The velleman marlin versions already use on/off for the heatbed.
It isn’t really “analog” but what I think you are looking for is called “pwm” - short for pulse width modulation. Look in Configuration.h of your Marlin firmware for the following line:
//#define PIDTEMPBED
If it is commented out (the line begins with “//”) then bed heater will be 1/0. This is how Velleman configures their firmware, and is suitable for using a relay to switch the bed heater. I think that is what you are asking to do.
If you were to un-comment that line…
#define PIDTEMPBED
…then the digital bed output would pulse rapidly between 1 and 0 to reduce the effective power output to the bed. This is more suitable for use with a solid state bed heater output, for example driven by a MOSFET, but is probably unnecessary even in that case.
[quote=“Dr. Vegetable”]
…then the digital bed output would pulse rapidly between 1 and 0 to reduce the effective power output to the bed. This is more suitable for use with a solid state bed heater output, for example driven by a MOSFET, but is probably unnecessary even in that case.[/quote]
Yes, Dr. is right.
The Heatbed generally doesn’t need PWM control. The thermal mass simply is that high that it would be of no advantage.
[quote=“raby”][quote=“sauntson”][quote=“edirol”]The original ribbon cable goes to the X Support. From there is use a 2x0.75mm2 cable along the cable chain.
Buttom of the X support
Connection to the heatbed[/quote]
Does anyone know if this item is availiable as an STL file. Just adding a upgraded heated bed to my K8200 and this looks like the perfect item for a neat completion[/quote]
I don’t remember where it came from but I still have it and you can download it here[/quote]
That looks awesome! Thanks for the link. I have to print and re-wire as I /scraped away & melted my bed wiring!
Hello all,
I’d like to implement edirol’s solution (as above) with an optocoupled separate PSU. Due to availability of parts I have lying around, I’d like to use a TLP504A as OK1 (datasheet at http://www.farnell.com/datasheets/58098.pdf) and an IRF3205 as Q1 (datasheet at http://www.irf.com/product-info/datasheets/data/irf3205.pdf). Can you guys please help me figure out if my calculations are correct and therefore whether it’s safe to make these changes? I consider myself a beginner when it comes to electrical engineering and I certainly don’t have any formal qualifications 
Here goes:
Part 1. Optocoupler’s LED
LED’s Forward Current I_F maximum is 60mA, recommended typical is 16mA. With 15V from controller at R1 at 1.2k ohm, that would give me 15/1200 = 12.5mA. I’m considering lowering R1 down to 1k to bring it up to 15mA, which is closer to recommended.
Part 2. Optocoupler’s Detector
Collector-emitter voltage V_CEO is maxed at 55V, and we’re not going to exceed 24V in any case so that should be safe. I’m not 100%-sure on the recommended condition of the Supply voltage V_CC of being 24V max – where do optocouplers take their “supply voltage” from? In any case it should be OK, as the R3/R2 thingies should be dropping that 24V to around 10V (if I’m not mistaken – 24V * R2/(R2+R3) = 10.77V). And even if they don’t, 24V is still good (although at the limit).
Emitter-collector voltage V_ECO is only 7V, but I guess that’s for a reverse polarization condition? Not sure what’s that about, but feels safe to ignore.
Finally the most important thingy is the collector current of absolute max 50mA, and recommended max of 10mA. Considering Q1 in a state which would be shorting the gate to GND, the max current there would be limited by R3, giving I_max = 24V / 2.7k ohm = 8.9mA – so just about good. Of course the Q1 would add some resistance there so should be even better.
Part 3. MOSFET
Here’s when I feel even less confident, but will give it a try anyway – what’s the worst that could happen 
V_GS Gate-to-Source Voltage is given as +/- 20V, which is the same as the recommended IRLR8743. I guess the purpose of R3 is to drop it to around 10.8V so should be good.
Continuous Drain Current, I_D, even at 100 deg C and V_GS @ 10V is 80A – assuming the heatbed is 5 ohm (for safety, I know it’s around 6 in reality) the total max I’ll be doing is 24V / 5 ohm = 4.8A. Excellent.
V_(BR)DSS drain-to-source breakdown voltage is 55V, I won’t exceed 24V ever – good.
R_DS(on) at 8 mOhm max is a bit higher than IRLR8743’s excellent 3.1 mOhm so I’ll surely be losing slightly more heat on this. At 4.8A that will be going on there, it looks like the heat dissipated via this will be (4.8 A)^2 * 8 mOhm = 184.32 mW. Without a heatsink (I’ll add a small one anyway), R_{\theta JA} Junction-to-Ambient thermal resistance is 62 deg C / W, which, if I understand correctly, would mean the MOSFET should heat up by 62 * 0.18432 = 11.42 degrees C above ambient. Sounds allright. I’m not considering any switching losses because 1. I don’t know how to, 2. I don’t anticipate any high-frequency switching going on there. Anyway a small heatsink certainly can’t hurt.
V_GS(th) Gate Threshold Voltage. The 2.0-4.0 V min/max from the datasheet looks quite different to 1.35-2.35 V of the IRLR8743. Not sure how that works, but I think as calculated earlier the voltage here will be around 10V, exceeding the threshold, which is what should be turning the thing on. So good.
That’s about all I can think of right now. Please let me know if I omitted anything
Thanks!
EDIT: one last thing: the datasheet for IRF3205 seems to omit the most important thing of all - the pinout Is it standard for all TO-220 MOSFETs, i.e. is it G-D-S left-to-right, as per IRLR8743?
Hi uukgoblin,
I think your electrical analysis is correct. You should be able to substitute the components you have into the circuit.
The emitter-collector voltage (V_ECO == 7v) should be fine for a largely resistive load such as the heat bed. I think reverse biasing would only really be a problem when switching off an inductive load, like a motor. So this voltage is not necessarily limited by your 24v supply, but in this application should pose no problems.
As far as the pinout goes, here is what I found:
el-component.com/transistor- … f3205.html
Good luck with your project. Let us know how you make out!
[quote=“edirol”]
When of interest, I will publish my cable chain solution, but i will have to prepare it first.[/quote]
did you ever publish it?
[quote=“boelle”][quote=“edirol”]
When of interest, I will publish my cable chain solution, but i will have to prepare it first.[/quote]
did you ever publish it?[/quote]
Is this what you are asking about?
[quote=“raby”][quote=“sauntson”][quote=“edirol”]The original ribbon cable goes to the X Support. From there is use a 2x0.75mm2 cable along the cable chain.
Buttom of the X support
Connection to the heatbed[/quote]
Does anyone know if this item is availiable as an STL file. Just adding a upgraded heated bed to my K8200 and this looks like the perfect item for a neat completion[/quote]
I don’t remember where it came from but I still have it and you can download it here[/quote]