Ramps Mod w/ Custom heated PCB, Stuck!!!!!

It sounds like the mosfet is running waaaay too much current, which could be a short in your bed wiring or the bed itself. Running this too much will kill the mosfet on the RAMPS board, which I'm sure you don't want to do!

Disconnect the heatbed at the RAMPS board, and measure the resistance of the bed at the connector. It should be in the order of a few ohms. The current draw will be 12 / R. If it's really low (close to 0 ohms) then something is wrong - look for bridged solder joints, crossed wires, loose strands, etc...

Yup, that will work.

Note that if you're using bang-bang control on the bed (default solidoodle firmware), then a regular automotive relay (voltage rating should be at least 12V DC (note the DC rating is important, it's usually a lot lower than the AC voltage rating), and current rating: measure the bed's resistance when cold, calculate the current draw and get something rated higher - 15-20A would be safe) will be fine, as it only switches every now and then.

If you've modified the firmware to use PID control for the bed, you'll need to use a solid state relay (SSR) of similar rating (many can be found on eBay etc), as these can handle the rapid on/off required. These are also silent, which may be of interest to you.

rcullan,   If you use a Solid State Relay (SSR) you     MUST!!!    mount it directly to the metal back of the Solidoodle or it will overheat and stay on constantly  this will cause major meltdown  ... Trust me I found out the hard way..    Bill D

I wouldnt necessarily say that. This is a custom PCB. So we do not know what resistance it ended up being. But the MK-II PCB are less than 1 Ohm. 144Watts+ which is equal to 12A+ @ 12V. And if you have a cheap multimeter, your not going to be able to measure the resistance enough to really tell what it is. Even the continuity beeper will go off on a good heater.

The best test to see if your PCB works correctly, is to wire it directly to the PSU using a 20 amp car fuse. If you blow the fuse, your heater resistance is too low (bad PCB). If the fuse survives and the PCD can reach 200 C then your heater is good, and the MOSFET should be replaced.

Most RAMPS1.4 were built with an inadequate MOSFET for the bed heater. Check to see if it a STP55NF06L. Also, these MOSFET require a real heatsink you bolt on made of aluminum if you run it at these currents. Do not use adhesive.

Thanks for all the input guys. I am not a novice at all in the realm of electronics (Engineer by day, hobbyist by night), and I appreciate everyone's concern. I tested this principle out very carefully this morning. I am not one to wire it up and just start printing. I want to be sure its a good setup before I walk away from the machine.

First off, I am probably not going to start with a SSR, I will only go there if the clicking of a mechanical relay drives me totally bonkers. We will see on that front.

I did have a 12V @ 20A relay sitting around, so I wired it up this morning and tested it out. It works great! The bed heats up to 105C in about 3 minutes, where my standard SD2 took about 30 min to get to 95C. What I didnt like was the wire gage, with the increased current I need to beef up my wire gage so thats #1. The relay also got somewhat warm, but not to the level I couldnt touch it. The one I had lying around was from an Arduino relay unit and has no feature to mount a heat sync of any kind, so its kind of hanging loose (dont worry I insulated it with Kapton). I'm headed out now to get a 12V mechanical car relay, rated for 40AMPS and with a heat sync that I will be mounting to the back of the machine itself. With a heavier gage wire I hope this is a good solution.

I'll post a follow up. But if any one sees something errant in my approach. Please let me know!

Thanks all for the input. A 12V/40A relay and 12 Ga. wire between the PSU and the Bed worked great. I am printing a calibration print as I type. NThe wires, the PSU, the relay, nothing gets warmer than luke warm holding steady at 105C. I am very happy. This heats up 10X faster than the standard bed provided with the unit.

I have one last question, I did measure the resistance of my custom PCB and its about 0.2 Ohm. I noticed the following lines in Configuration.h

// If your bed has low resistance e.g. .6 ohm and throws the fuse you can duty cycle it to reduce the
// average current. The value should be an integer and the heat bed will be turned on for 1 interval of
// HEATER_BED_DUTY_CYCLE_DIVIDER intervals.
//#define HEATER_BED_DUTY_CYCLE_DIVIDER 4

Have any of you used this for a similar issue?

It's been my experience that SSR s bought on Ebay tend to to be over rated by about 50 % so if you want to pull 25 amps get 1 that is rated for 50 or 60 amps the ssr s on ebay usually don't have UL or CE ratings so they tend to over rate them.

it's a FOTEK SSR-25 DD...input 3-32VDC; Output - 5-60VDC...

Thanks all for the input...I will consider a larger rated one...

But also, I truly wonder if I'm just wiring something wrong:

Any thoughts?

Will definitely order a larger one...regarding mechanical relay, doesn't that require switching off PID in firmware? And what about arcing and seizing closed?

Try heatsinking it. You are supposed to heatsink these relays if your using more than 25% its rated current. It may be overheating and causing it to limit the current.