No power to heatbed, how to test ramps for problem??

I'm getting no power to my heatbed, just stopped working. Tested with multimeter and i'm getting 12v to the ramps power input from the power supply to supply heatbed but i'm just not getting voltage from D8 to the heatbed.
Everything else on the board is working, i'm still reading a temperature from the heatbed termistor etc, just not power to the heatbed.
Anyone have any ideas what i can do to maybe check and see what components dead or am i just better buying a new ramps board???
Like i said it was working previously and just stopped, maybe burn out something etc.

Thanks

Check the fuse (the big yellow thing). Connect you meter to ground and both sides of the fuse in turn. You should get about 12v on both.

continuity test the heated bed with it not plugged in. is it possible you have an open in your circuit?

Thanks for the reply's, checked the fuses there working, getting continuity and getting 12v across both.
Checked the bed for continuity, not faults.
Any other suggestions?

what way is power routed on the ramps for the heatbed?
12v input on the ramps input rail->fuses->?->?->mosfets?->D8?

Sorry i'm not very familiar how the board works but if i know what other components to check.

Have you checked the cables? Also is the led coming on for D8?

ok think i found the culprit, any ideas why that blew?
I resume its the 12v voltage regulator for the heatbed yes?

Thats the mosfet.

I wounder is my power supply amps too high for the heatbed 12v input, its a universal 20a power supply i'm using.
Or just might have been faulty.

Is there any spec i should stick too when ordering one of these encase i cannot find the exact one?

It is odd for them to fail but not unheard of. They are rated to 50amps so it is unlikely you over loaded it. It may have over heated though. as it does not seem to have a heat sink, which is always advisable.

There are many suitable alternatives, just search the forum.

be sure to check all the tracks around it on the board too.

It probably overheated. Make sure that it's the correct MOSFET. If it is, I suggest replacing it with an ultra low RDS(on) MOSFET and also putting a heat sink on it. I don't understand why a lot of RAMPS boards don't come with heatsink on that MOSFET.

Those mosfets are rated for higher than 50A. The only way to blow them is if something shorted. I had an issue with one Ramps board where I accidentally wired a MK5 Plastruder hot end resistors into a short instead of parallel wiring. When power was applied, the trace fro mthe mosfet to the screw terminal smoked up and blew like a fuse. I soldered in some wiring and found that the nozzle heater was stuck on and wouldn't turn off with power supplied. I removed the D8 mosfet and moved it over to D9 and control was restored. I had smoked the original mosfet by shorting it. Are you sure you don't have a short somewhere in your pathway? How is the quality of all of your solder connections (one's you've made as well as those on the RAMPS board)? A bad connection can cause bad things to happen.

Edited 1 time(s). Last edit at 01/17/2014 07:33AM by umdpru.

I can't see any faults bar the mosfet, i remember i did see some smoke before but couldn't trace it. But the heatbed was still working after that.
I just seen the boards are only 12 euro on ebay so to save alot of hassle i bought a new one.
Thank you for your advice though!

My blown mosfet showed ZERO signs of failure and yet it was stuck in the on position meaning an internal short somehow. Swapping it out with a known good mosfet allowed the Ramps nozzle heater to work properly again further proving that the mosfet had blown.

had this exact same problem.

With the bed I found there are 2 issues at play here.

1. The standard MOSFET that is typically found in theese kits have an RDS that creates a fair amount of heat at 10A and at this level the datasheet says stick a heatsink on. Screwing a piece of metal to the MOSFET cures the issue I use an aluminium bracket because thats what I had laying around, thermal compound helps too if you have that laying around from a PC.

2. A lot of heat beds state that they are 1.2ohms but aren't ( just a variance in manufacturing prehaps? ) sticking a cheap multimeter across it says this but in practice it may not be reading correctly. I found this out because my bed kept tripping a 11A polyfuse but was fine with a 12A meaning its somewhere around 1.1ohms. good news is it means my bed heats up slightly faster!

but seeming as your transistor blew I imagine the polyfuse is ok, watch out though you may find it slowly does kick in when the MOSFET works properly because if its near its tripping point it can gently heat up and stop the current.

I'm having the same issue with my heatbed not working and no led. I smelled a strange burning smell all of a sudden but no smoke. My mosfets all look ok. But I tested the heat bed heater wires unhooked from the board and they were an open circuit. Should they not be? Could my heatbed bad? Since the mosfets look fine, do I still need unsolder it to test it out?
Thanks

If it is an open circuit then it is likely not the mosfet. You need to test each wire and the heat bed separately to find out which bit is faulty.

I'm sorry, my heatbed is not an open circuit. It's showing no resistance like a short circuit. Is that what it's supposed to be? If the heatbed is good, is that mosfet the first thing to check? Thanks

A short will surely blow your mosfet. What are you measuring it with? Is the reading accurate? It will be pretty low resistance but not zero.

Quote
foster182

what way is power routed on the ramps for the heatbed?
12v input on the ramps input rail->fuses->?->?->mosfets?->D8?

That's not the way it's wired. Check the schematic.

