resistor size?

Question

I have a 9 Volt Dc input. My led runs on 1.7 volts, at 20 milliamps. Waht size resistor should i use? I need a lot of explaining cause i don't want to ask again later for something else

it also has to be a 1/4 watt resistor

Answer 1

First determine the voltage the resistor has to drop. This is the difference between the supply voltage and the forward LED voltage.

9V - 1.7V = 7.3V

Use OHM's law R= E/I to compute the resistance value for your desired current 20ma (.020A)

7.3 / .020 = 365 OHMS

Answer 2

You need a resistor that will drop 7.3 volts at 20 milliamps(0.020 amps). 9 - 1.7 = 7.3v
E(voltage)= I(current) x R(resistance)
7.3volts = .020amps x 365ohms

I x E = VA(power, in this case heat)
0.020 x 7.3 = .146VA(watts) Use a 1/4 watt resistor

You may not be able to find a resistor of exactly the right value, get one a close as you can and don't worry about it too much. If you are using a 9volt battery for your supply the voltage will decrease as the battery discharges but most LEDs are not that critical about it. The LED will not be a bright if the voltage (and therefor current) are a little low but you will have a hard time seeing the difference.

Answer 3

Ok first things first. your total circuit current needs to be 20 milliamps correct? Your power source it 9 VDC correct?

First find what the total resistance of the circuit needs to be. by using ohm's law.

I (current) = E (voltage)/ R (resistance)
E (voltage) = I (current) x R (resistance)
R (resistance) = E (volltage)/ I (current)

Need to know what total resistance needs to be. so use:

R (resistance) = E (voltage)/ I current)

R = 9 (VDC) / 20 milliamps

R = 9 / 20 x 10^-3

R = 450 ohms is going to be your total resistance of the circuit from now on we will refer to it as Rt.

Now you have to find out how much resistance your led has. Again use the ohm's law formulas.

Again it you will use R= E/I but this time we just want the resistance of the LED.

Rled ( the reisitance of the LED) = 1.7 VDC since that is the voltage you want across the LED. / 20 milliamps since that is the circuit's current.

SO:

Rled = 1.7VDC/ 20 milliamps
Rled = 1.7/ 20 x 10^-3
Rled = 85 ohm's
Now we know both the total resistance and the resistance of the LED. our next step is to find the size resistor we need.

R resistor = Rt - Rled
What we are doing is substracting the resistance of the LED from the total resistance of the circuit to get the resistance of the resistor we are going to need.

Rresistor = Rt - Rled
R resistor = 450 ohms minus 85 ohms
R resistor = 365 ohms.
Your next step is to go to your resistor chart and select the resistor which is the closest to 365 ohms.

Now we need to see if 1/4 size resistor is going to be large enough or if you need a bigger resistor either a 1/2 watt or 1 watt resistor.

We do this by applying a modified version of ohm's law a lot of people call Wattls law.

Basic formulas for this are.

W (watts) which can also be expressed P (power in watts)
R =resistance in ohms
I = current in amps.
E = voltage in volts E can at times be expressed as V.

W(watts) = I x R^2
P (power in watts) = I^2 x R both these two are the same formula in one you are just calling it watts in the other power.

P (power in watts) = E^2/ R
W (watts) = E^2/R again both the same formula just calling it watts in one and power in the other one.

P (power in watts) = I x E (current in amps times Voltage.)
W (watts) = I x E (current in amps times voltage) Again these two are the same formula only in one power is being expressed as P and in the other a W.

Quick review of power formulas (Watts Law)

P = I x R^2
P = E^2/R
P = I x E

One quick reminder do not use derived values. Use instead given values when calculating.

We had a voltage of 9 VDC which is given. We have a Current of 20 milliamps which is given. These are the two values we are going to use since they were given.
The Watt's law formula which best fits is:
P = I x E since both current and voltage were given.

P = 20 milliamps x 9 VDC
P = 20 x 10^-3 x 9 VDC
P = 0.18 watts which is expressed as 180 milli watts.

Now comparing to a 1/4 watt resistor which is 0.25 watts or 250 milliwatts we see that a 1/4 watt resistor it plenty big.

Hope that helped you out. All that is left for you to do is go to your resistor chart and get the nearest resistor. I am going to leave that one up to you.

Answer 4

Your resistor must be 365 ohms to protect your LED and serve the proper voltage to it.

If you have a 9V input and need to drop all but 1.7V in a resistor, then you have to drop (9-1.7) 7.3V in the resistor. Your current is the 20 milliamps (0.020A) your LED can handle so: Voltage / Current = Resistance (V/I = R) and: 7.3V / .020 A = 365 ohms.

As for power, Power = Voltage * Current = 7.3V * 0.020 A = 0.146 watt so you will have plenty of margin using a ¼-watt resistor.

Answer 5

in case you needed to shrink the voltage on your halogen mild to 9V employing a resistor, the cost might could desire to be 0.672 ohms. linked directly to the battery, your mild is ingesting 40 two.seventy 8 watts of potential. With a sequence resistor, the mild might eat 28.a hundred twenty five watts, and the resistor might eat 6.fifty six watts, for a entire of 34.685 watts. that may no longer a great distinction in potential intake, yet you will possibly observe an significant drop on your mild's brightness. i might propose some form of switching potential furnish, rather.

Answer 6

Everybody is right. The closest commercial sizes will be 330 or 390 Ohm 10%. Or 360 Ohm 5%.