PC Power Supplies, RAMPS, and Resistor Hot End

Except for a hosed Pololu Carrier, and a missing thermistor, my Mendel Prusa (Prussia?) is built, wired up and ready to go, or so I thought...

I tried out the extruder manually to test the hot end, and it seemed to work fine, until I started the steppers moving. It ran for a couple of seconds, and then the voltage collapsed. Temperature started falling, and the steppers all went limp.

I thought maybe that my cheap PC power supply was not enough to drive everything (300W, 15A@+12VDC), so I bought a new power supply with more power (450W, 21A@+12), but it seems to perform even less well, as I cannot get the heater to even warm up without voltage cutting out.

I'm convinced that power is not the problem here, so it must be something else, but what???

Well, seems I've answered my own question. It turns out the new PC power supply does have more power. So much so that it melted, and subsequently shorted the cable from the resistor. Guess I should use heavier gauge wire...

Can anyone enlighten me on recommended wire when using the resistor type hot ends?

no but sounds like you had a good short in there,

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[mike-mack.blogspot.com]

I used standard kettle wire from what normally plugs into my wall (240v) and it's been fine for twelve hours of running, I would imagine any standard electrical wire for a big domestic appliance will do the job, I also left all the resistor wire on the resistors and connected using a crimped pin and header with two layers or heat shrink. Sounds like your hot end is getting two hot though, Mine is getting upto 200 C within a couple of mins and almost instant up to 50c from power on so there is some sort of high demand when it starts up. BTW I used this peek [reprap.org] design which contains heat very well when wrapped in kapton. I also use kapton around all the wires down in the hot end of the unit.

Cheers

Rob

Edited 1 time(s). Last edit at 03/14/2011 01:55PM by hexitex.

If you can figure out what current your hot end is drawing (as in Ohms law), you can google "wire gauge chart" and easily find out what the rated current for different type of wire is.

HOWEVER, I would assume that you supply wires heating up wasn't the problem (unless you used really fine wires) so much as cunducting heat away from your hot end. Finding and wire with insulation rated to somewhere around where your hot end runs is important, I would think.

Had I not left my multimeters all at the office, I would have seen that the resistor was in fact the short here. Nothing wrong with the wire. I have not installed the thermistor yet, and did not realize that RepSnapper does not turn off the heater when you unselect the "Switch Heat On", so I was "regulating the temperature by hand, not realizing that the thing was just getting hotter and hotter.

i have noticed that too, i hope they can get a new update out for repsnapper out soon, is the rest of it good, or did you cook any more parts of your extruder.?

i should see if i cant find a donation for them to help spur them along.

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[mike-mack.blogspot.com]

Repsnapper sends a M code to the Arduino FW to turn off/on the heater. Thus, if you check the communication log you would see the M106 or M107 sent. Now, if the FW "ignores" (in quotes because I don't know the specific FW so I don't know the logic it uses if ther eis no temperature probe) it then that's not a Repsnapper bug.

And it's Prusa.

The extruder heater is usually around 6-7 ohms and it's being driven by 12 V. So, that works out to around 2 amps (I=V/R). That's all you really need for the hot end.

I run every thing on a 30V 12A supply its less bulky than a standard PC supply.

I use 6.8ohms 5W @30V with without any problems PCB heated bed is 4ohms again with 30v supply.

The FeTS I use are 0.014 ohms RDS on for the Bed and 0.3 ohms RDS on for the extruder so they both have heatsinks.

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Bodge It [reprap.org]
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