0
\$\begingroup\$

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.

\$\endgroup\$
1
  • 3
    \$\begingroup\$ 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. \$\endgroup\$
    – DKNguyen
    Commented Jan 16, 2020 at 4:29

1 Answer 1

Reset to default
1
\$\begingroup\$

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 ?

200mA is the maximum current. The minimum is zero (device off), so your voltage will vary between 9-(20*.2) = 5v and 9-(20*0) = 9V. Since average current is usually a lot less than maximum, you'll mostly be giving the device too much voltage.

That might or might not be ok (most devices that use USB tend to be robust), but the best solutions here are to either get a 5v supply, or to get a voltage regulator (a device that takes one voltage and turns it into another). A linear regulator such as the 7805 is the most simple, but will waste some battery life.

\$\endgroup\$
3
  • \$\begingroup\$ thank you. so does a smartphone have a voltage regulator built in to be able to maintain the 5volt charging? \$\endgroup\$
    – st4rgut
    Commented Jan 16, 2020 at 5:41
  • \$\begingroup\$ why does the device use variable amount of current? Other than powering up or shutting down, shouldn't we expect the amount of current to be stable especially for a receiver? \$\endgroup\$
    – st4rgut
    Commented Jan 16, 2020 at 5:43
  • \$\begingroup\$ Current is consumed by individual transistors and resistors in the device. As they turn on and off different amounts of current will flow. It may reach a steady state current once running, but this is unlikely to the 200ma. The phone uses a regulator to convert the ~4v from it's battery into 5v. \$\endgroup\$
    – user1850479
    Commented Jan 16, 2020 at 14:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.