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RAMPS Fuse/MOSFET extremely hot, shut off print bed

Posted by mklinker 
RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 15, 2013 12:52PM
I have been running RAMPS 1.4 for about 6 months now. Yesterday after a 4 hour job I noticed a burnt electronics smell around the controller board, and now my heat bed wont stay for more than a minute. The fuse is too hot to touch, and the mosfet is also very hot.

Resistance of my heat bed (Mk2) - 2.6ohms
Voltage of 12v PSU for heatbed - 13.2 volts

The mosfet has a heatsink and the fuses are not touching. I assume something burnt out because of the classic burnt electronics smell even when it is off, but I can still control the heat bed and the fuse seems to still kick off when the pieces get too hot.

Got the board from 3dgadget. Also ordered some new MOSFET's last night...

Mike
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 15, 2013 07:53PM
I have no idea what sort of printer you have. On a 200mm square bed, you need something like one ohm rather than 2.6 ohms. If it was 1 ohm and it's now 2.6, something has gone wrong.

The fuse is rated at 11A. Your hot end should pull < 6A if it's 2.6 ohms. The only way to get that fuse hot is to run > 16A through it.

If you have a metal frame printer, having the heated bed short out to the (grounded) frame is the most likely explanation. Shorted / shorting wires are the next thing on the list.
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 15, 2013 08:32PM
I have a Mendel 90, so no way for it to be shorted by the frame.
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 15, 2013 09:44PM
I re-measured the resistance to be 2 ohms.

It is annoying because this whole thing worked fine... the heat bed appears to be fine (no burn marks or anything) but now I can only intermittently get the printbed to turn on.
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 16, 2013 12:16AM
The heat bed pulls about 7 amps.

The heat bed wont turn on anymore via the controller. The fuse seems fine (tested with multimeter).

The D8 pin (I think this was the pin on the arduino that controls it) still goes high and low when I toggle it.

I think it is just the MOSFET blowing up?? Is there anything else that could be going wrong?
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 16, 2013 04:40AM
Did you measure the heated bed resistance at the heat bed solder tabs or from the Ramps end? If its ok at the bed, then change the cables. Also, it's essential to have a fan blowing on the Polyfuse and Mosfet when using a heat bed. All the Mosfet does is provide a ground path (switched) for the return wire of the heat bed - you can check this from the wiring diagram and test it to see if its grounding ok.


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Waitaki 3D Printer
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 16, 2013 06:47AM
The standard MOSFET on RAMPS produces a lot of unnecessary heat. If it does turn out to be faulty, replace it with a better MOSFET like IRLB8743: [www.irf.com]

Drop-in compatible and produces a lot less heat at high current.

You should still have a fan on your RAMPS board though. Running the MOSFETs hot for extended periods will eventually kill them, not to mention issues from motors cutting out if the drivers get too hot, and those annoying PTC fuses..
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
December 17, 2013 01:18PM
If you only have 7A into your heated bed, something is indeed wrong. Check the voltage at the bed terminals when it's trying to warm up. You should have ~ 11V. If it's way less, you need to find and fix the problem.

If you have a significant drop on the PTC fuse, chuck it and put in a 15A auto fuse externaly. Run the heated bed hot end directly through it to the power supply. (note the qualifier *through* the fuse spinning smiley sticking its tongue out).
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
May 19, 2014 12:15PM
When i'm running the heat bed, i'm measuring 10.38v on D8. Is that ok?
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 14, 2014 01:49AM
Intosite that looks like it is getting close. I am only getting 7.6V to my Heatbed. I just checked how many amps were going through to the heatbed and it looks to be around 5.3A. aarrrgghhh....just measured Volts from the -ve terminal of heatbed to the psu -ve and got a reading of 3.5V. interestingly the voltage from the -ve heatbed terminal to GND starts high and gradually drops to 3.5v.
Anyone got any ideas on what is going on here?
My heatbed takes over 40min to get to 100deg and that is only achieved if I put insulation on top of it while it is heating up. After I remove the insulation the temp starts to drop and any print job soon fails to stick to heatbed...another arrrggghhh..... :-)
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 14, 2014 03:21AM
The power terminals to D8 often get heated because of the higher current which can cause burning and a contact resistance between the wires and the terminal or the terminal pins themselves. That resistance will drop a voltage across the terminal block thereby restricting power to the bed. So......check the terminal block for evidence of burnt plastic and the wires for security.


