Switching heat bed

Hi,
I have old rapman 3.1 for which I build heated bed. The firmware allow to connect the heat bed to I/O of extruder 3. I have beed alble to connect the thermistor and the temperature measuring is working but problem is power. The one-board electro,ics has 12v output for extruder heater which of course is not enought for heatbed. I need to connect external 12v power supply. Is it doable to connect switching transistor on output of another (smaller) switching transistor to somehow proportionaly switch the load?

Thanks.

Yes. One transistor can drive another although you may find it easier to integrate a solid state relay which is essentially the same thing in a tidy package.

Note that these MOSFETs (not exactly Transistors) don't switch 12V, but 0V. Heater turned off means you have 12V on both pins, not 0V.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Hello,

I have about the same problem with my rapman 3.2
I'm using I/O 1 & 2 for both extruders.
I just got me an MK3 ALU Heatbed, but i'm not sure how to connect it.

When configuring printer settings in Axon software, the RapMan 3.2 offers only choice for 2 extruders.
To get the 3rd extruder in the printer options, i need to choose a diff printer, which has not the same printzone as mine.
So how did you get it to work? Is the I/O active even when RapMan 3.1 or 3.2 is chosen in the printer options?

I do have a spare driver to connect to I/O 3. I also got a 100kOhm thermistor (MGB18-100K) and a MOSFET (IRFZ244N).
To power the bed i have a PSU of a computer, +12VDC / 60W output.

Could you please assist me on getting this to work?
Do you have a schematic on how to connect the whole thing?

Any help will be much appreciated.

Greetings

Quote
Traumflug
Note that these MOSFETs (not exactly Transistors)

I forget, what does the T in MOSFET stand for again?

Traditional transitors, or BJT or newer IGBT are devices which take input of current, meaning their function is current allowed at CE is = gain * Ibase. So are current devices, their output depends on a current input.
FETs input is purely voltage and their function is current allowed trough drain to source iDS = K/2 * (Vgs-Vth)^2. This function is now a curve and this has many implications. More important these are voltage devices, their output depends on a voltage input.
So imo traditional transistors and mosfets are quite fundamentally different.

Quote

So imo traditional transistors and mosfets are quite fundamentally different.

I didn't say they weren't different. BJT, IGBT, and MOSFET all have a T in their name, because they are all still transistors. A transistor at it's basic definition is a device that amplifies and/or switches the flow of current. It doesn't state that the amplification or switching is based on current or voltage, either can be used.

You don't say a delta printer isn't a 3D printer just because it operates differently than the original cartesian printers. They are all still 3D printers, just like BJTs and MOSFETs are all still transistors. They just operate differently to ultimately fulfill the same end task.

BJTs can work good as amplifiers because their function is linear, the only factor is gain and it has no exponent, and this function is suitable for amplification. So traditional transistors replaced lamps because they had same linearity sort of speaking. But mosfets can not do that on same level. One of the implications of the curve function of mosfets is that it can not work as amplifier of large signal. Because the function is curved (coz of the exponent) it distorts the amplification. Draw a graph with xoy and a curved function and see that a certain variation on X produces different outputs on Y, depending where the X variation takes place. Meaning some levels get more amplifications, others less so, which means amplification distorts the original signal. So when we want amplification of a signal, we want it to keep the original signal integrity, but only to amplify it across the board, amplification has to be same at all levels, which means we do not want a curved function, but a linear one instead.

Maybe sometimes are used as amplifiers but only on *small signal* where it can ignore the curved function distortion on the account of it being small because the signal variation was small to start with. So when are used as amplifiers its small signal only, and where application would accept *some* level of distortion. So generally speaking mosfets are not used as ampifiers, again except small signal amplification but thats rather rare - as a matter of opinion, my own.

What i see largely used as "definition" its current device or voltage device, depending on input type. That matters because our uC are voltage level outputs so uC outputs voltage, therefore when pairing devices we want to pair it also with a device which takes voltage for input, so they will speak the same "language". Say an uC output of 5v goes to a transistor base, but the base would require 50mA to get to the operating point, which the uC may not provide, that would be a case where the two devices speak different language. So if we need a switch to a uC pin, better to use a mosfet because they can "understand" each other better.

Otherwise to know what a device is, have to know its elementary function like that, and check the implications of it. Any device has an some input and some output, and "a device that amplifies and/or switches the flow of current" thats some unprofessional generalization that could actually suit almost any device out there. Including my wall mounted light switches, some old crt monitors or closer to the subject, old school vacuum tube, which are not transistors by any means, they were actually the devices which the transistors replaced. And there are more similarities between tubes and bjts in that their elementary function are almost the same, except tubes are still better for amplification so thats why a class A+ audio amplifier still has to use vacuum tube specifically for the amplification function. If it uses a bjt for amplification function, then its less than class A, it may be class D audio amplifier instead.

I'm not sure what you're going on about. Traumflug said "Note that these MOSFETs (not exactly Transistors)...". MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. Transistor is in the fricking name so no one can say that they are not exactly transistors. BJT also have transistors in the name. They serve very different purposes, but they are TRANSISTORS.

We're using them as switches. We don't care about amplification and gain and distortion. It's essentially irrelevant when it's either on or off.

And I'd also challenge your assertation that mosfets are only used as amplifiers with small signals. Amazon has 5,600+ examples of not-small-signals using MOSFETs. They're a different category of MOSFETs that what we use, but they are still MOSFETS none the less.

Sry, im not trying to win an argument, and there is nothing for me to win here. I was trying to be helpful and provide help where i think its needed, for what i consider a misconception about a label word. Imo yes mosfets have "transistor" in their name, but other than that, imo are quite different devices by any of the important considerations. Who knows, maybe the creators could not figure out a better name thats why they came up with the acronym. There are many situations of same word used in similar contexts even if not much related. For me labels and words are not important, the inner details are. Its perhaps a matter of opinion, but for me at least, these are different enough that the phrase "mosfets are not exactly transistors" is very very true, wanted to make others see that, so i tried to do just that. You dont have to agree, you can consider different, so ok you are right, and cheers.