Help with finding the right resistor

[MattG2826811]

so i need to reduce a dc 12v to dc 5v. do you know what type of resistor i should use? also could you please suggest an online calculator or an equation so that i can find the resistance i need in the future.

 

[billy_joule]

https://en.wikipedia.org/wiki/Voltage_divider

Note that:

Loading effect
The output voltage of a voltage divider will vary according to the electric current it is supplying to its external electrical load. To obtain a sufficiently stable output voltage, the output current must either be stable or limited to an appropriately small percentage of the divider's input current. Load sensitivity can be decreased by reducing the impedance of the divider, though this increases the divider's quiescent input current and results in higher power consumption (and wasted heat) in the divider. Voltage regulators are often used in lieu of passive voltage dividers when it is necessary to accommodate high or fluctuating load currents.

Applications
Voltage dividers are used for adjusting the level of a signal, for bias of active devices in amplifiers, and for measurement of voltages. A Wheatstone bridge and a multimeter both include voltage dividers. A potentiometer is used as a variable voltage divider in the volume control of many radios.

A voltage regulator like the LM7805 might be a better solution depending on your application.

 

[davenn]

so i need to reduce a dc 12v to dc 5v. do you know what type of resistor i should use? also could you please suggest an online calculator or an equation so that i can find the resistance i need in the future.

using a resistor is far from a good way to do it. This is because a resistor or resistive voltage divider as billy_joule suggested will also limit the current and that may be a bad thing for what you are trying to achieve

billy_joule's second suggestion is the best ... use a voltage regulator and ensure it can handle the current your project needs
And yes there are ways to boost current requirements via a regulator

for current requirements up to ~ 0.75A ( 750mA) a 7805 reg is ideal
for higher currents and much more efficient operation use a switching DC-DC buck converter
eg ...
http://www.ebay.com.au/itm/DC-DC-Buck-Converter-4-5-35V-to-1-25-30V-3A-Power-Module-24V-to-9V-12V-to-5V-etc-/171640130013?pt=LH_DefaultDomain_15&hash=item27f68c89dd

it can take a wide input voltage range and has an adjustable output and is capable of 3 Amps

cheers
Dave

 

[meBigGuy]

I like both previous posts and third the suggestion that you drop the voltage with a voltage regulator.

You need to compute the power dropped in the regulator and make sure it can dissipate the heat (that would be true for a resistor also).

The power is 12-5 = 7V times load current. If 100ma. then that is 0.7 watts. Thermal resistance junction to air (to-220 package) is 65C per watt. so the junction will be 65 * 0.7 = 45C above ambient without a heat sink. Max junction temp is 125C. How hot will the environment be?

Another way to look at it. Say a 50C ambient, that leaves 125C - 50C = 75C. 75C / 65C/W = 1.2 watts 1.2 watts/7V = ~160ma. Above that you need a heat sink.

 

[MattG2826811]

using a resistor is far from a good way to do it. This is because a resistor or resistive voltage divider as billy_joule suggested will also limit the current and that may be a bad thing for what you are trying to achieve

billy_joule's second suggestion is the best ... use a voltage regulator and ensure it can handle the current your project needs
And yes there are ways to boost current requirements via a regulator

for current requirements up to ~ 0.75A ( 750mA) a 7805 reg is ideal
for higher currents and much more efficient operation use a switching DC-DC buck converter
eg ...
http://www.ebay.com.au/itm/DC-DC-Buck-Converter-4-5-35V-to-1-25-30V-3A-Power-Module-24V-to-9V-12V-to-5V-etc-/171640130013?pt=LH_DefaultDomain_15&hash=item27f68c89dd

it can take a wide input voltage range and has an adjustable output and is capable of 3 Amps

cheers
Dave

https://en.wikipedia.org/wiki/Voltage_divider

Note that:
A voltage regulator like the LM7805 might be a better solution depending on your application.

thanks for the tip i think i will use the lm7805