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make my heated bed more efficient

Posted by GITRDUN 
make my heated bed more efficient
September 23, 2014 08:56PM
Ive been printing and building repraps about 2 years now. Only used PLA up til now due its simplicity with not requiring a heated bed. So wanting to switch to ABS i finally installed my mkII heated bed, wired it all up and it cant get over 75*C, and takes forever to even get that hot. My power supply is rated at 24amps on the +12V , the power wires from the supply are not even warm, the +12V measured at the heated bed is measuring 11V, at the connection to the ramps board i get about 11.5V. Wires to the heated bed arent warm either. I have the heated plate mounted on a 3/16" thick plate of particle board and have 1/4" thick glass plate on top of the heated plate. The temp sensor is taped to the top of the heated plate. I tried it with the heated plate facing up and also facing down. Also have a resistor wired to the +5V wire on the power supply.

So is my power supply to weak? What am i missing? Im surprised its dropping a full volt at half of its max amp draw. Should i put a layer of thermal tape or something similar in between the heated plate and the particle board under it?
Re: make my heated bed more efficient
September 24, 2014 03:41AM
1/4" thick glass? Wow!
That's probably double the thickness that you really need - dunno whether all that mass is contributing to heat loss but it's very unusual. Also, I would prefer a beefier power supply, maybe 400W to give you a comfortable overhead.

Waitaki 3D Printer
Re: make my heated bed more efficient
September 24, 2014 07:55AM
I'm printing on a 3mm aluminum plate and attached the heated bed directly against this aluminum plate on the bottom.

Underneath that there is isolation material so all the heat that gets radiated out on the bottom of the heatbed will be confined and reflected back up so there is minimal loss on the bottom. The Sides could be more closed, but I found that hard to achieve with my current heatbed design.

On top of the 3mm alu plate there is sticky kapton and that's all. Works great, however, the wires do get quite warm, so if yours don't, then maybe the PSU doesn't output enough amps?

I'm not familiar with measuring amps as it's not doable with a standard multimeter as those only tend to go up to 10A max. Also, I believe it should be measured in parallel instead of serial, but I'm not sure, dig into it.

With this setup I have two printers running, one with ABS and one with PLA, the ABS one takes about 10 mins to get to ~110ºC and the PLA printer takes only about a minute to reach the 60ºC. The PLA printer has the better isolated version of the heatbed including thicker gauge wires and thicker aluminum plates on top and underneath.

Re: make my heated bed more efficient
September 24, 2014 08:07AM
I was mistaking, my power supply is a 500W Inland rated at 28amps on the +12v. I can get a thinner piece of galss easily, ill start with that and putting some muffler tape or something under the heated plate.
Re: make my heated bed more efficient
September 24, 2014 03:24PM
My bed heater is home made, with nichrome wires, 24 gauge, about 350 inch, at 120VAC, its about 300watts. Since there is no power supply, its hooked up directly to the wall with a relay in between. I calculated that the max temp should be at 250-270°C, so it is fine wiring it with kapton tape. i did install muffler tape as well because i had a problem once when the thermistor got off the plate, my particle board started giving off smoke... hot smiley i was there at the time but it could've been much worse. i think i might look for a thermal switch as a safeguard as well. you can never be too carefull. smiling bouncing smiley
Re: make my heated bed more efficient
September 26, 2014 03:35PM
Hate to say it, but glass is an excellent heat-insulator. I'd stongly suggest you look at aluminum etc.

Also, building a box around the entire machine will assist by confining the heat being generated to a small local area. It will also make your printed items stronger as the melting and cooling rates can be better controlled.
Re: make my heated bed more efficient
September 26, 2014 04:59PM
the +12V measured at the heated bed is measuring 11V
Where and how are you measuring that 11V? You shouldn't be seeing a 1 volt drop if you are measuring between the supply and ground wire at the heated bed. If that's what you're doing, then your power supply isn't keeping up with what it should be regulated at.

Also what is the resistance of your bed by measuring the two leads that plug into your controller (or relay or however else you switch the bed on and off).
Re: make my heated bed more efficient
September 28, 2014 05:45PM
I was measuring the voltage at the wires where they are soldered to the PCB heated plate. And i was mistaking about it being a mark II, it is a mark I heated PCB plate.

I tried switching to another 500W power supply and got exactly the same results. I connected the heated plate to the ramps using 10gauge wire just to rule that out and same result. None of the wires are getting hot, nor the wires from the power supply. I also put aluminum foil beneeth the heated plate, same result. Took the glass plate off and started measuring directly on top of the heated plate and i did get a 20*C rise in temp up to 90*. But once i turn on the printing nozzle to print the bed temp cant get higher than 60*. So something just aint right.

I will measure the resistance on the bed and see what it checks at. If it had to much resistance i would expect the power wires to heat up. Might also just wire it direct to the power supply instead of going through the ramps board and see what i get.

Wouldnt it only need half the amp draw to run it at 24V rather than 12V? Unless im mistaken the only thing that could be damaged from running the bed at 24V would be the LED's.

Edited 1 time(s). Last edit at 09/28/2014 05:47PM by GITRDUN.
Re: make my heated bed more efficient
September 29, 2014 05:33AM
Did you use thick cables from PSU to controller, too? To measure the voltage drop, compare voltage at the bed with that directly at the PSU. Or with that on an unused string in case you have a PC PSU.

Running a 12 V heated bed at 24 V doubles current, so you have 4 times the power requirement ... and need even thicker cables.

Generation 7 Electronics Teacup Firmware RepRap DIY
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