LM317 Constant Current Calculator, Tool

Here is a handy LM317 constant current calculator/tool. It allows you to quickly calculate the set resistor and the heatsink (is needed).

Sections of the calculator

Constants

Ambient temperature – this is the temperature of the surrounding air / environment. You can increase this variable accordingly.

Thermal Resistance, Junction-to-case, θ_JC – taken from LM317 datasheet. Different packages have different value.

Thermal Resistance, Junction-to-ambient, θ_JA – again, see this value in the datasheet

Thermal Resistance, Case-to-heatsink(no thermal compound), θ_CH – this one is an conservative estimate. It’s better to calculate worst case scenario then optimistic scenario. It will be lower in real world if you use thermal paste.

Inputs

V_IN – the input voltage. To minimize the heat it’s recommended to use as low value as possible. Bare in mine that you still need about 2.5V across LM317.

I_SET – this one is clear. Input your desired current value.

R_LOAD – input the load resistance. This one will give you the V_F value which is the resulting voltage across the regulator. If resulting value is lower than 2.5V V_F is displayed in red. This means that the regulator will not work.

θ_HS – this is the thermal resistance of the heatsink. You should be able to get this value from its datasheet. The better the heatsink the lower the value.

Formulas

R1 = I_SET x 1.25V – calculated value in Ohms

V_OUT = I_SET x R_LOAD   – calculated value in volts

P_IC1 = (V_IN – V_OUT) x I_SET    – calculated value in watts

P_R1 = I_SET^2 x R1 – calculated value in watts

tº_{IC1 =} tº_AMB + P_IC1 x θ_JA     – calculated value in ºC

tº_IC1HS = tº_AMB + P_IC1 x (θ_JC + θ_CH + θ_HS) – calculated value in ºC

LM317 spreadsheet >>>LINK<<<