Actually the title should read power to the heat bed, make sure it is right! This also applies to the hot end as well.
Continuing my work on the 3d printer, I started checking out the heat run up. Using pronterface I connected to the controller for the printer and then set the hot end temperature to 185° C for ABS and turned it on. Looking at LCD screen and the real time graph on pronterface, I could see the hot end getting hotter. But unfortunately it did not get hotter than 125° C. I searched online for answers. I made some changes to the Configuration.h file for the Marlin settings. Nothing helped. I was going to do an autotune on the hot end, but from the autotunning example I needed to get the heat up to 230° C. So I messed around with it for a while more and still not getting anywhere. So Mary asked if the fans were cooling it off. She was talking about the fan from the PS and the fan I had keeping the controller board cool. I said no. But there was also the fan that was on the heat sink of the hot end assembly. It should be always on. But hmm, let me turn it off to see what happens. Well the hot end temperature climbed higher and higher. Soon it was over 125° C and then over 150° C heading to 230° C.
I was able to run autotune on the hot end since I could get it to go over 230° C.
But unfortunately not having the fan is not correct. I sent a message to E3Ds online support asking about this. While waiting I went on to the heat up test for the heat bed.
I was able to get the heat bed to make it to 60° C without a problem. In the settings for pronterface, 60° C is listed there for PLA. While some people say that you don’t need a heat bed for PLA, many say that’s not true. They say that nothing is better than a heated glass bed. It is the perfect print surface, PLA sticks really well and the parts pop-off as it cools. Just make sure it’s clean and the glass is at 60° C before you start the print. Well, no problem getting the heat bed ready for PLA! Next I try heating the heat bed to 110° C which is the setting for ABS. I get to 90° C without any issues and then the temperature slowly rises. It takes a long time for it to get to 100° C. It never gets higher than 101° C or 102° C. It took 30 minutes or so to get there. That’s not right. So I search on the internet for possible solutions. It all boiled down to what was the voltage that the heat bed was receiving.
So without further ado, let’s introduce the handy dandy digital multimeter (DMM). It slices, dices, and does a whole lot more! Sorry wrong intro, that was for the all purpose Ginsu Knife. 😉
A DMM is very handy while working on electronics. These are the DMMs that I have. The one on the left was about $20 from Amazon and the one on the right was included when I bought a Soldering Station (not the Hakko FX-888D) a few years ago.
But I am going to save my pennies and save my dimes for a brand new Fluke 409! Well there’s no such thing, it just sounded good for some reason. 🙂 But on my Amazon wishlist I have a Fluke 115 Compact True-RMS Digital Multimeter and also an EEVblog Brymen BM235 Digital Multimeter. The Fluke 115 is $132 and the BM235 is $125. Well for the time being I have to make due with what I have.
When I was wiring the power supply (PS) up, I adjusted the trim pot on it to set the output to 12v. The PS is a 12V 30a unit. For use with the Ramps 1.4 board you need at least 16a of power. That is 11a for the heat bed and 5a for the hot end. So this PS has plenty of power to spare. So I set to the task of checking the voltage at various locations on the printer. The voltage coming into the control board was just a bit shy of 12v, which was acceptable. That was for both of the inputs to it. When I turned on the heat bed, then measured the voltage at the output that goes to the heat bed (without the heat bed connected), the voltage was around 11.65v. When I measured it again under a load with the wires of the heat bed connected to the output on the controller, the voltage was 10.5v. The voltage at the heat bed was 10v. Not good at all. These measurements were the same when I used the second controller board that I have. No wonder why the heat bed wasn’t getting hot quick enough. The resistance of the MK2b heat bed was 1.5 Ω, which was more or less an acceptable value. The resistance for both of the thermistors for the printer were around 80K Ω. Which depending on the temperature would be fine. I took measurements of the thermistors later, checking both ohms and temperature. I took two readings for the heat bed which gave me 96.5K Ω at 25.4° C and 94.5K at 26.4° C. I took two reading for the hot end which gave me 98.3K Ω at 25.4° C and 92.5K at 26.4° C. The resistance for a thermistor changes as the temperature changes. They don’t measure an actual temperature but measure the change in the resistance. There is a reference table in Marlin that maps these values.
So what did the internet say about things? Various people said don’t up the voltage on the PS, while others said up the voltage on the PS. Both of the Ramps boards I have were made in China, so how closely they follow the Ramp 1.4 specs is a mystery to me. I upped the output voltage of the PS to 13v. That made a big difference in my output voltages and the time to heat the bed. On load, I was getting 11.75v on the output for the heat bed and around 11.5v at the heat bed itself. I was able to get the heat bed to 110° C in about 25 minute. Which is still too long. From what I have read it should take a little more than 13 minutes. But another issues I have is that the temperature that thermistor for the heat bed measured, it was lower than the value that I got from my hand held infrared thermometer. About 10° C on the average. So when the LCD screen showed that the thermistor reached 110° C, the hand held infrared thermometer showed 121° C. A big difference. Not sure which one is correct. Here is a youtube video about calibrating the the thermistor on your printer’s hot end.
Yesterday I got a reply from E3D and they said:
If it’s the heat sink fan you mean, as opposed to the part cooling fan, then it should be on all the time. There is no reason for that fan to ever be off, so you either have an under-powered heater cartridge, or the heat sink is touching the heater block. If there’s a visible gap between the heat sink and the heater block, then it’s most likely a faulty heater cartridge or the wrong voltage.
You can check by taking a resistance reading. It should read about 6 ohms for 25w and 3.5 ohms for 40w. Much higher, and it’s either a 24v cartridge or a faulty one.
It’s also worth checking your thermistor to make sure you’re not pouring massive amounts of heat into the V6 while your sensor is denying it. It should have a resistance reading of about 100K ohms, at room temperature (25 Celsius).
It was the heat sink fan I was talking about. I checked the resistance reading for the heater cartridge and it was 5 ohms. I have do have a 25w 12v cartridge. The resistance was about 90K Ω at a room temperature of around 24° C. So that is still in the acceptable range.
Reply from E3D:
At 5 ohms, your heater cartridge is slightly more powerful than its rated power. It’s actually closer to 30w. Thermistors have large resistance changes at the lower end of the temperature spectrum (and I meant to write k ohms, not ohms). So your thermistor is correct, if you took the reading in a room that was 27 Celsius. At 20 Celsius, the thermistor should read 125 k ohms, so getting 90 at that temperature would mean it’s out of tolerance.
At around 30 watts, you should easily reach over 200 Celsius. We’ve used 30w heaters to go above 400 Celsius, so there’s something not right with either the hardware set-up, or the electronics. Try heating up, and check that there’s 12v across the heater cartridge, by taking a voltage reading. You can then switch back to ohms to check it’s still at 5.
So I will stop this post here for now. I still have to resolve the heating issues. Looking into getting a replacement Ramps board.
To be continued