12V input on RAMPS -- fuse -- (PS1) + output to BED

GND -- Source[MOSFET]Drain -- (PS2) - output to BED
D8 ---------------- Gate

As you can see, the MOSFET is switching the GND connection to the bed. There will always be +12V on the + bed terminal whenever power is on as long as the fuse is good. There's an LED across the + and - BED terminal on RAMPS to indicate voltage. If that LED is turning on when you turn the heated bed on from the host software, then the MOSFET is good. If it's not coming on, they you might have a bad MOSFET or your +12V supply is bad. Verify using a voltmeter across the bed terminals and across the 12V bed supply input in RAMPS.

The Prusa heated bed PCB should have a resistance around 1.1 ohm.

Edited 1 time(s). Last edit at 01/24/2014 12:01AM by brnrd.

Well I tested everything and you guys are right, the bed itself is showing 1.5ohms. I do have power coming into the board and no power on the D8 bed rails. Is there a way to test the mosfet itself without pulling it out of the board? Also I saw mention earlier of ultra low rds mosfet. Can someone tell me which mosfet part number I should order? Thanks

You haven't mentioned checking the fuse yet. You should make sure that it's not blown. If RAMPS is still wired to the 12V power supply for the bed, you can check this easily witn a voltmeter between the + bed terminal and the ground wire from your power supply (any of the - terminal between the power supply and RAMPS). You should read 12V if the fuse is still good. If not, then the fuse is probably blown. If the fuse is good, then it's a problem with the MOSFET.

You should visually check the solder pads for the MOSFET to make sure that all the 3 pins are still soldered well. The chip can get hot enough to melt the solder resulting in broken connection. Then with the power on, turn on the bed heater in the host software and measure the voltage going into the gate pin of the MOSFET. That should be the outside pin on the same side of the MOSFET as the + D8 terminal. You should see TTL voltage there (probably close to 5V). If you do, then it confirms that the MOSFET is bad.

BTW, 1.5 ohm is probably too high for the bed resistance. You need to subtract the resistance that you measure when you touch your multimeter leads together. There could easily be 0.4 ohms there.

Thanks brnrd. You were right on about subtracting the resistance from touching the leads together. They were 0.5 when touched together. As for testing the fuse using your method, I wasn't sure if I needed to activate the heatbed before I checked, but a bad thing happened. I screwed up and tried to check the power at the fuse and the supply and I shorted something. Now I have no display at all when powered up. Do one of these fuses also protect the display or is my arduino board hurt?

But i believe the mosfet was the issue. And the solder underneath still feels solid and not loose. I wasnt able to check the voltage of the mosfet yet since I blew the display out, but i will when I get that fixed up. If I need a mosfet, would you know which mosfet part number i should get? Thanks

Not sure what you mean by display. Do you mean the host computer display? Or do you have an LCD connected to RAMPS?

As far as checking the fuse, another way would be to turn off all the 12V power to RAMPS and check for continuity between the + bed power supply input and the + bed output on RAMPS. If the fuse is still good, this should be a short (near 0 ohms). If you disconnect the + lead to the bed, then this would be more accurate.

I don't have a recommendation for an ultra-low RDS(on) MOSFET for RAMPS. I switched to using Azteeg controllers that use them instead of replaceing the one on the RAMPS. Perhaps someone else can help. Here's a product line that came up on a google search: [www.irf.com]

So with no power to the board and testing the +bed output and + psu input, I'm getting an open circuit and reading a steady 1 on the meter instead of 0. So I guess this means fuse then.

And yes, I meant my lcd display that's connected to my ramps is out. The blue back light shows up with power but it's blank with no letters or numbers. And when trying to connect my pc to the board via usb, it won't connect either anymore. Any ideas there?
Thanks

If you mean a "1" on the left side of the meter display, then that sounds like it's open. You probably have a blown fuse for the heat bed power.

There's also a separate fuse for the other 12V supply input to RAMPS that powers the rest of the controller, including the AT Mega. You should check that too in a similar way. With all the power off, measure the resistance between the leads of F1 on RAMPS. You can also double check the resistance on F2 (bed 12V fuse) right next to it.

BTW, you can check the AT Mega 2560 by itself. Just separate it from RAMPS and connect it to the host computer with a USB cable. The AT Mega will get it's power from the computer and you should be able to communicate with it using the Arduino sw or your host (Pronterface?). Pay attention to the LED indicators on the AT Mega.

I checked the resistance of the leads on both fuses at f1 and f2 and they're both showing 0 ohms (actually .4 ohms but you showed me that this amount is present in the leads alone). So I'll plan on new fuses for now I guess.

Unfortunately, I might have blown my arduino board. I've used two separate computers and neither one recognizes it's connected. I don't get any windows sound or change in the device manager. But I do get a solid green light on the arduino.

No resistance means that both fuses are good.

Sorry I'm not very electrical savvy, but yeah I wasn't even thinking straight and of course that makes sense that they should be no resistance.

Well I took the mosfet out and tested it as shown on YouTube videos and online and my meter stayed on 1 the whole time and didn't change as it looked like it should have.

And as for the arduino, I really believe I toasted it somehow since it's not being recognized with different PCs and different cables now.
Thanks again