# Stepping Current Down

Hi! This isn't strictly related to an Arduino, so feel free to remove, but I've lurked on this forum for awhile (for my other Arduino-centered projects) and I wasn't sure where else I could ask this question online.

I’m currently trying to power a DC fan through a USB connection (splicing the fan into the power wires of the USB) and I have a very basic question. My fan is max 5V and 65mA, but a USB 2.0 cord (which I’m using) is 5V and 500 mA. So the voltage is fine, but do I need to step down the current before I hook my fan up? If I do need to step down my current, does anyone know an easy way to do that?

I was looking at adding in another resistor in parallel to pull most of the current, but then I’m running into the problem of power ratings on resistors—I think I would need a 560 ohm resistor with a power rating of 107 W, which isn’t really realistic. And then I started playing with adding in more resistors in series and parallel to limit the power needed through one resistor and that was a whole other variable problem.

The fan will only “draw” 65mA when connected to 5V.

[u]Ohm’s Law[/u] says Current = Voltage/Resistance.

The current rating for a power supply is it’s maximum.

If you connect nothing you still have 5V but no current flows. If you connect 10 fans, you’ll exceed the current limits and “bad things” can happen.

I think I would need a 560 ohm resistor with a power rating of 107 W,

No… You don’t need a resistor, but…

Power (in Watts) = Current x Voltage
Or Power = Current2 x R
Or Power = Voltage2/R

5V across 560 Ohms is about 45 milliwatts.

DVDdoug:
The fan will only “draw” 65mA when connected to 5V.

[u]Ohm’s Law[/u] says Current = Voltage/Resistance.

The current rating for a power supply is it’s maximum.

If you connect nothing you still have 5V but no current flows. If you connect 10 fans, you’ll exceed the current limits and “bad things” can happen.
No… You don’t need a resistor, but…

Power (in Watts) = Current x Voltage
Or Power = Current2 x R
Or Power = Voltage2/R

5V across 560 Ohms is about 45 milliwatts.

PERFECT. Thank you for the sanity check!! That’s what I was thinking but then one of my friends made an offhand comment about the fan pulling too much current so then I got worried.

Ahhhh…I was trying to use the total current - the current through the fan (500 - 61 mA) and force that through the resistor. Power = (439 mA)^2 * 560 ohms = 107.9 Watts. BUT, that’s obviously not possible with a 5V source. It’s been waaaay too long since I tried to do simple circuit analysis.

Think of it this way. Your typical mains socket will drive a 5W night light, a 60W reading bulb, a 100 W room light, or a 1KW electric fire. All from the same socket who’s maximum current capability is, in the UK, 13 Amps.

but then one of my friends made an offhand comment about the fan pulling too much current

Try changing your friend for a less stupid one.

Grumpy_Mike:
Try changing your friend for a less stupid one.

Oh really!

Another Grumpy_Mike absolute Zinger!

I am compelled to give it a +1!