Figure out the Resistance Value With Known Device Specs

I'm learning electronics and trying to find the right resistor value for my device: a Eachine Rotg01 Receiver.

The specs

Working Current     200mA/5V
Power Supply        5V(by smart phone)


I don't want to fry my receiver so I was hoping to get some confirmation that my math is correct. Let's say I want to use a 9 volt battery as my power supply. From the specs, I am assuming 5V is the voltage drop so V = 9-5 = 4. Plugging this into Ohms Law 4 = R*.2 So that would work out ot a resistor of 20 ohms. Is this right ?

I'm not sure if 5V is the voltage drop of the device. I tried measuring with my multimeter, but got OL. Any help is much appreciated.

• You cannot rely on the current draw being 200mA all the time. At lower current draws the voltage drop across the resistor will be less causing the voltage output to be higher. People do not use resistors for this. Get a regulator. Your 9V battery won't be too good at supplying that much current either. Commented Jan 16, 2020 at 4:29