouldn't if you didn't want nusance trips by selecting your fuse at exactly expected draw.
Fusing is also only intended to protect the wire, not the device.
That is again, false. Install a fuse that is rated for ~10% more than the rated load. In OP's case, I'd use a 10 amp fuse. A 10 amp fuse exposed to 16A (resulting from one of the heaters dead shorting) will blow in under 3 seconds. I doubt you'll even be able to detect a noticeable change in temperature in that time.
This is where the OP may accept a reduction in safety magin if they wish, but my original suggestion of SSR/external mosfet is safer. Besides, at 10A you've aready gone above spec for the input.
A current chopper style circuit could make it safer, as after all most of our stepper motors rated voltages are much below drive voltage.
That's what a PID loop is.
I think you're wrong here. PID is a control method, not a means to directly control or limit current and has no way to detect an over current. I was referring to active current chopping like in the stepper motor drivers. It would open circuit if the current exceeded a set limit but not reset. Much like the system in the Tesla battery pack which blows a pyro disconnect in the event of overcurrent.
Easier to use a 24V heater! We're talking about less than a £10 here
He needs 100w. 100w heater cartridges are not easy to find, plus they result in a pretty extreme heat density.
Refer to my original commment regarding external ssr/mosfet. Could also do 3 40W cartridges which at a 100% duty will pull 5A so safe on the Duet spec. If you don't want 120W of heat adjust the max duty down.
Edit: Just re-read dc42's earlier post that said the inputs can pass over 5A safely and probably 10A. Looks like the board could take the fault current of one of the two heaters shorting. I still would prefer external switching and 12V over on board and two 12v heaters running in series from 24vdc.
Finding suitable 24V heaters or multiples thereof would be better still.
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@doctrucker said in Hot end maximum wattage possible:
@elmoret said in Hot end maximum wattage possible:
@doctrucker said in Hot end maximum wattage possible:
Well, that's just not true, or there wouldn't be fuses on printed circuit boards. Or in plugs. You can use a fuse to protect a wire or a device. But don't take my word for it!Plugs often have wires coming out of them. Printed circuit boards have wires coming out of them, and their traces are un-insulated wires. Fuses are not quick enough to protect the likes of the mosfets on the board which you are proposing the fuse protects in the fail-short failure mode.
As for the FET, the Duet boards use IPD036N04L, which are rated at 90A continuous. It'll handle 200A for 1ms. The fuse is gonna blow before the FET, look at the time response curves for fuses and compare to the FET.
This would be difficult to protect against with fusing as a fuses are often only guaranteed to quick fail at double rated current.
Which would be experienced in this case.
No it wouldn't if you didn't want nusance trips by selecting your fuse at exactly expected draw.
I agree! You should oversize by 10-15%. Covered this in my reply.
Much like the system in the Tesla battery pack which blows a pyro disconnect in the event of overcurrent.
Sounds like a fuse!
I don't wish to argue further, but hopefully this helps: have you ever looked at how a heater cartridge is made? It is a wire wrapped around a core, inserted into a metal cylinder. It could short in a variety of ways, resulting in resistances from nominal down to a few tenths of a ohm.
Knowing that, what is the realistic difference between that, and two heater cartridges in series? Put another way, what is the difference between two 20mm long heater cartridges in series, and one 40mm long heater cartridge? (nothing, aside from physical packaging)
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I'm a (retired) mechanical engineer, not an electronics expert but I've read all the posts above about FETs and fuses, and still running two 12v heaters in series from a 24v supply just seems an unsafe thing to do, unless I've missed something somewhere.
Just about every electrical fault I've ever had with my printer has been some sort of wiring issue (bad crimps, broken wires, faulty terminals) and almost never a component failure.
So in simple terms, if one used two 24v heaters in parallel and a wire came adrift on one of them and shorted to ground, it'd take out a fuse. That's providing you have a V1.04 board or later - mine is an early one without fuses AFAIK, so something would go pop instead of a fuse but no matter, it would still fail safe (more or less). But if one used two 12v heaters in series, and the same fault occurred on one of them, then it wouldn't be a short to ground which would take out a fuse or some other component, but a halving of the resistance and so the other heater would get very hot, very quickly and potentially be a fire hazard would it not? Or maybe even worse, the PID control would catch it and the user might never know until the MOSFET failed. In that case it would be fed continuous 24 volts, the firmware would detect the sudden rise in temperature but would be unable to turn it off because of the way the MOSFET has failed (or at least that's how I read it here https://duet3d.dozuki.com/Wiki/FireSafety).
I'm just going by the fact that when I tuned an 80Watt heater, the firmware gave me a warning that it was overpowered and potentially a dangerous fire hazard. I can't help thinking that a 12v 40 watt heater being fed 24v would in effect turn it into an 80Watt heater.
Maybe I've just missed something in all the above discussion, but my gut feel is that it's an unsafe thing to do.
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@deckingman As I've tried to explain a few times now, there is no guarantee that a heater cartridge exists only in a full short or a no short situation. It can also be partial shorted.
As such, running two 12v heater cartridges in series no more "dangerous" than experiencing a partial short on a single heater cartridge.
If you are running a single heater cartridge, and the bottom half of it shorts, how is that any different from the scenario you're fearful of? The other half of the heater cartridge would get very hot, very quickly (until the PID loop kicked in), just like the scenario you're concerned about.
And we all run single heater cartridges without worry.
And also, there's no need for a v1.04 to have a fuse in the hotend circuit. One can add any fuse they wish, to any board!
I need to find a way to disable notifications on this thread before I lose my sanity.
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@elmoret Yes, I hear what you are saying and I take your point but ...................well let's just agree to differ before we both lose our sanity
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The heating elements are just essentially just resistors. There are several watchdogs that take care of the issues you bring up..... and if you appropriately size the fuse, there is even less to worry about.
The only additional risk with wiring them in series is that it adds a additional possible point of falure.
