Help With LM338 Voltage Regulator

[electric_n00b]

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.

 

[KeepItSimpleStupid]

The average power will be (P1+P2)/(duty cycle)
A 50% duty cycle would be 0.5, and your Duty cycle would be (5/(5+30))
P1 is (Vin-4)/I1; P2 = (Vin-1.25)/I2
Compute I1 and I2 for the different values of V and your 2 ohm load.

You can put an optocoupler across the POT and turn the optocoupler on to get the 1.25 output voltage.

The actual calculations escape me for the moment, but you typically need the power dissipation from the above calcs, Junction temp for silicon (Usually about 150 C I think), and the ambient temp. Usually you also have to add the thermal resistances as well for the washer. etc. to select the heat sink.

So, do the calcs above for starters.

 

[Jony130]

Well the power dissipated by LM338 is equal to
Ptot = (Vin - Vout)*Iload

Iload = 7.125V/2ohm = 3.5625A

so

Ptot = (16V - 7.125V)*3.5625A = 31.61W

But if I understand correct you will ON the heater for 5s and for 30s heaters will be OFF.
If so, the power is equal to
P = 31.61*5/35 = 4.5W

Rth = (125 - 40)/4.5W = 18.9K/W

So you need a heatsink with RTh equal to 10K/W
https://uk.rs-online.com/web/p/heatsinks/2212154/

 

[KeepItSimpleStupid]

@joney

Your calculations would be correct if the load current went to zero. Agreed, we don;t know how the OP will turn on and off the load. Making the voltage 1.25 and 4 could make the regulator be the switch too.

 

[ChrisP58]

Heating elements are easily controlled by Pulse Width Modulation circuits.

Invert the output (to get <50%) of a 555 timer to the gate of an N channel mosfet.

You won't need any heatsink.

 

[electric_n00b]

Thanks for all the info guys, great stuff, just what I was after.

The power to the heater element will be controlled by a momentary switch that I operate manually when I want to take a drag on the atomiser. Typically I will have no more than a 5 second drag at no more than 30 second intervals -- it is usually a lot less than that.

I'm interested in the Pulse Width Modulation circuit idea, Chris. Could you possibly point me in the direction of some suitable info/schematics/components so that I could learn about this avenue further?

Thanks again.

 

[Jony130]

Here you have some info about 555 and PWM
http://www.reuk.co.uk/LED-Dimmer-Circuit.htm