PCB Heat Bed too Cold

Hello I have a PCB heat bed MK2. I am running it off of a RAMBo board with Repetier Firmware. I am using a 400W server PSU to power the board and heat bed.

My issue is that my bed only heats up to 94.7C, no matter how high I set the temperature. Can this be fixed by changing something in the firmware config? Or what are some other things to look for?

Measure the resistance of your heated bed when disconnected. Some PCB heated beds have too high of resistance, above 1.5 Ohms. This means there isn't enough power for getting above 100*C. The resistance should be less than about 1.2 Ohms.

OK so I tested the resistance and it is right at 1.5 ohms. Is this acceptable or should I get a different heatbed?

jdevmiller Wrote:
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> OK so I tested the resistance and it is right at
> 1.5 ohms. Is this acceptable or should I get a
> different heatbed?

At 1.5 Ohms you'll have trouble getting to 100*C, as that's only 96W of heating power. 1.2 Ohms gives you 120W, which will still take a while to get above 100*C but will do it.

You can try insulating the bottom of the heated bed, which will help, but ideally you'll want to get a new one. Make sure that if you do get a new one that the seller rates it at 1.2 Ohms or less, and that they will replace it if it comes above that.

I can second insulation. Pick up some cork tiles from your local hardware store. Place some aluminum foil over the top (that meets with your heatbed). I bonded mine to cork using Loctite contact adhesive in a spray can. Cut a second piece and put it over your heated bed when warming up. My heated bed has even higher a resistance than yours and I'm now able to hit 100C in 12.5 minutes in a 25C ambient room.

Also, how many amps are available on the 12V line? You'll need at *least* 10A for the bed alone with 1.2 ohms resistance, but better to get at least a 20A supply to be safe given everything else is running off it.

I have a 400W server PSU and the RAMBo has a 15 amp fuse on the heatbed. Not sure what exactly the heatbed can pull but I used a ohms law calculator and apparently it's drawing about 7.5 amps right now.

jdevmiller Wrote:
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> I have a 400W server PSU and the RAMBo has a 15
> amp fuse on the heatbed. Not sure what exactly the
> heatbed can pull but I used a ohms law calculator
> and apparently it's drawing about 7.5 amps right
> now.

The 400W PSU you have probably provides a number of voltages, including +3.3V, +5V, and +12V. Since it supplies power at all of these different voltages, it is difficult to tell how much power (or current) is available at 12V based on the information you've provided. Power supplies tend to have a sticker on the side that states the amount of current available for each voltage output. Would you mind sharing this with us?

You can also use a multimeter and check the voltage on the +12V line when the heated bed is on. If it sags considerably below 12V (say, 11.5V), then there could be an issue of the PSU not supplying enough current at 12V.

I wasted an incredible amount of time trying to figure out why my 400W PSU would not get my heated bed above 95 or 100C or so, even after 30 minutes or more.

I finally figured out that my 400W no-name PSU did NOT actually supply the 20A it claimed to on the 12V rails. I got an Antec Basiq 500W power supply for $48 at Fry's and now I can get the heated bed warmed up within 4-5 minutes.

You can also check your solder connections for the wires and make sure you have either doubled the wires carrying current to the heated bed, or you are using heavier gauge stranded wire. You also don't want unnecessarily long lengths of wire from RAMBO to the heated bed - if you feel the wires getting hot, you either have insufficient wire thickness, too much length, or some combination.

My conclusion is that there are many PSUs out there that do NOT supply the current they claim to on the 12V rails. Most power supplies (if they follow the ATX guidelines) should have TWO 12V rails so to get maximum current from the PSU you need to use both rails. Usually the 20 or 24-pin main board connector has 12V from one of the rails, and the separate 4 or 6-pin Molex connector (for a high-current video card) has 12V from the other rail.

I don't know how the high-current power inputs are wired on the RAMBO, I'm using RAMPS 1.4, but you should be able to figure it out. The main thing I wanted to point out is that not reaching a high enough temperature, or not reaching it quickly, was my number one symptom of an inadequate power supply...