Watch out for the RAMPS Heatbed Transistor!

Discussions about the electronics
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dognotdog
Posts: 181
Joined: Tue Sep 04, 2012 9:22 pm

Watch out for the RAMPS Heatbed Transistor!

Post by dognotdog »

I've been running a lot of prints over the past few days with the new heatbed, but the old electronics (Arduino/RAMPS), and the heatbed transistor died a gruesome death. Very weird, too as, it can be basically turned on once after power-on, and then it stays on, with no regulation.

Alas, it is an STP55N, and seemingly the v2.0 heatbed was too much for it. The RAMPS board has no heatsink for the FETs, which is a major problem with the STP55N, as it's not quite fully open at 5V, so sinks a significant amount of power.

I replaced it with an IRF3808. In theory, that transistor is better suited for this setup. The STP55N is rated at 15A for a 5V gate voltage, which is with a heatsink, so without a heatsink it can't really sustain the 10A without overheating. The IRF3808 has much better specs, so it should drive the heatbed without a sweat. We'll see how it does in the long run.

Long story short, if you run the new heatbed with the RAMPS board, I highly recommend replacing the heatbed transistor with a better one.

Sündi
Posts: 16
Joined: Fri Oct 12, 2012 7:32 pm

Post by Sündi »

Oh yes I think this Transistor is at the boarder to death. With or without the new heated bed. I recocnized this problem and solved it with a SMD Transistor witch is not nice to mount but at the moment with 10A it is getting hand warm at 10A. :-)

satoer
Posts: 436
Joined: Mon Oct 01, 2012 8:00 pm
Location: Netherlands

Post by satoer »

Even with the old resistor heated bed it became way to hot for my liking. A wet finger made a sizzling sound. I've attached a piece of metal to it:
Image
Stay's cool this way.

Now I'm using the new heatbed, and so far no problems. I just started a ABS print with a 75 degrees heatbed. Lets see if it survives :)

If you attach a heat sink yourself: Make sure the metal parts do not touch the other mosfets! If you do the correspondent attach device to that mosfet will activate.
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dognotdog
Posts: 181
Joined: Tue Sep 04, 2012 9:22 pm

Post by dognotdog »

Attaching a heatsink is almost futile, due to the way the transistor is placed on the board. Not for the faint of heart, as you have to take good care the heatsink doesn't touch any other metal parts. I decided against it when I got the printer, as I figured I'd just short it to something important at an inopportune moment.

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