Ramps 1.4 Heatbed Mosfet

Guys i have a problem that i can't understand, i messure the heatbed mosfet and get 11.88V all the time. Never close but this is other problem for later, my real problem is that when i connect heatbed i get 0 power.

If i messure negative from power supply to any of heatbed connections i have 11.97V.

I replace heatbed and still the same. I check fuses and power flow ok.




Thanks Panos

Edited 1 time(s). Last edit at 07/03/2015 10:40AM by powercreations.

The MOSFETS switch to low side of the circuit - IE, as you have noted, the positive terminal to the bed has power all the time. So, if you see the same voltage on both sides of the bed, the MOSFET is either not being commanded to turn on, or has failed.

- Tim

Thanks for your answer, so the best thing i can do is to change RAMP?

Is a way to bypass the circuit and run the heatbed directly from power supply?

Panos

Quote
powercreations
Thanks for your answer, so the best thing i can do is to change RAMP?

Is a way to bypass the circuit and run the heatbed directly from power supply?

Panos

Or replace the MOSFET - they are about $1 in the US . . .

And no, you can't run it direct, it would overheat.

You are certain that the firmware is trying to turn it on, right?

- Tim

I am certain only because everytime i say to open the led is turned on. But as i said the power is flow all time and when i touch the cables at heatbed is droped down.

Is normal that the heatbed have no resistance? Is new...

Thanks Tim

Panos

Depending on the heat bed, it might have a very low resistance like 1ohm but it shouldn't be a dead short (0 ohms) or open circuit.

Quote
ntoff
Depending on the heat bed, it might have a very low resistance like 1ohm but it shouldn't be a dead short (0 ohms) or open circuit.

You was right, i change multimeter which can read smaller amounts and the heatbed have 1.2ohm.

Unfortunally i can't order anything for now country policies (Greece) .

If i unsolder the mosftet and solder as contnue circuit work? At least to see if the board is ok.

Thanks Panos

Edited 1 time(s). Last edit at 07/05/2015 07:38AM by powercreations.

You want to remover the heatbed-FET ( D8 ) and short the Drain/Source pins? Don´t do that, you wouldn´t have any temp-control then....
If you don´t use the second extruder output and you are good at soldering, you could replace the D8-FET with the D9-FET.
-Olaf
PS: I hope yesterdays referendum in Greece turns out to a re-election. Varoufakis and Tsipras are not trusted anymore in EU...

Quote
o_lampe
You want to remover the heatbed-FET ( D8 ) and short the Drain/Source pins? Don´t do that, you wouldn´t have any temp-control then....
If you don´t use the second extruder output and you are good at soldering, you could replace the D8-FET with the D9-FET.
-Olaf
PS: I hope yesterdays referendum in Greece turns out to a re-election. Varoufakis and Tsipras are not trusted anymore in EU...

Thanks for answer, i change the mosfet and work but...still the power is continued (can't control from board) and the mosfet get hot like hell.

Probably is not the same, the old one was irf3205 and the new one f3205z.

That was a good idea, i will try to put it at D9.

PS Tsipras and Varoufakis is not trusted because they not obey them! Who cares what they think, just i hope for a good agreement for all EU.

Don´t use D9 for the heatbed!
It is only fused for 5Amps...
If you can´t get it under control with D8 connector, you have a second problem on your boards.
When the FET is getting hot, you should check the 5V somewhere on the RAMPs board. You can measure at the endstop - or servo connectors.
If the 5V is more like 4.5V, the 3205 FETs are not fully switched.
-Olaf

I messure at limit switch and i have 4.97V, i change the heatbed mosfet with a IRFZ44N and no more hot but the Volts goes there but not passing .

I messure the mosffet legs and i get 11.97V. I am geting tired and i probably make a controller with a relay from PS to heatbed directly until i buy other board.

Thanks Olaf for help if you have more suggestions i am willing to try it ( until i burn the board completly )

Could the firmware have something to do with that?

-Panos

Edited 1 time(s). Last edit at 07/07/2015 10:40AM by powercreations.

