Heated Bed isn't working

[LonV]

So my friends heated bed stopped working the other day...I went through a bunch of tests, and I thought that the actual heater was bad. So we bought a new one, wired it all back up and it still doesn't work.

Without the heater connected, I see 12v. The moment the bed is connected it drops to 0v. Any ideas why?

-Lon

[PcS]

My first guess would be to check the ATC auto style fuse when load is applied as this fuse is for the heated bed.If that passes I would think you lost the heated bed output from the rambo.Are you testing right at the rambo when the heated bed is plugged in ?

[ednisley]

Without the heater connected, I see 12v. The moment the bed is connected it drops to 0v. Any ideas why?

It depends on what you're measuring and how you measure it.

The heaters connect between the power supply (+12 V for the original platform) and the MOSFET, so one pin in the connector will always be at +12 V relative to ground. With the MOSFET turned off, the other pin will also be at +12 V relative to ground, because it's connected to the +12 V supply through the heater's low resistance and the MOSFET is an open switch.

Turning the MOSFET on connects that pin to ground, current flows through the heater, and the voltage on the pin drops to (almost) 0 V. It's still connected to the supply through the heater, but the MOSFET is now a closed switch with a lower resistance.

However, if you measure the voltage across the connector pins, not from pin-to-ground, you'll see 0 V with the MOSFET turned off and +12 V when it's turned on. When the heater sees 12 V across its pins, it gets hot!

Actually, you'll see less than 12 V, because the connectors and wiring will drop several volts. In your situation, you'll probably see only a few volts, not enough to warm the platform, because the typical failure involves a break in the wires at the red connector near the platform or a charred connector at the RAMBo end of the cable.

If the platform isn't heating, measure the voltage across the wires, starting at the MOSFET on the RAMBo board, with the heater turned on: you should see 12 V. As you work your way toward the platform, the point where you see 0 V will indicate when you've passed the break. Once you know the general location, the problem should be easy to find...

[LonV]

I thought maybe it was the red connectors and I got rid of them temporarily. I get 12v (when it's on) at the heater until I connect it up and then it's 0v...I'm pretty sure it's not wiring.

I will check the automotive fuse though. Visually it looks fine, but maybe there's something wrong with it.

-Lon