Hey guys, how do I check the power of my heated bed??
I was never able to make the bed reach proper ABS print temperature (over 100º), just recently found out that my power supply had to low amperage and changed it for a 24A (each 12v channel, total of 48A) power supply.
Although I’m now able to maintain stable 85º (and print faster :D), it steel far away from the proper temperature.
The only thing I can think of now is the cheap chinese heated bed.
What do you think?
You might try running an autotune on the heated bed to get new PID values. Bad PID’s can keep the bed from heating up.
There is a relation between wattage and maximum temperature. Every heated bed dissipates energy in the form of heat. If the bed has not enough power in relation to the size of the heat dissipating surface, you can’t get it above a certain temperature. So it all depends on the wattage, and how big the surface is.
One way to overcome this, is isolating the bottom-side of your heated bed. another way is to increase the wattage of the heater. With 12 V DC there are certainly limits.
The wattage is the voltage times the current, and the current is the voltage divided by the resistance.
Suppose you would need 200 Watts, your current should be 200 divided by 12, which results in 16.7 Amps. The resistance should be 12 divided by 16.7, which comes to 0.72 Ohms. This is a very low value.
If you would switch to a mains supply, lets say 110Volts, the total resistance for 200 Watts comes to 60 ohms at 1.8 Amps.
For 230 Volts, it comes to 264 Ohms, at 0.87 Amps, which is far easier to accomplish.
So my advise is to isolate the bed as much as possible, use thick wires, or switch to a completely different type of heater, connected to mains power trough a solid state relay.
If you go for the mains option, be sure to ground your heated bed and the rest of your printer properly. Mains power kills.
I have the same issue with my heated bed. I can get it to 120C initially by putting a piece of cardboard over it. It takes about 45 minutes though. As soon as I take the cardboard off the temp rapidly drops. When my cooling fan turns on the temp goes down even further and is around 75 C during printing. There are a few things I have tried that has helped but the best is cover the print plate with elmers max glue stick. ABS sticks really well to it and most prints won’t release until the bed temps drop below 60C. I also cover my printer with a garbage bag to help keep out drafts. I have not tried some of the recomendations above but maybe this weekend I’ll give a few of them a try.
This may be a bit of a digression, but here goes…
The fastest way to heat up the bed is to pre - warm it with a microwave heat pack that has been heated in a microwave for about 3 minutes - much longer than they are meant to be heated for.
WARNING: The heat pack put in the microwave for 3 minutes will be too hot to touch with your hand - it will likey burn you. I wrap mine in a plastic bag and carry the bag from the top.
Also, the hot heat pack tends to give off steam, so tying a knot in the bag is essential to prevent lots of moisture condensing on the bed.
Put the plastic bag with the hot heat pack directly on the bed, and within a minute the bed will be very hot. Leave it there for about five minutes for a thoroughly heated bed.
My Printrbot Simple Metal, with a 3mm solid steel bed, heats up to nearly 80 degrees Celsius on the top, and the bottom is also very warm. The steel has a large thermal mass so it stays pretty warm for a while.
This is what I call my “poor man’s” heated bed ;-).
I don’t have a real heated bed, but this method should also apply to one by giving it a head start - significantly reducing the time to reach starting temperature.
Because beds, heat packs and microwave ovens are all different, results may vary.
Good luck!
Several good recommendations already. I’m assuming that you are using rerap heated bed. I’ll throw in insulation between the bottom of the heated bed and the bed carrier and kapton tape to keep out drafts . I use oven insulation. Maybe you know where ovens go to die around you? Also, ABS likes a heated chamber. Cardboard is a better insulator then a plastic bag. Can you enclose the print area and keep the control electronics out? Around 45 deg C works for me. depending on room temp you may need to play with how much air you to let circulate through print chamber. If you get the print chamber up to 45 then you will need to lower your nozzle temp. I’m trying to decide if i need a circulation fan in my chamber to avoid stratification.
Ok! So many suggestions in such short time, thanks.
@Fred_Baumgartner I don’t know how to do that but I’ll check it out.
@Mark_Rehorst that was exactly my question, how to check that? Mine is one of those generic MK2 heated beds, I’m assuming now that it is a low wattage one.
@Rieni_Puspita I’m a computer engineer so I do understand the principles. If what you say is correct, everyone using a RAMPS board would have the same problem as I"m having and I would have found A LOT of posts about it. There must be something else happening here.
@Travis_Cage I’m experiencing that EXACT problem. Insulating the bed was fairly intuitive, I’m using fiberglass tape. And for sticking the prints I use hair spray. But I really can’t print anything box shaped or very slim.
@Mark_Rehorst 11.14V x 0,28A = 3,12W
That seams really really low…there must be something wrong with my measures or the hardware…or the firmware…
Is there any advanced configuration on Marlin for power distribution???
@Andre_Frazatto about 12 volts and 0.28 amps means approx. 40 ohms of resistance in the heater. That’s an unusually high resistance. Something seems to be way off with that heater based on your measurements.
