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Heated bed amps

Posted by carmojos 
Heated bed amps
April 08, 2014 12:43AM
Hello reprap gurus,

I was investigating why my heated bed was not holding temp when I discovered that it was only pulling 4.37amps. Not sure what my problem is.

I pulled everything else off the ramps board just in case my PSU was underpowered and got the same results. I also replaced the 10a fuse.

Any ideas?
Re: Heated bed amps
April 08, 2014 05:14AM
Has to be a higher resistance somewhere? Are your bed cables thick enough - not getting hot are they?

Waitaki 3D Printer
Re: Heated bed amps
April 08, 2014 09:14AM
The resistance I am getting is 1.2. I am using double wires (2 at each terminal) and I have not noticed them getting hot. The heated bed came assembled with the wires connected.
Re: Heated bed amps
April 08, 2014 12:24PM
1.2 ohms appears to be a reasonable value for bed resistance.
I'm also assuming the reported bed temperature is a sane value. If it's not, the firmware will prevent heating.

Depending on your firmware settings, I believe there is a PID setting that limits the maximum current going to bed.

It's something along the lines of PID_MAX. You will want to increase this if it's less than 255.
If you have your bed configured for bit bang, then look for something like BITBANG_MAX and increase if less than 255.
Re: Heated bed amps
April 08, 2014 12:35PM
I am using Marlin as the firmware. The BED_MAX_POWER setting is at 256, but I did find the the bed was using PID. Could the wrong PID setting cause the issue?
Re: Heated bed amps
April 08, 2014 11:54PM
Tried using bang bang instead of PID with no luck.
Re: Heated bed amps
April 09, 2014 06:23AM
What kind of result do you get if you connect the heat-bed directly to the power supply?

Waitaki 3D Printer
Re: Heated bed amps
April 09, 2014 03:12PM
By ohm's law, 4.37 amps times 1.2 ohms would mean 5.2 volts. Are you seeing 12V across the heatbed terminals?
Re: Heated bed amps
April 09, 2014 03:14PM
Actually I am only seeing 4.something volts across the terminals.
Re: Heated bed amps
April 09, 2014 08:17PM
When I connect the heated bed directly to the 5volt power I get 4.7amps.
Re: Heated bed amps
April 09, 2014 09:54PM
Ah... most people put 12V through their heated bed. With only 5V you'll get less than 1/2 the amps and less than 1/4 of the power out of a standard PCB board heater.

On the other hand, to get 120W out of a heater at 5V, you'd need 24A, and a resistance of 5V/24A=0.2 Ohms. I don't advise it, but if you want to get experimental, it might be possible to get 100W with what you have, but you should beef up your wiring and system to handle 24A, and then you might be able to turn the parallel double-loop trace of the typical PCB board heater into a parallel 10-loop circuit with some trace cutting and shorting.

Edited 2 time(s). Last edit at 04/09/2014 09:56PM by DaveX.
Re: Heated bed amps
April 09, 2014 10:07PM
I'll have to swap the pins around in the marlin firmware. I hooked up my RAMPS 1.3 as the wiring instructions said to and that puts the heated be on D8 which is only 5volts, but allows for 11Amps.

Any suggestions on how to wire it to 12volts other then changing the pin settings in Marlin and swapping it with the fan wiring?
Re: Heated bed amps
April 09, 2014 11:01PM
I think RAMPS takes 12 V:

1.3: [reprap.org] [reprap.org]


1.4: [reprap.org] and [reprap.org]

Edited 1 time(s). Last edit at 04/09/2014 11:02PM by DaveX.
Re: Heated bed amps
April 09, 2014 11:36PM
As a test I hooked the bed up to the 12volt rail of my power supply going into the RAMPS board. My power supply shorted. The 5volt rail is rated at 16A. the 12 volt is 5A. I'm using an old ATX power supply.
Re: Heated bed amps
April 10, 2014 11:02AM
12 V through a 1.2 ohm heatbed would try to draw 10amps, dissipating 120Watts.

A 12V x 5A power supply is only capable of emitting 60W, which will be insufficient for a 120W heated bed. Your power supply is incapable of putting the full 120W or 10A through the heatbed.

When you hook the 5V x 16A power supply to the 1.2 ohm heated bed, it pulls the 4.37 amps for about 21W. The power supply won't fault because it is within its capacity, but it won't heat the bed up very much.

See [reprap.org] or [reprap.org] or [reprap.org] or

Edited 1 time(s). Last edit at 04/10/2014 11:07AM by DaveX.
Re: Heated bed amps
April 10, 2014 11:54AM
your power supply didn't short - it overloaded. It is probably not rated for the 10 amps that the heated bed draws. What everyone else is saying is true. There is no 5 volt rail on ramps. There IS a 5 AMP rail and there is also an 11 AMP rail. Both are meant to take 12 volts. You'll never get your heat up to the required levels with 5 volts.
Re: Heated bed amps
April 10, 2014 03:14PM
Your PSU will never be able to provide enough power with those specs. Even if you max out the 5v rail, you're only going to get 80 watts out of it which still isn't enough and it would require modding the heatbed. Save yourself endless frustration and get another PSU. Hell, go to a computer repair store and they'll probably give you a broken one. The most common mode of failure with those is the 12v standby which you don't need for reprap anyway. I've been using my trusty "broken" Antec for over a year now and it's still going strong! smiling smiley
Re: Heated bed amps
April 10, 2014 07:20PM
Thanks for all of the guidance on this issue. I had an ANTEC ATX power supply rated at 35 AMPS in my pile of computer parts. Now the bed heats to 60c in about 3 minutes and stays there. I might actually get to start using my 3D printer soon.
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