General Question About Hot End Cartridge Heaters

[gmay3]

Hey guys!

I think the ceramic cartridge heaters that are being used in place of resistors for hotends are pretty neat. I just want to try and understand them a little more and haven't been able to find info or datasheets on them.

I have a question about their operation. From what I gather from a few web searches, it seems like they are ran at 12V and the temperature is controlled by changing PWM values which increase the percentage of current it receives and using a thermistor to regulate the PWM to maintain temperature.

What happens if you run it at 12VDC and dial the current to a certain value with a potentiometer to set the heater cartridge at a fixed temperature?

Why we would prefer to depend on a thermistor to regulate the temperature instead of setting it to a fixed current and temperature?

[Mjolinor]

Because the amount of heat it needs varies with the amount of plastic it has to melt. If you fix the voltage and current then it will cool when you extrude and overheat when you don't. The amount of heat needed will also vary with distance from the table if you have a heated bed and with the size fo the print, the heat loss form the printer and any draughts you may have. It has to be an active feedback system preferably using a PID type controller to maintain the temperature required.

You can do what you suggest for testing things as I do when I am messing with such things.

[printbus]

Having a print cooling fan is another significant variable in how much power needs to be applied to the heater to maintain the nozzle at temperature. I've seen the temp drop a couple of degrees for a short period when my print cooler kicks until the automatic loop can compensate for it.

The cartridge heaters are essentially just wire-wound resistors.

gmay, in your scenario are you proposing having a potentiometer between the 12V source and the heater? That leads to a pretty high power potentiometer because of the current involved. It'd have to be something like a 15-watt 8-ohm speaker L-pad control. Feeding a potentiometer setting into an analog circuit that drives the heater would push the power dissipation into the drive circuit, not eliminate it. PWM is used because it is easy for the AVR controllers to deal with, and the only power dissipation you have is from the on-resistance of the transistor or FET being used as the on/off switch applying power to the heater.

[gmay3]

Thanks guys, that's very interesting and I understand now why there is a need for PID control with the thermistor. I really appreciate your responses! I'm trying to go back in reprap history a bit to make sure I understand what I have under the hood!

Some specs I've seen floating around characterize these as 12VDC 40W cartridges, so 12V/40W = a 3.6 ohm resistor? This means to me that you can input up to but not exceed 12VDC and there maximum power limitation is 40W.

Since they essentially just wire-wound resistors could you also put in 6VDC to lower the temperature? Is there a reason they are spec'd at 12VDC or is this just for reprap convenience? I'm guessing the power dissipation has a linear relationship to the temperature of the cartridge heater just like a resistor?

[Mjolinor]

They come in all sorts of voltage and wattage ratings. Generally it is safe to use a lower voltage if you are controlling the current but as the resistance generally goes up as the temperature goes up you need to be careful running them at lower temperatures, the VI curve will not be linear.

My Wanhao uses 24 volt ones, my Stratasys uses 110 volt ones.

[gmay3]

Ok thanks for that info Mjolinor! So is it reasonable to expect that if I have it set it to a certain power dissipation level and the resistance has stabilized at that certain temperature (running for a few minutes), it should stay at a constant temperature?