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Waitaki 3D Printer
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 14, 2014 08:48PM
I will double check the connections Waitaki. I am thinking maybe the mosfet may need to be updated so I am checking my options on what mosfets may be able to be used. the local Jaycar store does not seem to have a wide variety available. Just doing a crash course on mosfets and how they work and what parameters are important...lol
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 15, 2014 07:29AM
Just thought I would pop in an update on my heatbed issue. I got an IRF405 mosfet from Jaycar today and put that in place of the other mosfet feeding D8 and now I have a mosfet that runs cool and I get around 10.6 V to the heatbed which allows it to warm up to 110deg in just under 20min.YAY Now printing and finding out how much calibration I need to do to get my printjobs looking good. smileys with beer
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 16, 2014 05:35AM
Yeah, those numbers look good!


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Waitaki 3D Printer
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 16, 2014 06:02PM
Well, time for another update....
Not much calibration has been needed so far, but looking at my bed temps after 30min of printing they are at around 80deg C. I had some prints lift off the bed and thus fail. One print I noticed last night had a warped base and it was only 2cm long. I have been using Hairspray on the bed to bind the prints....maybe I need to change the Hairspray brand LOL.
Seriously, I suspect that my heatbed temp of 10.6V may be too low still and is not enough to maintain a high enough temp to stop warping of the prints. I have a car charger I built as an apprentice that probably kicks out 13.?V so I am thinking maybe I might use that to power the 11A input part of the ramps board.
Anyone out there got any ideas???
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 17, 2014 07:59AM
What is the wattage of the power supply you have been using?
I assume you are printing with ABS? 80C will not really hack it for that plastic!
You can improve bed temp by insulating the bottom of the bed pcb - a piece of 2mm card or anything that stops the heat escaping downwards.


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Waitaki 3D Printer
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 17, 2014 08:16AM
I have been using a 300W atx PS, I also have roofing insulation under the heatbed to stop heat escaping. I am just wondering about my heatbed itself, it is only pulling about 3A and has 10.5V across it which when calculated out means that the resistance is well above the 1.2 ohm it is supposed to be. In saying that though, the terminal wiring is a bit strange, the bed can take 24V or 12V depending on the pins you solder the wires too. It is a sainsmart heatbed and I have shorted solder pads 2 & 3 and connected the -ve wire to them and the =ve wire to the solder pad 1...maybe this is not right but the wiring table etched into the heatbed surface certainly looks to guide you this way I think...
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 17, 2014 03:58PM
Quote
happygoluckyaussie
I have been using a 300W atx PS, I also have roofing insulation under the heatbed to stop heat escaping. I am just wondering about my heatbed itself, it is only pulling about 3A and has 10.5V across it which when calculated out means that the resistance is well above the 1.2 ohm it is supposed to be.
Unplug the wires from the electronics. Actually measure the resistance of the wires going to the heated bed. V=I*R so if indeed it's 10.5v and pulling 3 amps you should measure around 3.3 ohms which is 3x what it ideally should be.

How are you measuring the 3 amps? Is your power supply large enough? Note that if you're using a PC power supply, the listed wattage likely is to be a combined wattage across all rails and the available power on the 12v rails may be less the the nameplate suggests. If you're using a discrete 12v power supply, then that doesn't usually apply.

