Heat beds are always problematic.
Thoughts from me:
1. What voltage do you get on the following places with the heatbed off (referenced to Gnd on the PSU input to the RAMPS 11A connector):
a. 11A PSU input to RAMPS?
b. D8 + pin (nearest the 11A connector)?
2. What voltages do you get on the following places with the heatbed on (referenced to Gnd on the PSU input to the RAMPS 11A connector):
a. 11A PSU input to RAMPS?
b. D8 + pin (nearest the 11A connector)?
c. D8 - pin (the other D8 pin)?
3. What voltages do you get across the connector of the heatbed with the heatbed on, at the heatbed end?
From comparison of 1 and 2, you can see if your PSU is drooping under load (eg: putting out less than 12V).
From the values given by 2 and 3, you should be able to map the rough voltage drops in the circuit, which may help identifying further issues if the PSU is not drooping under load.
Note: There are always going to be voltage drops in PCB tracks, cabling, components, etc. It's just a matter of mitigating issues to see if you can improve things.
PS: A standard RAMPS setup will run happily on 13.8V (such as a CB/HAM radio PSU), and the extra 1.8 volts can sometimes make all the difference. The "loss" in your wiring and components (the fuse and the mosfet) will be offset by the extra voltage. I would avoid going over 14V without precautionary measures being taken, though a standard Arduino Mega should be OK up to at least 14.5V, maybe even 15V. As always, YMMV.
Thoughts from me:
1. What voltage do you get on the following places with the heatbed off (referenced to Gnd on the PSU input to the RAMPS 11A connector):
a. 11A PSU input to RAMPS?
b. D8 + pin (nearest the 11A connector)?
2. What voltages do you get on the following places with the heatbed on (referenced to Gnd on the PSU input to the RAMPS 11A connector):
a. 11A PSU input to RAMPS?
b. D8 + pin (nearest the 11A connector)?
c. D8 - pin (the other D8 pin)?
3. What voltages do you get across the connector of the heatbed with the heatbed on, at the heatbed end?
From comparison of 1 and 2, you can see if your PSU is drooping under load (eg: putting out less than 12V).
From the values given by 2 and 3, you should be able to map the rough voltage drops in the circuit, which may help identifying further issues if the PSU is not drooping under load.
Note: There are always going to be voltage drops in PCB tracks, cabling, components, etc. It's just a matter of mitigating issues to see if you can improve things.
PS: A standard RAMPS setup will run happily on 13.8V (such as a CB/HAM radio PSU), and the extra 1.8 volts can sometimes make all the difference. The "loss" in your wiring and components (the fuse and the mosfet) will be offset by the extra voltage. I would avoid going over 14V without precautionary measures being taken, though a standard Arduino Mega should be OK up to at least 14.5V, maybe even 15V. As always, YMMV.