Heat bed problem (tried everything)

Hello,

I have a prusa i3 8'' bought from makerfarm.
All the prints are in ABS. I got Ramps 1.4, atx power supply 500 watt (20 amps on yellow wire, i got 2 in each slot).
I got a problem since the beggining; i couldnt heat the bed very much, just between 85-93... But sometimes the temp stays at 65 other at 70 it's just strange. (in ºC)
I had a similiar problem with the hot end, in the first 4/5 prints the temp went to 220-230, but after that the temp stoped at 190, other times 170, other 200, it depend!
I thought it was the office, but i have all doors closed and without conditioned air. The temp in the office is arround 23ºC.
On the hot end, i put some insulation and now it goes fine up to 230 ºC. I tried the same with the hot bed, i put insulation used in ovens between the wood and the pcb and the max i can get after like 1 hour is 93ºC.
I don´t know what to do more, i checked all the wires many times and the thermistor. I bought a laser termic sensor to have sure the temperature is the same and it is.
I dont know what can i do more. My next step is to solder an extra wire to the bed and connect to the RAMPS.
This could be a problem with RAMPS or heat bed, but the thing is if i sent it again to USA it will take 2+ months, the shipping costs are huge and i will have to pay costume services again (i am from Portugal).
I really need to try fix the problem in here, anyone can help?

just a thought but is the ATX power supply modded with a load resistor? See this page for a description. I guess its possible that your power supply could be having problems without the load and shutting down periodically?

I use a modded ATX power supply with no issues in getting high temperatures.

as I say, just a thought...

thank you for your repply! I tried with 3 different a power, 2 from computers i have and other that i bought. But the temp issue is always there!
I solder 2 more wires from the heat bed to ramps and the problem is still there any ideas?

well depending on the age of the ATX power supplies from the machines you have, they could all exhibit the same problem (its more likely in older ATX power supplies).
Does the LED on the RAMPS that indicates the bed is heating stay on or is it switching on and off? If its switching off, it might suggest that there is a problem with your thermistor - (i.e. RAMPS is reading that its up to temp when it isn't and turning off the power). If the light stays on but the bed doesn't get hot - start looking at the power supply. Maybe measure the current with a meter and see if its actually delivering current when RAMPS asks it to.

One power supply is new with 500 watt and 20 amp on yellow. I connected 2 yellows and 2 blacks in each slot in ramps.
The led in ramps is always on. The max temp i ever got from it were 93ºC, never passes this.
Can be a problem with ramps or bed?
You are saying to measure current in yellow and back wires from the power supply to ramps?

Edit: the hot end heats nice now; i am just wondering but if the power supply can give current to heat until 230 ºC it can give to the bed to reach 110 ºC no?

Edited 1 time(s). Last edit at 09/12/2013 10:01AM by freskhu.

hi freskhu,

the point of the article is that regardless of the age or potential current abilities of your ATX power supply, sometimes they are unable to provide the power you need on the 'yellow' 12V wires as there is an expectation of a load on the other wires which, if not present, can limit or halt the current on the yellow wires - hence the mod the article talks about. Some supplies do this, some don't. If you haven't modded the supply or if your kit (if it is a kit) doesn't provide a load its hard to tell if thats the problem without measuring the current when your RAMPS led says the heater is on.

The hot end would be on at the same time as the bed so its hard to say that if it can do one, it can do the other as the total current is cumulative. You could have a problem with the bed itself I guess - but I'd start by measuring the current being drawn by the bed.

I did this,

-> disconnected the 11 amps wires from the ramps and connected directly to the heat bed, result, nothing happened.
-> disconnected the 11 amps wires from the ramps and connected the bed to other power supply directly, result, the bed started to heat! i Cant wait to see if this will go up to 110, only tomorrow, but probably it will so can be a problem with a brand new power supply!

More news... I got an old power supply, connected it to the heat bed and it reached 100ºC in like 10-15 min and 109 in like 20 min.. So probably or is the power supply or the ramps.
I will connect this old power supply to ramps ans check if the bed heats.
I will let you know.

I connected to the ramps and it heats upp to 110ºC, but when printing the first layer i setup to 90 and the others to 110ºc.The thing is that the bed never leaves the 90ºC. Any idea?

I think normally you'd want to start at 110 ( for good adhesion to the bed) then reduce to 90 or whatever you want.

what are you trying to print with ABS or PLA ? In pronterface you can manually adjust the temp during the print

Good luck