Please, need a ressistor to reduce 8.5 V to 4.5 V

[bambino]

Hi all,

Im not sure this is the right forum for this question

Im trying to adapt a laser pointer to my RC Battle Tank that shots bb bullets.

Well, I wanted to install it a laser pointer that normally is powered by 3 button cells (1.5 volts). I consider getting power from the air gun that gets 8.5 volts from the battery (a 9.6 volts NiCad). I think I will need a ressistor to go from 8.5 volts to 4.5 volts.

The issue here is, how do I choose the color code for the ressitor?

I remember back in my school days there was this law of Kirchoff and the triangle and so on and will need to determine how many Ohms do I need and then match it with a color code.

Can anybody give me a clue?

Thanks all!!!!

[Rodney]

I'm assuming that this will be used to power the LED's so will be a constant current. If so the value will be 8.5 minus 4.5 or 4 divided by the expected current. Say the current is 30 milliamperes, then the resistance would be 4/.03 or 133 ohms. The closest standard value would be 120 ohms. A 1/4 watt resistor would be adequate power rating. Color code would be brown,red,black.

[bambino]

Ok, yes, it will be used to power the laser pointer (continuous voltage) that is normally powered by 3 button cells wired in series (4.5 volts total) and yes, I think 30 miliamperes is ok to expect from a 1.5 volts button cell.

Thanks so much!!

[Alison F]

The triangles that you speak of...

PVC and VCR. ;)

VCR being the one you want. Voltage at the top, and then Current to the lower left, and Resistance to the lower right.

PVC being Power (Watts), Voltage, and Current (Amps).

Just spent ages trying to find this, as it seems to have been forgotton in this modern age; http://ourworld.compuserve.com/homepages/boyce_smith/formula.htm