Ohms law, electronics math question

[cauzomb]

Is this correct?
One finds resistance required to get a circuit down to 20mA by V/P=R

Inputing supply voltage minus v-drop across the component then, devide the desired mA to get required R?

say I want to kick my 30mA rated component down to 12ma;

I have a spec'd vdrop of 2.27v.

Supply is spec'd 2.5v

Supply measured 2.545 at full charge and 2.47 at low charge.. Going on the safe side and using full charge as my value for supply voltage.


(VS - Vdrop)x.012 = P
2.545-2.27=.275 "measured .31"
.31x.012=.00372
P=37.2 uW
R power rating minimum I could find was 1/4 watt 5%

then
V/P=R

.31 / .012 A = 25.8 Ohms

[TurcoLoco]

This sounds more like a Physics question!!

[cauzomb]

A physicist would probly know the answer... I'm fairly certain it's correct, but now I need to look and see how to switch this circuit on using a transistor pulled up from a lower power circuit, without crossing the wires from one side of the low power circuit over to the other side of the high power circuit... sort of like a relay..

[Mathster]

for the most part, current isn't controlled.
there are some special circumstances where it is.

What is the equipment your working with?

[cauzomb]

led circuit

[Mathster]

What is the voltage rating on the LEDs?
What is your source voltage?

Typically, an LED has a 2 to 3 volt drop. You can get LEDs with different voltage ratings, they are usually either clusters or have a resistor in series with them.

The easiest method is to buy a potentiometer and dial in the brightness you desire.
Mathematically, measure the resistance on the LED (forward).
I=V/R (use this to find the theoretical current or your target current. R that you measured on your LED, V at its rated voltage)
RR=V/I - RL where RR is your resistor, V is your actual voltage, I is calculated above, and RL is the resistance of you LED.

[F1]

Try this page for your formula's, Caz......

http://www.angelfire.com/pa/baconbacon/page2.html

[cauzomb]

What is the voltage rating on the LEDs?
What is your source voltage?

Typically, an LED has a 2 to 3 volt drop. You can get LEDs with different voltage ratings, they are usually either clusters or have a resistor in series with them.

The easiest method is to buy a potentiometer and dial in the brightness you desire.
Mathematically, measure the resistance on the LED (forward).
I=V/R (use this to find the theoretical current or your target current. R that you measured on your LED, V at its rated voltage)
RR=V/I - RL where RR is your resistor, V is your actual voltage, I is calculated above, and RL is the resistance of you LED.

My supply voltage is 2.545v full charge and 2.47 low charge, not to worried about the brightness, but I do have a basic stamp pwm to fool around with.. I can't reprogram it "haven't done that b4" so I don't really know.. But I can hook it up as a pulser to be more noticable.

Supply voltage measured across the LED = .31, or a total vdrop of 2.235v

The led's are Vf 3.2-3.6, but I'm ignoring this for my application; I'm just worried about the current because that can kill the LED fast and the circuit "wireless 360 controller" runs off 2.5v at a higher current rating than the typical watch battery LED circuit.. Strange thing is, with the controller on, the draw on the controller battery seams to either drop the voltage a little too low for my LED's or there's a transistor inside the battery that switches it to another leg of the contact.

Oh a new twist to an old trick, sticking axial caps in the wall plug is fun!/noisy and dangerous... the new twist on this is the LED.. they go off like firecrackers and send out shards of polycarbonate or whatever the lense is cast out of. But you know... You always gotta see what stuff does when you plug it into home AC

:P blown up capacitors, LED's, potatoes, uhm oh one time I hooked a 9v battery into the wall outlet... NASTY!!!! red hot melted stuff.... What little I knew when I was a kid, didn't realise I could have burn the house down... now all these things get plugged into an isolated psu at relatively low volts, but higher current :O

[Mathster]

http://www.theledlight.com/LED101.html

plugging diaelectric caps in backwards is fun too.
You really wanna see fun, get a van de graf machine and you can put a localized static charge on those clear plastic sheets teachers like to use as transparencies.
Another good one is to leave 1F caps laying around charged.

[cauzomb]

Another good one is to leave 1F caps laying around charged.

LOL! happened a couple times with broken camera flash caps. We used to charge them up, then use alligator clips to apply about 590v 55kuf to either end of various food items from the school cafeteria...The best was turning cooked rice into arc-lamps.