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Electronics => Electronics => Topic started by: Xyver on May 16, 2010, 11:02:45 PM

Title: Simple resistor question
Post by: Xyver on May 16, 2010, 11:02:45 PM

I should know the answer to this, but how do I calculate the resistor value?  I have a 9.6 volt battery at 1000mAh, and i need to run a servo. 6 volts and 150 mA.
 
Isnt it (9.6 - 6)/0.15 for a resistor value of 24 ohms?  that seems kind of small...

What am I doing wrong?

Title: Re: Simple resistor question
Post by: Razor Concepts on May 16, 2010, 11:21:28 PM

Do not use a resistor to "lower" the voltage. It will not work and your servo will fry. The voltage into the servo will depend on the current it draws, and the servo will not draw a constant 150mah, so the voltage received will fluctuate greatly and damage the servo.

This is why voltage regulators exist  ;)

Title: Re: Simple resistor question
Post by: billhowl on May 17, 2010, 03:50:13 AM

For supply to servo, you should use more efficient switching regulator.
read more about it from here http://www.dimensionengineering.com/switchingregulators.htm (http://www.dimensionengineering.com/switchingregulators.htm)

Title: Re: Simple resistor question
Post by: Cristi_Neagu on May 19, 2010, 08:21:26 AM

If you plan on powering something that draws more than 1-2A of power, then, yeah, go with switching supplies. But for 150mA... A good ol' 7806 will work great. Or you could get an LDO. A bit more expensive, but it's more efficient.
Since the servo won't always draw 150mA, when it draws less, the voltage drop on the resistor will be less (Ohm's law again) and it will supply your servo with more than it can take, thus frying it.

Bottom line: go with a 7806.

Good luck.