Technology 3 course

How to Calculate the Value of Resistor for different types of LED’s Circuits

This tutorial will help you to find the proper value of resistor (or resistors) for one or more LED’s to connect with battery. If you pick this topic, you will be able to:

• Calculate the value of resistors for different LED’s Circuit diagrams
• Calculate the Forward Current of LED’s
• Calculate the Forward Voltage for different LED’s Circuits
• Connect LED’s in Series with batter
• Connect LED’s in Parallel with battery
• Connect LED’s in Series-Parallel Circuits

Update: You can Also use this LED Resistor Calculator for this purpose

Typical LED Symbol, Construction and Lead Identification.

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Before we go in detail, we will try to get ride on below simple circuit, so that the other calculation will be easier to understand.

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 This is the Simplest LED Series circuit ever.

Here, the supply voltage is 6V, LED Forward Voltage (V_F) is 1.3 Volt and Forward Current (I_F) is 10mA.

Now the Value of resistor (which we will connect in Series with LED) for this circuit would be:

Resistor Value = (V_supply– V_F)/ I_F= (6 -1.3) / 10mA = 470 Ω

Current draw = 20mA

Resistor Power rating formula for this circuit

Resistor Power Rating = I_F²x Resistor Value= (10mA) ² x 470 Ω = 0.047W = 47mW

But This is the minimum required resistor value to ensure that resistor will not overheat, so its recommended that to double the power rating of resistor that you have calculated, therefore, choose 0.047W x 2 = 0.094W = 94mW resistor for this circuit.Resistor power rating (Value is doubled) = 0.094 W = (94 mW)

Also keep in mind that:

• It is too difficult to find the exact power rating resistors that you have calculated. Generally, Resistors come in 1/4 watt, 1/2 watt, 1 watt, 2 watt, 5 watt, and so on. Therefore, select the next higher value of power rating. For example, if you’re calculated value of resistor power rating is 0.789W = 789mW, then you would select 1W Resistor.
• It is too difficult to find the exact value of resistors that you have calculated. Generally, Resistors come in standard values. If you are not able to find the exact value of resistor that you have calculated, and then select the next coming value of resistor that you have calculated, For Example, if the calculated value is 313.5Ω, you would use the closest standard value, which is 330 Ω. if the closest value is not close enough, then you can make it by connecting resistors in series – parallel configuration.
• I_F = Forward Current of LED: This is the amount of maximum current that LED can accept continuously. It is recommended that provide 80% of LED forward current rating for long life and stability. For example, if the rating current of LED is 30mA, then you should run this LED on 24mA. Value of current over this amount will shorten LED life or may start to smock and burn.
• If you are still unable to find the LED forward current, than assume it 20mA because a typical LED’s run on 20mA.
• V_F = Forward Voltage of LED: This is the forward voltage of LED i.e. the voltage drop when we supply the rated forward current. You can find this data on LED’s Packages, but is somewhere between 1.3V to 3.5V depending on type, color and brightness. If you are still unable to find the forward voltage, simply connect the LED through 200Ω with 6V battery. Now measure the voltage across LED. It will be 2V and this is the forward voltage.

Formula for finding the value of resistor(s) to connect LED’s in Series:

Below is another simple LED’s (LED’s Connected in Series) Circuit. In this circuit, we have connected 6 LED’s in Series. Supply Voltage is 18V, The Forward Voltage (V_F) of LED’s is 2V and the forward Current (I_F) is 20mA each.

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Resistor Value (LED’s in Series) = (V_supply – (V_F x No. of LED’s)) / I_F

Here, Total forward voltage (V_F) of 6 LED’s = 2 x 6 = 12V

and forward Current (I_F) is same (i.e. 20mA)

(Note: this is a series circuit, so current in series circuit in each point is same while voltages are additive)Now, the value of resistor (for Series Circuit) would be:

= (V_supply – (V_F x No. of LED’s)) / I_F= (18 – (2 x 6)) / 20mA

= (18-12) / 20mA = 300 Ω

Total Current draw = 20mA

(This is series circuit, so currents are same)Resistor Power Rating

= I_F²x Resistor Value= (20mA) ² x 300 Ω = 0.12 = 120mW

But This is the minimum required resistor value to ensure that resistor will not overheat, so its recommended that to double the power rating of resistor that you have calculated, therefore, choose 0.12W x 2 = 0.24W = 240mW resistor for this circuit.Resistor power rating (Value is doubled) = 0.24 W = (240 mW)

Formula for finding the value of resistor(s) to connect LED’s in Parallel (With Common Resistor):

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In this circuit, we have connected LED’s in parallel with common resistor. Supply Voltage is 18V, The Forward Voltage (V_F) of LED’s is 2V and the forward Current (I_F) is 20mA each.

Resistor Value (LED’s in parallel With Common Resistor) = (V_supply – V_F)/ (I_F x No. of LED’s)

Here, Total forward Current (I_F) of 4 LED’s = 20mA x 4 = 0.08A, and forward Voltage (V_F) is same (i.e. 2V)

(Note: this is a parallel circuit, so voltage is parallel circuit is same in each point while currents are additive).

Now, the value of resistor (for parallel Circuit with common resistor) would be:

= (V_supply – V_F)/ (I_F x No. of LED’s)

= (18 – 2) / 0.08

= 200 Ω

Total Current draw = 20mA x 4 = 80mA

(This is parallel circuit, so currents are additive)

Resistor Power Rating = I_F²x Resistor Value= (20mA) ² x 200Ω = 0.08 W = 80mW

But This is the minimum required resistor value to ensure that resistor will not overheat, so its recommended that to double the power rating of resistor that you have calculated, therefore, choose 1.28W x 2 = 2.56W resistor for this circuit. Resistor power rating (Value is doubled) = 2.56W (280 mW)

Formula for finding the value of resistor(s) for connecting LED’s in Parallel (With Separate resistor)

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This is another way to connect LED’s in parallel with separate resistors. In this circuit, we have connected 4 LED’s in parallel with separate resistors. Supply Voltage is 9V and the Forward Voltage (V_F) of LED’s is 2V and the forward Current (I_F) is 20mA each.

Resistor Value (LED’s in parallel with separate Resistor) = (V_supply – V_F)/ I_FHere, Total forward voltage (V_F) of LED’s = 2 and forward Current (I_F) 20mA (i.e. 20mA)

(Note: this is a parallel circuit, but we are finding the value of resistor for each section, not for whole circuit. So in each section, the circuit becomes in Series position (refer to the Series Circuit formula or the 1^st simple circuit above, you will find that these are same)

Now, the value of resistor (for parallel Circuit with separate resistors) would be:

= (V_supply – V_F)/ I_F= (9 – 2) / 20mA = 350 Ω

Total Current draw = 20mA x 4 = 80mA (This is parallel circuit, so currents are additive)

Resistor Power Rating = I_F²x Resistor Value= (20mA) ² x 350 Ω = 0.14 = 140mW

But This is the minimum required resistor value to ensure that resistor will not overheat, so its recommended that to double the power rating of resistor that you have calculated, therefore, choose 0.14W x 2 = 0.28W = 280mW resistor for this circuit.Resistor power rating (Value is doubled) = 0.28 W (280 mW)

There is another way (Series-Parallel Combination) to connect LED’s with battery; if you understood this simple calculation then I’m sure that you can easily calculate the value of resistors for Series-Parallel Combination LED’s connection circuit as well.

 By: Engr Wasim KhanCopyrigt @ http://electricaltechnology.org/