Help no heat bed power no LED tried everything super frustrated

I got a new ramps 1.4 which I proceeded short while the power was on (super stupid) by touching hot iron to HBP wire pad.

Before the heat bed light(on ramps not HBP) would come on but no heating(old ramps worked until a weekish ago). Ive never had the heatbed working with this ramps.

I am getting 12v to ramps. I have tried two different power supplies no dice(500 watt 20ish amp)

The fuses resistance and voltage(12v on each side with ground) appears ok. The Mosfets have all been swapped out with new IRLB8743's. I've tried swapping out the heatbed one 3 times. I swapped out the 11amp fuse. No dice.

I bought a new 2560. No dice. D8 works fine on both boards(no ramps) with the blink sketch.

Swapped out thermisters. No dice.

Ive had all of this working in the past, just not with this ramps


The extruder heater seems to function perfectly. Ramps LED turns on and thermister registers temp.

Ramps test code flashes LEDS, Seems to be working.

Power appears to be reaching the capacitor and LED prior to the mosfet.


Oddly enough if I touch heatbed mosfet metal at the top the LED will silghtly light. Was thinking it was a loose connection but it appears now as if im conductive.




Any ideas, hints, hacks????

Ive swapped out everything except the new(ish that I shorted) Ramps. I'm at a complete loss here.

Power connections to ramps are solid.

I have to be missing something.

Edited 2 time(s). Last edit at 11/03/2014 11:00PM by handles25.

Check the input resistors

mega d8 -> R15 -> MOSFET
-> R4 ->

especial R15, its only 10 ohm and could be easily burnt out

Further down the rabbit hole?

R15 and R4 seem to be fine.


Then things got weird. I measured R23. 3k. Figured id make sure and measured R24 and R25. Also 3k. Schematics show 1.8k

Old ramps board R23, R24 and R25 measures 1.8k


On new ramps board R3 measures 20k. schematics and old board have it rated same as R4, 100k.





"Before the heat bed light(on ramps not HBP) would come on but no heating(old ramps worked until a weekish ago). Ive never had the heatbed working with this ramps."


These resistors seem way off on new board. They are numbered the same but different resistances? Bad product? Did I blow a bunch? Never seemed to power HBP properly.



None of this is adding up at all

Edited 1 time(s). Last edit at 11/04/2014 02:51PM by handles25.

Quote
handles25
Then things got weird. I measured R23. 3k. Figured id make sure and measured R24 and R25. Also 3k. Schematics show 1.8k
Old ramps board R23, R24 and R25 measures 1.8k
On new ramps board R3 measures 20k. schematics and old board have it rated same as R4, 100k.

R23, R24, and R25 just are there to limit current through the LED, higher the value, the lower the intensity. The LEDs your board was made with may have been speced differently and therefor needed a different resistor value. These resistors will have zero impact on the operation of the mosfet and the actual function of the circuit other than a visual indicator that current is flowing.

R9, R2, and R4 are pull down resistors. They hold the line low when the arduino is first started and the pins are floating, not low and not high. Once the microcontroller is initialized, they really don't do anything aside from guarantee that a low pin really is low. Could be 10k or 100k and work fine.

R13, R14, and R15 prevent a "ringing", spikes, or interference from switching the mosfet on and off rapidly. Think of it kind of like a buffer or dampener. Not likely that is the issue but is easy to check. It should be a low value resistor.

I'd start with basics. Check continuity. Go between the - power INPUT (from power supply) for the heated bed to the MOSFET source (top pin). Should be near 0 resistance. Check between pin D8 in and the bottom pin of the mosfet. Should be whatever the resistance of R15. Check between the - going to the heated bed and the drain, the center pin. Should also be near 0. If all those test fine, I'd try a new MOSFET.