Hotend fan

Greetings. This is my first post here. I bought my Ormerod from RS a couple of weeks ago.

My hotend fan, which seem to be running constantly, also seem to vary in RPM when the heating elements go on. Is that normal?
When both bed & extruder elements are on, it would seem the RPM is down at about ~50%. Doesn't that affect cooling? Since
the bed is also going on & off every 30 second, it feels like the cooling efficiency may be variable?

Does anyone have any insight on this?
/Michael Ljunggren

The way the hotend fan is wired according to the documentation, it should run at constant speed since it's connected straight to 12V. The black/red cables are connected to pins 31 and 35 of the "motor loom" header, which is ground and +12V respective (as per the schematic for the Duet). The +12V pin is actually labelled as the power pin for the heater, but it's all the same +12V essentially. The way both the fan, the bed heater and the hotend heater works is that the + lead is wired straight to the +12V rail, and the MOSFET transistors (the big black components just above the FAN0 and E0_HEATER headers) turn the fan on by grounding the "negative" output (E0- or FAN0-), and off by basically disconnecting it. I haven't looked closely at the actual circuit board layout, but I guess there is just one +12V trace going from the PSU connection (the big green screw terminal) down to the various connectors on the edge of the board.

The reason it drops down in speed when the heater turns on is probably because there is some voltage drop on the +12V pin on the Duet board. If the copper trace across the circuit board isn't thick enough it will have some resistance to it, there might be a sizeable voltage drop over it (basically heating the circuit board a bit). The other cause might be the power supply, but it's a fairly beefy one (should be able to do >30A on 12V) and even with all motors and all heaters running it should be nowhere near that. However, some ATX PSU's give a voltage that is a bit too high when it isn't loaded, and then stabilizes at the correct level once you draw a couple amps. Do find out more on what's going on, I would measure the voltage over the fan, as well as on the +12V input (the green screw connectors) with the heater on and off. That will tell you where the voltage drop is.

I tried connecting the fan to it's apparently intended place (the FAN0 header at the very corner of the board, or pins 39/40 of the "motor loom" header, requires a more narrow connector). It ran at full speed when powering up and throughout printing. Not sure if the gcode file I had had any "fan control commands" in it though, and I haven't tried it specifically. However, connecting the fan to a different +12V source might just improve the voltage drop situation.

The best fix for the voltage drop (if it as I suspect is in the +12V trace across the circuit board) would probably be to connect the +12V lead from the heater closer to the PSU input (just as the bed heater is right next to it). Different fans react a bit differently to lower voltage, but as long as it's spinning fast enough to make some "air noise", I don't think it would have too big an effect on cooling when printing.

Hi Michael,

It's normal for the fan RPM to drop slightly when the bed heater is on, because the 12V supply drops a little under load. The reason for this is mostly that the 12V output from the ATX power supply isn't very well regulated. Some ATX power supplies are better than others. Also the wires between the ATX power supply and the Duet have significant resistance and contribute a little.

The fan RPM should only drop slightly when the bed heater comes on. If it is dropping a lot, then check the power wire between the ATX board and the Duet. In particular, check that the wires are secure in the terminal blocks.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the info. I measured the voltage on the fan when the heaters went on & off. Obviously ~50 must have been my imagination, because the voltage drop is nowhere near that. In fact the 12 V varies between 15-17 V, so you might even say it's not draw enough to even bring it down to 12.

I'm also toying with the idea of leading the exhaust air from the PSU via a hose to get rid of the fan all together. The noise levels must go down at least 20dB. I'm not concerned the air would be too hot. It's less than 30C anyway. Have anyone done such a thing?

1. If you are happy doing your own electrical wiring, then you can use a fanless PSU. See [forums.reprap.org] and [github.com].

2. The voltage on the 12V line should be nowhere near 15V. I think your ATX power supply may be faulty.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

The PSU fan is not my main source of noise - it's actually comfortably quite. It's the hot end fan I was planning to make redundant.

I've used old ATX PSU's as "generic lab PSU" now and then, and I've seen unloaded ATX PSU's give out anything between 10.5 and 13.5V on the 12V rail when under no load (which sort of never happens when attached to a computer). While 15V sounds a bit high, I wouldn't be too surprised by it. It could be the fan is actually running "too fast" on 15V, and then slows down as the voltage settles down to 12V.

Just drawing an ampere or two from the +5V or the +12V line usually brings it to stable values, but I guess the Duet + the fan (no stepper motors or heaters running) will only do a couple hundred mA. You could try attaching a "dummy load" and see if that stabilizes the voltage, a 15-25W car lightbulb (like a brake or indicator light) would draw about 1.2-2A, which should do it.

Edited 1 time(s). Last edit at 04/01/2014 06:40AM by jstck.

Quote
dc42
2. The voltage on the 12V line should be nowhere near 15V. I think your ATX power supply may be faulty.

It's actually 17.5 when idling. A few volts are to be expected surely, but you suggest this is too much? Could someone please check their own? I measured at the input to the duet board.

Edited 1 time(s). Last edit at 04/01/2014 06:45AM by michaelljunggren.

Quote
michaelljunggren

Quote
dc42
2. The voltage on the 12V line should be nowhere near 15V. I think your ATX power supply may be faulty.

It's actually 17.5 when idling. A few volts are to be expected surely, but you suggest this is too much? Could someone please check their own? I measured at the input to the duet board.

While I haven't personally seen 17.5V no-load voltage in an ATX PSU, it might very well be just one of those things ATX PSU's do, they're just not spec'ed to handle such low loads. If voltage comes down to 12V when loaded with a couple amperes, I don't think it is actually broken.

Aaaaallllright.... the battery in my multimeter just died... After replacing it, the PSU voltage is at a stable 12.1V...

Sorry about all this.

Quote
michaelljunggren
Aaaaallllright.... the battery in my multimeter just died... After replacing it, the PSU voltage is at a stable 12.1V...

Sorry about all this.

Thats better

should be something 11.7 to 12.5v

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Another RS Ormerod Mk1 meets the world

Retired now but I used to make....
CNC Machined Mk1 aluminium bed support plates for the Ormerod
CNC machined X-plates and ribs for Mk1 & Mk2 Ormerods
CNC machined bed support arms for the Mk2 Ormerod.
Dual Hot-End heatsink blocks.