Help pls. I am currently using a 12V relay to switch my heated bed as the mosfet on my Rumba board can’t handle the 270W POWER of the 300x300 silicone pad I’m using. The relay is triggered by a 5v source I’ve pulled off a spare output on the RUMBA and fed back into the Rumbas HB mosfet. I would like to swap the relay for a mosfet so I can do proper PID on the bed - I’m assuming the relay won’t like me flicking it on and off 20 times a second.
So, because I’m a bit of reluctant student when it comes to electronics I’m a little uncertain of what mosfet I can use to do a direct swap for the relay. Can someone less ignorant help me out?
I am using a BUZ 11. I use it with bang bang tho. Its works well with my 350W silicone pad. I use a BIG heatsink. This link is great place to reashearch transistors, mosfets ect http://alltransistors.com/search.php?search=buz11
The BUZ11 isn’t a great choice if you’re driving it directly with a 5V controller as it is not a logic-level and will therefore dissipate a whole lot of power for nothing.
To do things properly, you can either use a logic-level mosfet or add a simple gate driver - I’ll add two examples here in a bit.
Unfortunately without ordering in i have to use whats available in my area.
I was looking at this as it’s on a nice little shield already.
Well, 20A at 12V is 240W, less than the 270W (22,5A) you’ll be pushing through it. So it’ll probably get really toasty, at least while heating up.
I use the IRFZ44N for pretty much anything as they’re dirt cheap for their performance, and combined with a simple driver like this: Google Workspace Updates: New community features for Google Chat and an update on Currents it’ll handle anything up to the rated 55A (with a small heatsink, that is).
If you don’t want the hassle of having to build a driver, you can always use the logic-level version IRLZ44N - the L designates logic-level compatibility, so if all you can get are IRLZ34N or IRLZ54N, you can use them as well.
I agree with @Thomas_Sanladerer , for our first board we look at the ultimaker board schematic and adopt the STP55NF06L (order by RS components is about 1,5 euros) it can handle 55A and it needs only a diode on the output (probably optional) and a resistor on the gate to be driven directly by the Arduino. if you check the ultimaker board schematic you can see how it is used. I use it with a small heatsink with a 10A and it works perfectly.
Hm, i looked at the STP55NF06L, which is pretty much the standard RepRap transistor as it’s widely used e.g. in the RAMPS, and while it’s not specifically marketed as a logic-level FET, it has all the characteristics of one and seems to offer a solid all-round performance.
Logic-level mosfets require no additional components only a pulldown resistor when driven by a microcontroller. If you have extremely long leads to the load, a freewheeling diode parallel to your load can’t hurt.
Yes, in the design I used, there is a freewheeling diode (an 1N4004 if I remember well) and a 100k between gate and source.
you can easily put a led in parallel to the load to see when the heated bed is driven.
Right, a pulldown resistor should always be used.
O-Kay. So, yes correct, that one I linked to was only 20A so not viable. RS seems to think the IRLZ44N is a discontinued product 
and suggests this instead:
http://newzealand.rs-online.com/web/p/mosfet-transistors/5410086/?origin=PSF_430864|alt
@Tim_Rastall that is the IRLZ44N, or am i missing something?
The STB55NF06L and IRLZ44N… are very similar transistors (the IRLZ might have slightly favorable transfer characteristics). Both will need a compact heatsink.
@Thomas_Sanladerer I dunno - this one has ‘NPBF’ on the end but it looks to have the same characteristics. So, ill get one of them. I already have 100K resistors and heatsinks. What about diodes? RS has stacks of IN4004 flavours… http://newzealand.rs-online.com/web/c/?searchTerm=1N4004+&sra=oss&r=t
Apologies for my ignorance - I studied mechanical engineering and avoided electrical like the plague - bad idea in hindsight.
Try look the schematic (I hope the link works)
The diode act as a freewheel to counteract current coming from inductive load such as the long wires to the bed (the bed itself is resistive) as Thomas said. I think quite any diode will work also 1N4007 etc… A thing to be careful (if I remember from my errors) is that the heatsink is not GND, so watch out for short circuit touching the heatsink to ground.
The NPBF suffix denotes technological details, like qualification for lead-free products, halogen content and, in other cases, the chip’s package. Since IR only uses TO-220 for these chips, there’s not much relevant information in that suffix for the hobbyist.
So, since you’re ordering anyways, i’d use a diode that’s a bit beefier, like this one: http://newzealand.rs-online.com/web/p/rectifier-schottky-diodes/7484979/ EDIT: G’nah, the RS site is horrible to use. I suppose you need one, not fifty, but any 3A diode should be fine.
The 1N400x are rated for 30A non-repetitive forward current, which you’d be awfully close to with your bed. That being said, i’m not even sure you’d really need the diode, but it’s just good practice to throw one in there anyways.
Why not trigger the relay with the RUMBA’s heated bed output? It’s at the power supply voltage, and the relay won’t draw too much current.
That’s what I’m doing on mine, although my output is to a 120volt solid state relay hooked up to an AC bucket heater 
@Tim_Rastall honestly I recommend you use a solid state relay (can handle pid) hooked directly to the 12v heated bed output. Works well with ramps 1.4 they come cheap on eBay. They even have through hole solderable ones that can push mains at over 30a kiss keep it simple stupid. The most convenient efficent way is usually best.
@Paul_Chase uh, that’s what I’m doing at present except with a mechanical relay.
@D_Rob there’s not much in it between solid state relay and mosfet really. I’ll have to put the mosfet on a bread board and do 5 mins soldering but it’ll cost much less than an ss relay.
@Tim_Rastall in the op it appears you were using a 5v power rail. That probably contributed to Paul’s not thinking you were triggering the relay with the 12v bed connection. And although you are using a relay, solid state is required for pid.