Better thing to check is what voltage you get at the heatbed, what voltage is entering your electronics, and what voltage is coming out of your PSU.
You can lose a lot of power in cabling between your PSU and the electronics, and the electronics and the heatbed. Thicker wire will help, as it has a lower resistance.
Also, the component on your electronics that switches the heat bed on/off may be overheating, which once again will reduce the voltage at the output. If so, you want to put a heatsink on this component if at all possible, and cool your electronics with a fan.
Last thing of note: PC power supplies need a load on the 5V line (usually) before they'll give a stable 12V at high current. If you have a PC PSU, put a 10 Ohm 10W resistor between 5V and Ground. This will draw enough current on the 5V line to prop up the 12V line.
Edited 1 time(s). Last edit at 11/27/2013 04:52PM by Cefiar.
