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24 volts to 9 volts


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t&t tech 
Platinum - Posts: 2,601
Platinum spacespace
Joined: October 05, 2008
Location: Trinidad and Tobago
Posted: July 19, 2010 at 6:37 PM / IP Logged  
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!
JW.ORG.
COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF.
PSALMS 37:5
cpalmer 
Copper - Posts: 109
Copper spacespace
Joined: March 18, 2007
Location: United States
Posted: July 19, 2010 at 7:46 PM / IP Logged  
If you have a constant current draw that is predictable and does not fluctuate, then yes. What is the product your trying to feed?
KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: July 19, 2010 at 8:18 PM / IP Logged  
a voltage regulator would be better suited for your needs. Look for an LM7809.
Kevin Pierson
Ween 
Platinum - Posts: 1,364
Platinum spacespace
Joined: August 01, 2004
Location: Illinois, United States
Posted: July 19, 2010 at 9:34 PM / IP Logged  

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

t&t tech 
Platinum - Posts: 2,601
Platinum spacespace
Joined: October 05, 2008
Location: Trinidad and Tobago
Posted: July 20, 2010 at 5:38 PM / IP Logged  

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

JW.ORG.
COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF.
PSALMS 37:5
t&t tech 
Platinum - Posts: 2,601
Platinum spacespace
Joined: October 05, 2008
Location: Trinidad and Tobago
Posted: August 01, 2010 at 12:05 PM / IP Logged  

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 !

JW.ORG.
COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF.
PSALMS 37:5
KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: August 01, 2010 at 12:42 PM / IP Logged  
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.
Kevin Pierson
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: August 02, 2010 at 12:31 AM / IP Logged  
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.
t&t tech 
Platinum - Posts: 2,601
Platinum spacespace
Joined: October 05, 2008
Location: Trinidad and Tobago
Posted: August 02, 2010 at 1:53 PM / IP Logged  
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!
JW.ORG.
COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF.
PSALMS 37:5
KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: August 02, 2010 at 2:50 PM / IP Logged  

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? 

Kevin Pierson
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