Why does my FP-1600 take a 24V .5 Amp Power Supply and drop the Voltage to 11V at .5 Amps?

Question

I have a FP-1600 connected to 4 FP-TB-1 bases. 2 FP-AI-100, 1 FP-TC-120, and 1 FP-AO-210 modules. I am powering the FP-1600 with a 24 V .5 A power supply. There are no devices currently connected to any of the modules. The power supply should be supplying 12 W, which should be enough to power the FP-1600 and the 4 modules according to the power requirement calculation in the manual P = 7 W + 1.15*#modules. When I install the third module, the FP-1600 stops communicating. Checking the power supply I see that the voltage drop across the power terminals on the FP-1600 is only 11V and then measuring the current being drawn it is .5 Amps, which is only 5.5 W, not enough to power the devices. When I unhook the power supply and measure the Voltage it is 24 V.
Why is the FP-1600 dropping the Voltage to 11V? Should I just purchase a 12V 2 Amp power supply?
Thanks for any help
Jeremy

Answer 1

Hello jqk,
You got it figured out. The maximum current that can be drawn is .5 which is too small. As the current being drawn approaches the maximum allowable current, the voltage will begin to drop. Eventhough the current you measure is .5A, the fact that the voltage is 11V means that the circuit requires more current to function properly.
As a rule of thumb, you want the maximum current your power supply can deliver to be at least the double of the system's maximum current consumption.