Quote

In saying that though, the terminal wiring is a bit strange, the bed can take 24V or 12V depending on the pins you solder the wires too. It is a sainsmart heatbed and I have shorted solder pads 2 & 3 and connected the -ve wire to them and the =ve wire to the solder pad 1...
That is correctly wired.
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 20, 2014 10:10PM
Sorry for the ddelay in getting back to you cdru. I have taken the effort to measure the resistance of the heatbed and found it to be approx 1.5ohms when it was sitting at 60deg between prints (this printing is so addictive. spinning smiley sticking its tongue out
The voltage at the bed just after connect wires and turning it back on is approx 10.31V now with 3.8A current being measured to the bed via a clamp meter for current measurement. The Voltage at the supply terminals ia approx 11.45V. My suspicion is now leaning on the PS not being able to keep up. So, do I go for a 24V system or a new 12V supply. If I go for the 24V supply, what do I need to change as far as parts go? I know I will have to change the wiring of the bed. I am sure the MOSFET I put in is OK for 24V as well. All comments welcome. grinning smiley
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 23, 2014 06:25PM
If im not mistaken, if you double the voltage on a resistive load you quadruple its power consumption. So if it was [email protected] it is now consuming [email protected] You sure your not going to have a melted bed or possibly a fire?


My Personal Blog. Build blog.
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Modicum V1 sold on e-bay user jaguarking11
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 24, 2014 06:10AM
Nope, P=V x I
So if you double the Voltage you double the power.

Regards
Nige
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
June 24, 2014 10:44PM
No, @happygoluckyaussie, you're wrong here.

The only thing fixed about the bed is it's RESISTANCE. They don't just draw a fixed current.

So, V=I x R, and P=V x I, you get P = (V x V) / R

Doubling the voltage increases the power four-fold.

FWIW: I'm one of the early adopters that's been running 24V systems for ~4 years.

Note: Yes, you can do it, but expect twice the current, which means if you're running a bed that draws 10A at 12V, at 24V it will draw 20A. Sure you can PWM this down to whatever you want, but the PEAK current you'll be pulling is 20A.

PS: If you're just after overcoming a small hump, maybe look for a 13.8V or 15V supply. This will allow you to overcome any losses and still is within the limits of various things (like the max input voltage on the Mega).
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
July 02, 2014 06:04AM
Yes @cefiar, you are totally correct in the power calcs.....lol....I am really out of practice on these....must stop trying to do calcs in the middle of the night....smiling smiley Gave myself a good smack in the head after that dumb mistake.
Thanks again for your valuable advice. I have a Battery charger I built as an apprentice, I may have a closer look at it in the morning to see what sort of current I built it to handle.(it was a long time ago after all...hee hee) I am sure it has a voltage output greater than 13V as it was built to charge car batteries.
Will get back to everyone with the results very soon now I am back from a my break in the snow.

regards
Nige
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
July 05, 2014 08:20PM
Hmmm....Battery charger I was going to use as the power supply for the heatbed is showing a 6V difference on the -ve compared to the -ve that is powering the rest of my RepRap Mendal i3.
I think I will search out another power supply that has a bit more giddy up that I can use to power the whole setup rather than using two different power sources. Any suggestions???

Regards
Nige
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
July 05, 2014 09:00PM
Nige: Not cheap, but Radio Parts Group sells a 23A, 13.8V PSU for $125 inc retail (atm). It's actually made by Manson (SPA8230G) and meets all the safety regs, etc. Has a tiny trimpot on the bottom that allows you to adjust the voltage a tiny bit. Should do at least 14.5V, maybe 15V.

While a lot of people recommend the SMPS boxes that come out of china, the issue with these tends to be:
1. Quality. They don't always deliver the current they say they will, or have very good soldering inside.
2. Safety. I've had at least one that the AC input was mislabelled and earth/active was swapped.
3. Shipping. They can easily get damaged in shipping, and you may not even know until something goes wrong.

Also, they tend to need a box or something around them (or at least, a cover over all the exposed connectors, which I know you can print one of, but it's not quite as safe IMO).
Re: RAMPS Fuse/MOSFET extremely hot, shut off print bed
July 06, 2014 04:39AM
Hey Cefiar,
That Radio Parts PS looks good. Not been a big fan of anything from China lately, parts have been taking around 2 months to get to me here in Newcastle and I have had to chase some orders up with suppliers only to be told after the fact that customs returned the item to them.....they were not going to tell me I would think, just hope that i never enquired and keep my $$$...arrgghh

Regards
Nige
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