Help With LM338 Voltage Regulator

[Nusku]

Hi,

I would be grateful for any help in calculating how much heat an LM338 (TO-220 Package) voltage regulator will generate and what heatsink is required in the following application:

I want to build a variable voltage power supply for an electronic cigarette atomiser using items that I already have.

The input voltage for the LM338 circuit will be from an old laptop PSU rated at 16V, 4.5A. The load on the LM338 output will typically be a 2 Ohm Kanthal wire coil powered for, say, 5 second bursts with a minimum interval of 30 seconds.

This is the circuit:

Also, will there be any problem replacing R1 with a 2K resistor to limit the max voltage to 4.188V if I deem it necessary?

Thanks.

 

[erikl]

Here's a (zipped) Excel calculation sheet for your application:
View attachment Power-calculator.xls.zip

You can change the green fields for new calculations.

 

[chuckey]

The power dissipated by the LM338 is Vin - Vout times the current times the duty cycle. Vin - Vout = 12.4 V I = 24.8 W. Your duty cycle is 5/35 ~ 14% . Because the transistor is on for 5 seconds, it would get to its final temperature in this time. If this unit is going to be at different voltages, then sample calculations must be done to find out what the maximum point of dissipation.
From :- https://www.ti.com/lit/ds/symlink/lm338.pdf, max junction temp = 125 deg C, Tr j-c = 4 degsC/W, Tr j-a = 50 degsC /W. What about if the heater gets left on? either design the heat sink to carry the whole power or put a thermostat on the heatsink/thermal trip to trip the power if the heat sink gets too hot. So whats your ambien temperature - air conditioned lab = 20 degs C, industrial area or electronic cabinet = 40 degs C. At a power of 25 W the case drops 25 X 4 degrees from the junction, so now the maximum case temperature can only be 125 - 100 = 25 degsC. So at a continous dissipation the LM338 needs its case to be kept to less then 25 degs C !!! This is a bad design, get rid off some of the excess input voltage by putting a resistor in series, 8 ohms would halve the dissipation in the regulator , needs to be > 16W rating. The LM388, now has to drop 4V at 2 A, i.e. 8W, so its case now has a thermal drop from the junction of 4 X 8 = 32 degsC. So the its case can now be at 125 - 32 = 93 degs C. Given an ambient temperature of 40 degsC, then the heatsink has to get rid of 93-40 = 53 degsC at a power of 8W or 6 degs/W ,like :- https://uk.farnell.com/fischer-elektronik/fk-201sa-3/heat-sink-to-3-6-c-w/dp/4621359, this is for TO-3, you need TO 220.
Frank

 

[Nusku]

The power dissipated by the LM338 is Vin - Vout times the current times the duty cycle. Vin - Vout = 12.4 V I = 24.8 W. Your duty cycle is 5/35 ~ 14% . Because the transistor is on for 5 seconds, it would get to its final temperature in this time. If this unit is going to be at different voltages, then sample calculations must be done to find out what the maximum point of dissipation.
From :- https://www.ti.com/lit/ds/symlink/lm338.pdf, max junction temp = 125 deg C, Tr j-c = 4 degsC/W, Tr j-a = 50 degsC /W. What about if the heater gets left on? either design the heat sink to carry the whole power or put a thermostat on the heatsink/thermal trip to trip the power if the heat sink gets too hot. So whats your ambien temperature - air conditioned lab = 20 degs C, industrial area or electronic cabinet = 40 degs C. At a power of 25 W the case drops 25 X 4 degrees from the junction, so now the maximum case temperature can only be 125 - 100 = 25 degsC. So at a continous dissipation the LM338 needs its case to be kept to less then 25 degs C !!! This is a bad design, get rid off some of the excess input voltage by putting a resistor in series, 8 ohms would halve the dissipation in the regulator , needs to be > 16W rating. The LM388, now has to drop 4V at 2 A, i.e. 8W, so its case now has a thermal drop from the junction of 4 X 8 = 32 degsC. So the its case can now be at 125 - 32 = 93 degs C. Given an ambient temperature of 40 degsC, then the heatsink has to get rid of 93-40 = 53 degsC at a power of 8W or 6 degs/W ,like :- https://uk.farnell.com/fischer-elektronik/fk-201sa-3/heat-sink-to-3-6-c-w/dp/4621359, this is for TO-3, you need TO 220.
Frank

Thanks for all the info, Frank. That's just what I was after.

I get that it's a pretty horrible design, I just want to quickly knock something up with what I have to hand...

Reducing the input voltage with a series resistor is an excellent idea, I think I will see what power resistors I have and pursue this avenue further.

Many thanks.