Bed Heater Control

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Bed Heater Control

Postby dave3d » Sun Sep 15, 2013 7:52 pm

Just about completed my Ord Bot build however I have a problem now with the bed heater.
It does not shut off when it reaches the set point. The temperature keeps on climbing regardless of the setting.
Bed thermocouple reads OK and shows on the LCD panel. No problems with the nozzle heater which controls fine.
I am running RAMPS 1.4. Any advice where to start looking appreciated.
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Re: Bed Heater Control

Postby bdring » Sun Sep 15, 2013 10:57 pm

Does it read correctly as the temperature is rising. What host software are you using?
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Re: Bed Heater Control

Postby dave3d » Mon Sep 16, 2013 9:55 am

I am using Marlin and the bed temperature appears to read OK as it is rising. I am using 6mm glass.
I have not checked it with an IR thermometer. Maybe I should do that first.
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Re: Bed Heater Control

Postby bdring » Mon Sep 16, 2013 12:37 pm

If you see the temperature exceed the setpoint in your host software, that is strange. It is either a wiring problem or a software problem.
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Re: Bed Heater Control

Postby dave3d » Mon Sep 16, 2013 2:58 pm

Confirmed the bed temperature with an IR gun. Heater is wired to D8 on the RAMPS board. Tried various set points but the temperature continues rising. What should I check in the software? Everything else controls and works as it should.
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Re: Bed Heater Control

Postby Digitalmagic » Mon Sep 16, 2013 3:28 pm

Apparently, the PID acts as if the temperature is not yet reached.
Perhaps, you attached your thermistance/TC to the bad RAMPS input?
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Re: Bed Heater Control

Postby bdring » Mon Sep 16, 2013 4:01 pm

What is your host software?
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Re: Bed Heater Control

Postby dave3d » Mon Sep 16, 2013 6:35 pm

Bart: I am using it in standalone mode with Marlin firmware.

Digitalmagic: I damaged my Ramps board a while ago and had to repair the 5v supply circuit. I got help from other forum members. Everything appears to be working OK otherwise and the bed temp indicates OK on the LCD panel. There is a possibility the board is still damaged I guess. The problem is the temperature carries on rising beyond the set point.

Will try it connected to a PC. I use Repetier Host software.
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Re: Bed Heater Control

Postby cvoinescu » Mon Sep 16, 2013 10:50 pm

Can you turn the heater off at all, for example by setting the temperature setpoint to zero? If the MOSFET is shorted, you would not be able to turn it off at all.

Carefully measure the voltage on the gate of the MOSFET (vs. ground); it should be a few volts while the temperature is below the setpoint, then it should go close to zero; it may oscillate a few times around the setpoint. If that happens but the heater keeps going, look for a short, a bad MOSFET, or a wiring error. Note that the MOSFET switches the ground (negative pole) of the heater; the positive pole is tied to the supply voltage (via a fuse). If the gate voltage stays high, then the problem could be with the microcontroller, the firmware, or, again, a short-circuit somewhere on the Arduino or the RAMPS. Are you sure you're set to use the right MOSFET output? Are the thermistors wired to the correct inputs? Are you looking at the right temperature measurement on the display? The same problem can occur if you miswire thermistors and not realize you're looking at what the firmware thinks it's, say, the second hot end temperature, but it's actually the bed.
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Re: Bed Heater Control

Postby Digitalmagic » Tue Sep 17, 2013 9:14 am

Can you switch the bed heating off and on ? regardless of temperature control.
The mosfet could be damaged, having current flowing though it permanently.

Marlin FW version?
bang bang regulation for heatbed (standard) or PID regulation?

Should you issue deserves to externalize the power path, or finding a defective mosfet, there is a workaround to use an external MOSFET controlled by your ramps, using either digital output, or an avialble mosfet output.
Image

Have you an available mosfet output on your ramps? this is also a workaround by reconfiguring Marlin.
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