The Ramps wasn´t able to switch the FET, it neither will be able to switch a relais.
It is weird, that in the beginning you couldn´t switch the bed off and now you can´t switch it on?!
-Olaf

I can't because as at the begining if the bed is connected the V is 0 and the mosfet keeps the V if i disconnect it then i will have 12V all way to the end of cable.

I am thinking a relay with a thermostat so when reach a temperature to close and when droped to open, like this i can monitor the temperature from ramp just to see it :p and i overide the ramp for heat the bed.

-Panos

Edited 1 time(s). Last edit at 07/07/2015 12:39PM by powercreations.

Please show a clear picture of your ramps board including the mosfet orientation. Also make sure the tabs of your MOSFETs aren't touching. This includes any heatsinks you might have installed.

With bed disconnected:
1. With D8 commanded off, you should measure 12V between D8+ and PS-
2. With D8 commanded off, you should measure 0V between D8+ and D8-
3. With D8 commanded on, you should measure 0 ohms resistance between D8- and PS-
4. With D8 commanded on, you should measure 12V between D8+ and D8-
5. With D8 commanded on, you should measure 0 ohms resistance between D8- and PS-

With bed connected:
6. With D8 commanded off, you should measure 12V between D8+ and PS-
7. With D8 commanded off, you should measure 12V between D8- and PS-
8. With D8 commanded on, you should measure 0 between D8+ and D8-

Does your bed have an LED and does it light when it's suppose to and turn off when it's suppose to as well?

The measures are with the new Mosfet IRFZ44N
With bed disconnected:
1. With D8 commanded off, you should measure 12V between D8+ and PS- --------------------------Right
2. With D8 commanded off, you should measure 0V between D8+ and D8- --------------------------11.85V
3. With D8 commanded on, you should measure 0 ohms resistance between D8- and PS- ------I have resistance, is not 0.
4. With D8 commanded on, you should measure 12V between D8+ and D8- --------------------------11.82V
5. With D8 commanded on, you should measure 0 ohms resistance between D8- and PS- -----Same with 3

With bed connected:
6. With D8 commanded off, you should measure 12V between D8+ and PS- -------------------------Right
7. With D8 commanded off, you should measure 12V between D8- and PS- -------------------------is 0
8. With D8 commanded on, you should measure 0 between D8+ and D8- ----------------------------is 0

On bed i don't have a led, but the led on ramps works ok.

Thanks Panos

For #3, that should have been with D8 commanded off, not on. You should have very high or infinite resistance (open)
For #4, 11.82 is close enough to 12 so that's ok.
For #5, what is your resistance? You should have very little, enough to be considered a short.

With the D8 MOSFET, what is the resistance between each lead. So the resistance between 1-2, 2-3, and 1-3? Lead 1 is the one closest to the fuses. Your board acts as if the drain and source pins (pins 2 and 3) are shorted, either with solder or a bad trace.

D8 MOSFET Leads resistance is:
1-3 1.4Ohm
2-3 2.34Kohm

Only leads that are shorted is 1-3.

The resistance for D8+ D8- wasn't stable and was to big.

Now i make a small circuit with a auto thermistor so i work it directly from PS with an external fuse. At 92 degrees stop and 80 start again, for now i work like that.

Thanks a lot Panos

Not ideal, but in your situation, the best you can do...
Also look for coffee machines, they have a bimetal-thermostat, too. More in the range of 110°C for ABS printing.
Old cloth irons use adjustable thermostats, that would be a winner!
-Olaf

Good to know but i am not in a mood to mess my iron/coffee to fix this .

For no good reason the temperature goes to 105 and start again at 80, good for me as i have only ABS to work with.

Thanks again -Panos

The hysteresis is too big, I´m afraid you´ll get warping.
-Olaf

Yes i think you right, until now the bigest item i make was a 137mm at Z axis and was ok. X Y axis not so big yet, we will see .

The goal is that we are back in business

-Panos

Quote

The goal is that we are back in business

Are you talking about the new negotiations about the 51.000.000.000€ for the third Greece "care pack"?
I wonder, how much of this money will end up in Greece mafia pockets?
-Olaf

No , we are back in business that the printer works

Probably the 50.999.999.999

-Panos