Prusa i3 bed not heating

Hello all,

I have a Prusa I3 that I use for 3d printing, during last months.

Yesterday while running I observed that the printed part started to move on the heated bed , looking closer to the LCD the temperature was close to 20C ( from 50C ) and decreased to 14C (room temperature).

I have tried several power off-power on , different parts to print but the bed is not heating ( it remains at room temperature).

Reading on forums I have check using a multimeter voltage in different places after printer power on :













If I load a gcode from card and I want to start printing ( LCD message BED HEATING) the voltage is changing :





If my understanding is the right one, the heated bed is powered with 12V but as soon as I load a gcode part and I want to start printing the heated bed is not powered anymore ( 0v).

Any ideea regarding the potential root cause and how can I fix it?

Thank you!

Very nice testing and Documentation.

As you have probably worked out the +12 is connected directly to the heated bed (threw a fuse) like you have shown, the -ve line is controlled by the controller.

It looks like the mosfet controlling the headed bed is working or nothing wouldn’t change when you turn it on.

My guess would be a shorted bed or the 11amp polyfuse is heating up really fast and disconnecting the 11amp rail

Try unplugging the heated bed from D8 and put something else in there (a 12v fan would do)
Test you can tun it on and off using bed commands

Following your advice, I have removed the heated bed from D8 and I have connected the + wire from cooling fan.

Now measuring the voltage at D8 is 1.9v and if I load a gcode part to start printing is 0v.

Another difference is that after connecting the cooling fan to D8 there is a red led near Q3 mofset that is on , before ( with the heatbed connected to D8 ) was off.

I do not remember the led status when the printer was working properly.

When the bed is turned off, you should measure 12v on 'both' wires to the bed with respect to 0v.
When you heat the bed, then pin 2 of the connector should read 0v.
the red led should be lit when the bed is heating

Hello all,

The problem was fixed : after I connect back the heated bed to D8 the bed is heating ( red led on) , so I think that the root cause was poor connection between the wire and D8 due to vibrations.

Thank you all for your advice !

Thanks for the diagram and helpful explanation, Obewan.
My bed suddenly stopped heating up while troubleshooting a x-motor problem. Following your diagram and tips, I found that PS1 and PS2 turns to 0V when heat bed signal is on (from 11+V). I understand that it's only PS2 that's supposed to turn 0V. Could it be that my Q3 is shorted? What are the other possible solutions?
Thanks very much.

Yes ps1 should stay 12v all the time.
If Q3 was short circuit then the bed would heat to max as soon as power is connected.
If ps1 is going to 0v, then either the resteable fuse is tripping or your power supply is giving up,or maybe the wires from ps to ramps is not making a good conection.

Sorry for late update.
I disconnected the heat bed wires from pin 1 and pin 2. When heat bed signal is "on", pin 1 = 1.5V and pin 2 = 0V.
However, once while heat bed wires were disconnected, the heat bed signal LED went on. ONLY ONCE.
When it happened, I quickly reconnected the heat bed hoping that it will work from then on. It did not.
So again I disconnected the heat bed wires, and the LED won't turn on anymore and I keep getting P1 = 1.5V and P2 = 0V.

Btw, where is the resetable fuse located? Thanks very much.

the yellow/orange square thin things on the ramps are the polyfuses