How do I calculate what resistor I would need?

Ok, so I'm working on a project. I need to know how I can reduce 18volts down to 12volts. If someone would explain how to calculate this myself I can figure out what resistor (or series of resistors) I need.

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TimC1151 year ago

I need to take the voltage down from 12v to 2v with a resistor.

Which do I use?

you need LM 317


Hey, Tim,

one way is to connect a 1,000 ohm and a 5,000 ohm resistor in series across your battery, And the LED in parallel to the 1,000 ohm resistor. This is not the most efficient circuit.

I've drawn the circuit, and will try to share the pic tomorrow

A more efficient method would depend on the specific LED specs (current). If you can find that info, post it.

here's the voltage divider, Tim.

gmoon Toga_Dan1 year ago

A voltage divider usually isn't a good choice for a current source. To get any useful current from that divider would require very low resistance values (like 2 ohm & 10 ohm), which makes the divider very inefficient. In the drawing above, the most current the 5000 ohm resistor will pass will be around 2 milliAmps (0.002 amps).

Dividers are good as voltage sources (like a reference), but that's for high impedance stuff that draws little current.

A series resistor with the load can work, but that requires a constant load, and series devices always draw the same amount of current. A voltage regulator, as others have suggested, is probably best.

Toga_Dan gmoon1 year ago

It sounds like Tim is an electronics newbie, and I'm guessing that the LED he wants to drive is a little one. like an indicator on a panel. An LED draws little current, so I made a guess on resistance. A slightly larger LED might require 500 and 100 ohm in place of 5000 and 1000. The divider circuit is easy for someone new to electronics.

If anyone wants to explain voltage regulators, zener diodes, etc to Tim, feel free.

here's a basic volt reg. Anyone care to explain how to choose components for it? If I once knew, I've forgotten. Actually, I'd like to know this stuff, too. GMOON?

volt reg.jpg
gmoon Toga_Dan1 year ago

T_Dan: The pic is there, if you click on it. Just the thumbnail is clipped.

Those work well. They make other output versions, including a 3.3V. They even make small ones the size of TO-92 transistors, like a standard 2N2222, for lower current requirements. An LM317 would be slightly more complicated, 'cause it's adjustable over a wide range.

I think the first capacitor would be much larger if it followed a transformer/bridge type power supply. Not so with a battery. The second cap (0.1uF) is probably for decoupling, shunting higher frequencies. Pretty standard.

The 1K resistor is there to make the LED work correctly with 5V.

TimC115: I hate to ask you for more details. It sounds like a proprietary thing you'd like to protect, and I'm guessing you're not here as rep for a major corporation. So maybe it's inappropriate to ask for more info...

Does adding / adjusting the size of a current limiting resistor change your LED effect? Does differing voltage really change the color? I'd have thought an LED hue is fixed, and only the brightness would change...unless you're using RGB LEDs...

phoey. pic got clipped. supposed to have 9v batt on left, 1k resistor and LED on R.

I have made and sold a simulated firefly for 20 years that comprises a 12 volt CPU fan on 2 spun 32 gauge wires, that pulls around a green LED that is 2.2 volts. To get the proper yellow/green hue, I push up the voltage, closer to 3, but right at 3 makes it pure yellow. I need to have it a little less. It's a novelty item, and cannot be comprised of a lot of heavier components.

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