RAMPS 1.4 & Mosfet Relay (Wanhao i3)

Hi all, I have a Maker Select v2 (Wanhao i3) that had its Melzi board go bad (connector issue). I opted to upgrade/sidegrade to a RAMPS 1.4 board. The board I got came with a bad on board Mosfet so I opted to use a relay to circumvent it. The good news is that the relay does in fact power on the bed and there is no smoke/burning from the RAMPS board itself. However, the moment I power on my printer the bed immediately starts to heat. I am unsure just why this is happening. I have tried multiple external Mosfets (I bought two) and each have the same issue. I seemingly have everything wired properly, but I'm not sure why this issue is happening.

Any thoughts or ideas on things I should try?

EDIT: Wanted to add that I am running a recent version of the Marlin firmware.

Here are some pics of how I've got my stuff wired. I also provided a picture of the unwired mosfet so you can see what it looks like.

Imgur Album: [imgur.com]





Edited 1 time(s). Last edit at 02/23/2017 08:03PM by Wuzseen.

One of 2 things: your relay is bad or the control signal is always on. I'd check number one first by seeing if it conducts with the control voltage low, if it doesn't the control signal must be constantly positive. That would indicate either that the RAMPS is faulty or you wired it to an always-on positive signal instead of to the bed control. Not sure which it might be without more info on your wiring.

You say you have tried multiple external MOSFETS and a relay...?? Perhaps you should draw a diagram of exactly how you have wired your RAMPS board to the heated bed? Perhaps the original 'faulty' MOSFET wasn't the only fault with the RAMPS board? Have you stuck a meter on the MOSFET gate to see if the CPU is doing anything?

I got some help from the Thingiverse group and figured out what to check with my multimeter--as anvoice proposed, the control signal was always on (even if nothing was plugged in to D8). I went ahead and just ordered a new RAMPS board since they're so cheap--hopefully it isn't faulty. For anyone who sees this in the future here's a link to that thingiverse topic:

[www.thingiverse.com]

And a tl;dr: Check with a multimeter if D8 is always sending an ON signal even without the mosfet plugged in. If so there are a still a couple things you could try to get more specific, but it seems likely it's an issue with the RAMPS itself.