I run the auto PID calibration and it did improved a little, temperature raises faster now. But the returned values are way way off.
DEFAULT_bedKp 498.06
DEFAULT_bedKi 82.58
DEFAULT_bedKd 718.16
I guess all this data points to the cables as the bad guy…which kind of cable and size are you using?
0.5 ohms
Now I’m completely puzzled.
It means the bed should be working at 249,53w, which is good!
I’ll have to try and measure the current again later.
@Andre_Frazatto you have metered the heater at 0.5 Ohms, now just to be sure that it is OK, remove it from the controller circuit and direct-connect it to a DC 12 V supply that is capable of at least 22 Amps.
If the heater gets hot really quick (it should be quick dissipating a 1/4 of a kilowatt!) then that’s some good news because it is working just fine. If it stays cool… there’s the problem.
But if your heater is fine, then you need to check the controller software (PID etc) and hardware (MOSFET etc).
Note: The controller is only going to give the heater 12 V (approx) or 0 V - it isn’t capable of any other voltage in between, like 6 volts. This is because the controller output is actually digital, not analog. At 50% power the controller is switching 12 V for a short length of time, followed by 0 V for the same length of time. It can’t actually deliver 6 V at 50% power, but from the point of view of the heater, it’s just the same effect as 6 V. This is called pulse-width modulation (PWM).
Maybe your meter has picked up the peak voltage from the controller output - about 12 V?
To really see what the controller is doing to the heater, you will need an oscilloscope.
But a very crude way to read what proportion of 12 V is being sent into the heater is to put a capacitor across the heater output. The capacitor will roughly average the PWM voltage, allowing a simple DC volt meter to be used the see what percentage of 12 V the PWM is delivering at any given moment.
Sorry sorry, I’m making a mess here. Back to Circuits 101:
This revised:
There fore the bed would be working at 9,6W. That is pretty low…again.
I think this conversation is going no where.
@Paul_Gross I tend to agree with you. Marlin configuration.h says so and it would explain this weird results I’m getting from direct measure.
Maybe we could go the other way around. @Travis_Cage you have the exact same behavior on your printer, can you help validate my results?
Measure the resistance of your wires please.
@Andre_Frazatto I don’t know how you are measuring current and voltage, but most multimeters cannot properly measure the output of a PWM.
You can do it if you have a special multimeter with a PWM mode or frequency mode, such as this one:
The DC voltage and current settings of a standard multimeter will likely give you wrong readings if you use it to measure a PWM output.
If you don’t have an oscilloscope, and don’t want to invest in a fancy fluke multimeter, the are a few guides you can read that show you how to make a DIY oscilloscope from and Arduino:
@Paul_Gross Everything is really expensive here in Brasil 
Even an arduino is out of my budget right now.
Lets see if @Travis_Cage can help us a little and I expect we will get to the bottom of it all.
I tested the bed connected straight to the power supply as someone suggested and it’s no better. Temperature still raises slowly.
Is it possible that my bed is meant for 24v operation? How can I test this (without burning anything)?
@Andre_Frazatto in your case I would use the simple way and put an electrolytic capacitor between the controller output to the heater, and ground.
The capacitor will turn the PWM 12 V square wave pulses into a fairly good DC voltage, proportional to the duty cycle, and it will be readable by a DC multimeter. Thus when the controller output is at 50%, you should read 6 V, 75% will be 9 V etc.
Make sure that the capacitor’s negative lead is grounded, not the other way!
You can read here about using the capacitor technique to smooth PWM output to a DC motor. The same principle applies to your heater:
http://forum.arduino.cc/index.php?topic=19674.0
Good luck!
@Andre_Frazatto … and a simple test for 24 V, get a truck battery (24 V) or put two car batteries (12 V) in series and put them across the heater. For just a short time you won’t do any harm to the heater.
@Mark_Rehorst you have just described the effect of a correctly tuned PID controller that uses PWM.
It is possible that a PID controller that is very poorly tuned, particularly an unsuitable Proportional parameter, will not demand 100% heating on startup, in which case the output will be pulsed even when the heater is at room temperature.
I’ll take a resistance measurement of my wires tonight after work and let you know what I get. As I recall the voltage measurement at my ramps board was about 10 V the measurement at the heated bed connection was about the same. The ramps seems to be causing a loss of 2 V for me. I have planned to directly power my bed and use a mosfet to allow the ramps board to control the temp. I never got around to implementing that though.
Guys, I think this answer your debate, found on configuration.h:
// This sets the max power delivered to the bed, and replaces the HEATER_BED_DUTY_CYCLE_DIVIDER option.
// all forms of bed control obey this (PID, bang-bang, bang-bang with hysteresis)
// setting this to anything other than 255 enables a form of PWM to the bed just like HEATER_BED_DUTY_CYCLE_DIVIDER did,
// so you shouldn’t use it unless you are OK with PWM on your bed. (see the comment on enabling PIDTEMPBED)
#define MAX_BED_POWER 255 // limits duty cycle to bed; 255=full current
You can use PWM, but I’m not 