Voltage Drop at Ramps 1.4 / Heatbed

Hi,
I've recently finished building a Prusa Mendel i3 Rework bought from Replikeo. It came with ABS so i've been struggling with the heatbed to reach 110 / 115 C. After some insulation, it gets there, but holding 110 C is a struggle. I've read some postings here and decided to measure voltage and resistance of both heatbed and cable i've used.

It turns out i have a significant voltage drop at the D08 output and Heatbed.

Power Supply is a Switching PSU 12A 33 A, and i'm reading 12.34 V on output. At RAMPS D08 with the heatbed on i get 11.2V and at Heatbed (MK2 i get 11.1 V. The resistance of the heatbed is 1.3/1.4 Ohm. I've using 14 AWG wire i bought for my quadcopters, Turnigy brand, and i get 0.5 ohm resistance when measuring the cable alone (if i'm doing it right).

So with 11.1V and 1.3 resistance i'm getting only 8.53 A at the heatbed.

¿Any thoughts?

Thank you!!

Measure voltage directly at the PSU. If the voltage is low there, too, you need a more capable supply. If not, you need thicker (or additional) wires between PSU and controller as well as between controller and bed. It's possible you need both, of course.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

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Bugo
Power Supply is a Switching PSU 12A 33 A, and i'm reading 12.34 V on output.

so you loose around 1,2V within the RAMPS
did you check the clamps? check if they fit really tight.
Maybe your FET got damaged .... please check the Vds of the FET as well as its Temperature.

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mantus

Quote
Bugo
Power Supply is a Switching PSU 12A 33 A, and i'm reading 12.34 V on output.

so you loose around 1,2V within the RAMPS
did you check the clamps? check if they fit really tight.
Maybe your FET got damaged .... please check the Vds of the FET as well as its Temperature.

Ok, so it turns out i was not subtracting the resistance of the meter leads themselves which is 0.4. So my MK2B HB has a resistance of 1.0. Being that said, i should be getting 11.1 A which is not bad. On other post some stated that there's a PTC fuse which is rate 11A, so i guess i'm pretty good actually?.

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Voltage from both sides of D8 with reference to ground.

This last one will give you two voltages: One high, one low. The difference should be the voltage sent to the heated bed. The lower one is the voltage across the FET, the higher one is the voltage across the larger PTC fuse (F2).

The way PTC fuses work tends to be rather annoying when you're at the edge of their current load.

For instance, if the PTC fuse is 11A and you're drawing very close to that (say 10.75A) then it can start dropping volts to reduce the current slightly. It does this as the PTC fuses work internally based on temperature, so if they're already warm the trigger current is reduced.