Use fuse with lower current rating?
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This is for a 24V system. I moved the hotbed to be controlled by a SSR. Is it reasonable to replace the 15A fuse with a lower current one, e.g. 5A? The rest of the system include 5 standard Nema17 steppers, 40W heater, and two fans.
Also, what is the fuse type? https://en.wikipedia.org/wiki/Fuse_(automotive)
Edit: do I need a fuse in series with the 100A SSR or is it OK to rely on the MEAN WELL RSP-500-24 current limit (~20A)? Heatbed is 24V 350W.
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@zapta said in Use fuse with lower current rating?:
RSP-500-24
Your Mean Well power supply is both short circuit and overload protected, so adding a fuse would be unnecessary. If you favour the approach where you use several safety systems, it won't harm anything if you do add a fuse on the Mean Well's output.
It's remotely possible that the SSR's input might fail short, although as the input to the SSR is an opto-coupler, the chances of a failure drawing significant current are extremely small. The 15A fuse will protect your Duet in that case, if the wires to the SSR are thick enough to actually take 15A without melting. Replacing it with a 5A fuse would save your wiring and isn't such a bad idea (not that I've done this myself as I reckon the opto-coupler would go open circuit very quickly if a short failure happened). The fuse type is a "mini blade fuse".
Note that the 15A fuse only protects the bed heater output. The fan outputs have a 1A fuse and the rest of the board is protected with a 7.5A fuse, so changing the 15A one for something smaller will only affect the bed heater output. Your steppers and hot end heater are protected by the 7.5A fuse.
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Thans @grizewald, that's very useful. I measure the current through the 15A fuse and its 25ma, going to the SSR input so I ordered a 1A fuse for it. In the past, I reversed the polarity of the SSR input and it went up in smoke, literally. I wonder if a 1A fuse would save it.
Also ordered a 20A fuse + inline holder for the Mean Well to SSR connection. It's a short wire so I can easily replace it with the inline fuse holder. I doubt though if it will react before the Mean
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@zapta I doubt a 1A fuse will save your SSR if you wire the input up with reversed polarity.
If you will be pulling 20A from the Mean Well power supply, use big beefy copper wires and use at least two negative and two positive terminals. Pulling the full rated load from a single terminal is likely to overheat the terminal and PCB and bake them, eventually leading to failure.
If your 350W 24V heater is anything like mine, they are actually about 440W. That's fine with a 500W power supply and you get nice fast heating. If you want to be sure, check the resistance of the heater.
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@grizewald, I measured the bed heater with a small power supply, 3.95A at 6V so ~380W @24V ignoring resistance change due to temperature. As for the SSR input, I also added a series diode and a parallel 10k for to handle any leakage current, just in case I will reverse it again. For wires I am using the the heater's wires and this https://www.amazon.com/gp/product/B07588953X
How did you connect a wire to two terminals? Can you post a picture?
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I use a passively cooled, DIN rail mounted supply for my bed heater. It has screw terminals for ferrules instead of spade terminals, but the principle is the same:
Two beefy wires with ferrules into each pair of outputs:
Then the wires go to a terminal block and both get connected to the single wire from the heater, or to the SSR's switching terminals.
In the power supply's data sheet, Mean Well specifically warn against trying to pull the entire rated current from a single terminal.
Your 14AWG wire is just perfect for the job.
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@grizewald , that's a very nice power supply. I presume that passively cool means no fan noise which is very significant on my 500W one. I will see if I can fit this one in my frame.
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Yep, it's totally silent. I also have a second 120W supply of the same series sitting next to it on the DIN rail to power the rest of the printer.
The loudest thing on the printer is probably the hot end heatsink fan!
