Cannot get headbed above 95^c. SOLVED

Hi folks,
at the final stage with the printer, calibrating the heatbed and nozzle.
I'm using a seperate ktype thermocouple to generate some accurate
values and make a new temptable, assuming createtemptable.py is ever
going to work.

I set bed temp to 150 theoretical, the ramps temp is out of wack seriously, and monitor
by holding the ktype thermocouple against the bed. After 15 minutes the highest
the bed gets is ~95^c, actual

My hardware is 12v MK2a, 400w(alledgely, never trust anything on ebay) psu, 4mm Al
print bed.3MM silicone insulating mat under the heatbed.

Settings,
All steppers disabled
HEATER_BED_DUTY_CYCLE_DIVIDER disabled.
Room temp ~25^C

So what am I doing wrong? All suggestions appreciated


Many thanks,
Andrew

UPDATE
Okay, I can get to 105^ to 110^ but it takes 30 - 45 minutes. No quite ideal

Edited 2 time(s). Last edit at 01/10/2016 04:04AM by friarfish.

Check for losses in cables and plugs, you should have 12v on the bed connectors, even a little loss will badly effect heating
If you have losses check your cables to the bed, they should not get warm, and also check the screw terminals are nice and tight (not doing this leads to the terminals melting and catching fire)


Also insulate on top of the bed while heating up (just to speed things up)

400w!!! no they are about 132w (oh power supply, not heated bed... never mind)

If the bed was 400w at 12v its would draw 33amps! Ramps is only designed for 11amps.

Max temp is about 115c, I would not expect it to reach 150c

Edited 2 time(s). Last edit at 01/09/2016 07:06PM by Dust.

Actually, if it's the psu you're referring to (and it appears to be that way) and not the bed, 400w is pretty normal. I agree with Dust, though-- 150c is pretty high, I've never done anything over 90 (but, as said above, it can go a bit higher). Is there any reason you need it to get that high? Because if there is, I would suggest looking into a higher temp bed altogether. Also, you said that after 15 minutes the highest it got to was 95c; did it take all those 15 minutes to get to 95, or is that just the time limit you gave it?

First, check that the power supply is actually delivering 12V to the electronics input. Next, measure how many volts appear on the bed terminals. If there is more than about 0.2V difference, find out where on the electronics the volts are being lost. On RAMPS this is usually either the large square PTC fuse (which can be replaced by an auto fuse) or the bed mosfet (which can be replaced by a better one).

Even with 12V on the bed terminals, PCB heaters are often underpowered for printing ABS. Given that you have already insulated under the bed, the solution is to increase the power supply voltage a little. If your PSU is an LED/CCTV-type supply then it will have a voltage adjustment pot at one end of the terminal block. The snag is that Arduino/RAMPS has an inadequate voltage regulator if you power any external device that draw much current from the 5V rail, such as a servo or a graphical LCD. With either of those, the voltage regulator is probably overheating already, and will overheat even more if you increase the supply voltage. However, if you have no external devices taking power from +5V then you can safely increase the voltage to 14V, which will increase the heating power by 36%. I don't advise more than 14V because the capacitors on RAMPS are generally rated at 16V.

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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].

Hi folks,

To clarify i was initially setting the heatbed to 150 as i knew the thermistor was reporting around 30 to 40^c higher then actual. I was reading the true temp from a k type mounted directly mounted in the middle off the bed. I would adjust the bed thermistor/temp till the k type would report close to the temp i wanted, grab the adc value and use the k type/adc pairs to generate a new thermistor table.

Dc42 was pretty close to the prob. My "high quality regulated 400w" psu isn't.
As soon as the bed would come on the psu would drop 1.5v or more. I've dialled the volts up to 14.5v and the Al bed heats up in around 15min. I'm not worried about frying the rest of the board as i pulled the 4004 diode ages ago and get the 5v from the usb. I know it is close to the trip for the polyfuses and am thinking of upgrading them. Maybe even try going to 24v.

Not to fussed about blowing the FETs. Replaced them ages ago when in early research i saw a lot of poor opinions of the stock ones. Do you think Ids=100A at Vgs=5v was a bit of over kill? :-)

Anybody else experimented with 24v? What are reasonable heat up times for Aluminium beds?

Thanks for all the responses,
Andrew

Glad you solved it. Regardin the mosfet selection, ignore the rated current, that's just a headline to put on the datasheet. The figure to look for is Rds(on) @ Vgs=4.5V and Id larger than the maximum current you want it to carry. That determines how hot the mosfet gets, which is the practical limitation. Lower is better, with 0.01 ohms being the maximum you should accept as it would drop 0.1V and dissipate 1W when passing 10A.

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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].