24 volts to 9 volts

So, i'm not accustomed to resistors at all, so, is is possible to use resistors to accomplish this, and if so how? Or if there are any other suggestions that are more practical then feel free to chime in! Thanks in advance guys!

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

If you have a constant current draw that is predictable and does not fluctuate, then yes. What is the product your trying to feed?

a voltage regulator would be better suited for your needs. Look for an LM7809.

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Kevin Pierson

2nd that on the 7809.  how much current do you require?

a LM350 or LM338, while adjustable, may suit you better if you need much more than 1.5 A.

don't forget about possible heat sink requirements.

m

1.5 amps should be more than enough, it's just a couple of leds, so could i have a diagram? Thanks again guys?

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

No feedback! Could someone describe how to calculate resistance using resistors to acquire certain voltage! just an example would be great and i should be able to take it from there! Thanks !

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

The 7809 is pretty basic - three pins. Voltage in, ground, voltage out. The tab of the regulator is common to the ground pin and can be grounded to the chassis of your project to increase heat dissipation.

The problem with using resistors to try and control voltage is the fact that resistors don't resist voltage, they resist current. You can set up a basic "voltage divider" circuit but as soon as you start to load the voltage it starts to drop because you are changing the parameters of the circuit. Resistors are good as voltage references but not for voltage regulators.

Another simple way to go is to use a resistor to limit current to a certain amount and then use a 9v zener diode. I, personally would just get a hold of a 7809 and be done with it.

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Kevin Pierson

Ditto - 7809.

But if it is only LEDs, then why not resistors?
Resistor size is R = V/I where V is the voltage you want dropped, I is the (max) LED current, & R is resistance. VIR is in Volts, Amps & Ohms (or Volts mA & k-Ohms if easier).

There is a big different between needing a certain voltage for equipment/loads as opposed to LEDs etc.

Wait wait! I'm gettin confused! So if i wanted one amp of current at 9v from a 24 volts source what size of resistor would i need? To KP thanks for the reccomendation but it isn't for me, just tryin to help a friend out and learn at the same time! Oldspark i'm following you but still kinda hazy, lol!

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

You are not going to get 1A of supply power out of a voltage divider circuit.  A voltage divider circuit is the only way that you are going to get variable voltage through resistors (remember, they resist current, not voltage). 

If you need a few mA of current to power an LED then you can get by with a resistor setup. 

What exactly are you trying to run?  You mention a couple LEDs but say you need 1A of current.  Are they raw LEDs or an LED array or a module of some sort? 

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Kevin Pierson

Raw leds! About two blue ones! Can't remember the specs on it off hand!

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

If you only want to run two LEDs you don't need to reduce the voltage as much as you need to restrict the current.  Because of this, a resistor would be a great solution.  Figure out what the forward current is on the LEDs at hand and then calculate a resistor using the formula R=V/I!

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Kevin Pierson

What i don't understand is how i'm coming up with the value of the resistor! Ok assuming the leds are 5watts each what would be the value of the resistor! Thanks for being patient i know i can get irritating! show me how you calculate it and i shouldn't have to be a bother to you again!

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

Do you have a link for the led's you purchased?  All that is needed from the spec sheet is their Vf...forward voltage and If..forward current.  Blue led's typically have a Vf of around 4 volts.  Let us know what you have and we'll go from there.

m

Thanks KP for the night shift.

LEDs are not 5W unless they are for lighting (torches, headlights etc).

If they are typical "bullet head" LEDs they are probably 20mA. Hence 0.02A x 2V or 4V = 40mW or 80mW.

A 5W LED would require a 10W dropping resistor (24V to 9V).


Find their current rating. The the resistance required is easy...
V=IR hence R = V/I (Volts, Amps, Ohms; or Volts, mA & k-Ohms)
Where V is the voltage drop required & I is the current through it.

EG, 20mA blue 4V LED on 24V; R = (24-4)/.02 = 20/.02 = 1,000 Ohm = 1kOhm (1kΩ).
If 2 blues in series, drop 24-4-4V = 16V, hence 16/.02 = 800 => 820Ω. (In practice, probably the 1kΩ would also be ok.)
If 9V@20mA, R = 9/.02 = 450Ω => 470Ω.

The power dissipated by each resistor is P=VI (Watts = the Voltage across it x the current Amp through it).
EG - for 1kΩ at 20V/20mA, P = 20 x 0.02 = 0.4W hence a 0.5W (1/2 Watt) resistor (or bigger) is required.

The others will consume less (same current, lower voltage).

FYI shortcut: From V=IR & P=VI we get P=VV/R or IIR.
EG - for 1kΩ at 20V/20mA; P = 20x20/1000 = .4W or P = .02 x .02 x 1000 = 0.4W.

Ahhhh! Thank you old spark! That's all i wanted! Thanks fellas for your time and info, it's appreciated!

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

Sorry - WRONG ANSWER!
You are supposed to knock my long explanations.
(LOL! I write for novices etc, not for those in the know.)

And though you may have underspecified your OP, I (we?) should have checked before continuing...
Yes - we know better, but when 9V is mentioned, from experience we assume some 9V device that requires regulated voltage...

The key was "LEDS" (only).
As to the V=IR bit, a search would have been quicker (excellent tuts & references on this site as well as elsewhere; and I find Wiki invaluable).
But the "5W" LED assumption was one well suited to this forum.
And I could have added that many ignore the LED voltage when calculating for 12V or more - ie, 2V has little impact; 4V may impact - but round resistors UP to then (next or next-next) preferred value, and LEDs are not that touchy anyway... But KIS... (?)

Thanks for the thanks. And to the other repliers (good stuff; as usual).