Resistor Value..

I know there's a formula, but for the life and death of me, I can't remember what it is..

I need to drop 12V down to 1.2V (for easy. 10% of original). I think current needs to be a known, and unfortunately, I'm not sure. (It'll be powering a very small light bulb. Not a LED.)

Anyone?
Thanks!
Me.

Yes you will need to know the current.

Resistance = voltage (volts) / current (amps).

12V - 1.2V = 10.8

Resistance = 10.8 / current.

Rod.

If you know the wattage of the globe you can calculate current through the globe.

Wattage (watts) / voltage (volts across the globe) = current (amps).

Rod.

If you know the wattage of the globe you can calculate current through the globe.

Yeah, unfortunately I don't know the wattage. (Don't know much of nything on it, unfortunately. Except that it's a 1.5V light. I'm loweiring voltage to make it last a little longer.)

Thanks for the foruma, tho. I'm filing it away.

Thanks!
Me.

Thanks for the forumla! I'll probably have to.. come up with a small value. Margin of error is pretty darn large, relatively speaking..

Thanks!
Mike.

Also, consider the heat generated in the resistor: P=U*I (U=voltage over resistor, I=current through resistor). Can also be calculated as P=U^2/R or P=R*I^2.

You need a resistor that is rated for at least that amount of power dissipation, or you risk overheating it.

/Michael

Is it not marked on the bulb? Unless it's one of the grain-of-rice style ones it's usually etched into the metal fitting part.

Maybe some electronics guru can explain this, I've read about the resistor drop thing- so we is it I can't use a resistor to drop the 12V from a car battery to 5V so that it can be used to power a GPS or other 5V electronic device? All the schematics I've seen involve using some regulator either a 7805 or something more expensive. Am I misunderstanding something about what the voltage drop means in the context of a resistor?

As I understand it, you can use a resistor, but it just throws energy away as heat. A voltage regulator reduces the voltage much more efficiently. I believe that someone on this board explained how, but I don't remember who or how now.

Also, as the forumula does show, the resulting voltage out of the resistor varies as current changes. So, while you may get 5V from your GPS, hooking up an external player with HD to the save 5V source may give you 7-8V...

Me/.

No, it's not. It's an extremely tiny bulb. I mean.. TINY.
Me.

I've read about the resistor drop thing- so we is it I can't use a resistor to drop the 12V from a car battery to 5V so that it can be used to power a GPS or other 5V electronic device?

If the power (current) draw is constant you can use a resistor, but if the current draw varies, so will the voltage dropped across the resistor - and thus the voltage across the item you're powering. For instance, a portable CD player will consume different amounts of current depending of whether the motor is spinning or not, or how loud you turn up the volume.

A string of diodes (connected in series) offers a slightly more useful solution. If the item in question is designed to operate from small drycell batteries (ie AAA, A, etc., it already accepts a fair range of input voltage (voltage of fresh vs almost totally depleted - but still runs the item - batteries). Each diode typically drops 0.7 to 1V, across a fair range of current, and the sum of voltages, and the variance, can be enough to power a device. An old portable CD player I bought ran on 4 AA cells, but also came with a car adapter with simply had 7 1N4001 diodes in series inside.

A 78xx regulator isn't more efficient than a resistor solution, it just monitors and regulates the output voltage which offers a stable output voltage - it'll still turn to heat the product of the dropped voltage and the current (P=U*I) the device uses, plus a little extra used by the regulation circuitry.

To get better efficiency you need a switching power supply (essentially chopping up the DC into AC, feed that to a transformer and then use the output AC from the transformer to create DC again, at another voltage level). A switching power supply can both step up or step down voltage - a resistor or diode solution only step down. A switching power supply can use either AC or DC as input (with AC input, just skip the initial chopping up the DC stage). You can then regulate the input side so that you get "just enough" voltage and current on the output side.

/Michael

Sorry. I was apparently conflating regulators and switching power supplies. I guess I should avoid talking about things I know very little about. (Cheering ensues)