by bryanandaimee, published
Update: Made some measurements to help the designers out there. With three strips ganged together as in the PDF soldered in a long series trace across two stripboards the current at 12V is 10 amps cold and 8.2 amps hot. That gives you 120 Watts cold and 98 watts hot. So that means that the total resistance cold is 1.2 ohms, and individual strips have a resistance of 0.13 ohms. To calc. that I had to assume that the two wide buss strips were equivalent to 3 ganged central strips. So 0.13 per strip and about .04 ohms for each of the two wide strips on the outside.
So 4 ganged together should give about 0.13/4 * 9 = .3 ohms + 2*0.04 = .38 ohms per board or .38*2 = .76 ohms for a prusa bed giving 16 amps cold and 189 watts at 12V.
5 ganged together would be .13/5*7 = .182 ohms +2*.04 = .262 per board and .52 ohms total for the heated bed. Giving 23 amps at 12 V and 275 watts.
6 ganged might be "OK" for 5V at about .3 ohm and 17 amps (5V) but it might not get that hot at only 85 watts. (I'm thinking at 5 or 6 you will probably want to think about pairing the bus strips on the sides with a couple normal strips to keep the heat even across the board.
7 ganged would probably work pretty well at 5V at about .22 ohms 23 amps and 115 watts. (same here use 2 or 3 strips ganged with the wide bus on the side. )
Well, that was quite an extrapolation so don't trust the numbers too much way up in the 5,6,7 gang calculations.
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Ive tried another approach using a high density PCB with lots and lots of parellel traces... and managed to get it up to 30 Ohm resistance (oh boy): that works out at 0.4 amps or 0.8 amps depending how its wired up (which in turn works out at 4.8W or 9.6W depending on how the PCB is wired)
Reckon that's too low to do the heating?
Aww the PLA's great - prints without heating at all xD (in fact, some say it prints bettwer without heating xD)
Anyways - I'll look into it. I've got a ready supply of stripboard - so doubling up again won't be too heard (though I've not got a lot of space inside.)
I've also got a couple of grip heaters (which I gracefully rejected upon realising they have horrific design and will put a live current through my aluminium bed (I have insulation measures but I'm not putting these anywhere near it: they're from some yung hung low unbranded workshop and have all the engineering precautions of a rock on a stick)
Ganging up additional boards is probably the best option: I'll look into it xD
100x100 is about 1/4 the area of a regular prusa bed. You need about 120 W to heat that so 30W might be adequate. Less than that and you'll likely have trouble with ABS. You would still likely be OK for PLA. You could try making the heated bed double sided if you still want less wattage. Just tape another stripboard to the back and make it twice the resistance. Or a 2.5 ohm 25 W resistor. Or get a bigger supply. :)
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2. Solder the stripboard into a long series-parallel trace. For me it worked well to wire it up in groups of 3. Each trace is about 0.13 Ohm. There are 35 traces and two thick buss traces on each board. So if you wired it up as a single long trace one trace thick it would have about 9 ohms resistance and only generate about 16 watts. So I ganged 3 traces together and wired those in a long trace. Seems to heat up pretty well at 12V. Haven't printed on it yet as I'm still finalizing the Prusa.
3. Measure the resistance. As pictured in the PDF below the total resistance across two boards is 1.2 ohms. Giving 10 amps cold and 120 watts.
4. Calculate the current required.
5. compare to your power supply subtracting the load already in use.
6. If step 4 is less than step 5 cross your fingers and plug it in.
8. Still here? Want a 5V HBP? Try 7 strips ganged together or possibly 8. (at your own risk) A 24 V HBP? Maybe 2 ganged together or even single strip all the way through. None of these configurations is tested except the one described. (And even that I don't know exactly what temp you'll get. About 100 is my guess.)
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