RAMPS 1.4 Delivering 9.5 V to heated bed.

Hello All,

I am using the MK2A heated bed, RAMPS 1.4 and the Taurino Classic to drive my system. I am using a 400W ATX power supply (http://www.coolermaster.com/product/Detail/powersupply/elite-power/elite-power-400w.html)

My heated bed struggles to reach temperatures over 100 C degrees. I tried trouble shooting the issue and found out that the power supply is delivering about 9.5 V to the heated bed when it is on. (the LED on the bed lights up fine). My leads to the bed are 'thick' wires but not as thick as they could be.

Yesterday I found out that ATX power supplies need to be loaded on the 5V line so that the power supply can deliver the voltage and current on the 12V line. So I went out and bought a 12V 50 W bulb.

I hooked up the bulb to the 5V rail and measured a current draw of about 2.5 amps, but the voltage at the heated bed was still 9.5 V.

Do you guys have any idea of whats going on ? My other theory is that the MOSFET used to toggle the heated bed (Q3) has some internal resistance when on which is cause a voltage drop of about 2.5 V.

Any suggestions ?

Where are you measuring the voltage?

If it is at the bed, you may be reading a PWM voltage.

Also yes, all semi-conductors have a voltage drop, so there will be a voltage drop across the MOSFET.

What are you trying to heat up as well as the MK board? Where are you measuring the temperature?

The voltage at the pin terminals in the RAMPS is 9.5V
The voltage at the heated bed pads are about 9.1 V so there is a slight voltage drop through the leads.
The resistance of the heated bed is 1.3 ohms.

I am directly controlling the heated bed by toggling the arduino pin on high. So i am certain it is not a PWM signal.

I am measuring the temperature at the surface of the Bed using a infrared heat gun ( [www.ebay.com]) which is accurate to the nearest 1.5 degrees.

I find it very strange that there will be a 2.5 V voltage drop against a MOSFET that is in its on state.

I'd check and look at the following while on:
Voltage into the RAMPS board (the two power in pins near D8).
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.

Make sure that the PTC fuses aren't touching each other, or anything else, and that they have adequate airflow.

If all else fails, remove them and replace them with bits of wire. IF YOU DO THIS, put a fuse in line with your PSU (eg: 15A) so that if it short circuits you protect your PSU and most particularly, your board.

Also of note: I have seen some boards that had fairly thin copper on the tracks that lead from the PTC to D8, and from the FET to D8. You can solder pieces of wire on top (to carry extra current), or layer them with some solder (requires removing the solder mask from those tracks first so you can tin them). This reduces the voltage drop across those tracks, which could also be part of your issue.