LM338 Resistor Wattage - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 4th March 2013, 10:39 PM   #1
dsime42 is offline dsime42  United States
diyAudio Member
 
Join Date: May 2010
Default LM338 Resistor Wattage

Dumb question but please send me link or answer. :-(

I find calculators to find R2 for a given R1 but I don't find anything telling me how to calc the current involved. Probably saw it and just didn't recognize it.

I see that the LM338 has a max adj current of 100uA but also see that current flows from output to ground. (real shocker there)
So is current through R2 100uA > that the current through R1?
Is the current through R1 = Vreg/(R1+R2) ?
  Reply With Quote
Old 4th March 2013, 10:41 PM   #2
dsime42 is offline dsime42  United States
diyAudio Member
 
Join Date: May 2010
R1=240 Ohms
R2=5126 Ohms
Vreg=+28 Volts

So, How many watts?
  Reply With Quote
Old 4th March 2013, 11:11 PM   #3
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
Your answers are in the datasheet, using Ohm's law and some understanding of how the regulator functions (which is easy enough even I understand it). The LM338 maintains a typical 1.24V potential from its output to the adjust pin. So R1 will have 1.24V across it. Ohm's law then determines the current flow through R1. That same current then must flow through R2. So the R2 value can again use Ohm's law to determine the voltage desired, V=I*R or R=V/I. So far this has ignored that current flow out of the adjust terminal, 45μA typ and 100μA max. This "represents an error term" in the device, and normally the current through R1 is much greater than that and so it can be ignored. It is included in the standard formula,
Vout = Vref (1 + R2/R1) + Iadj*R2.
The max 100μA will have even smaller consequence in determining the R2 power rating. Calculate the current through R1, add 100μA, and use that sum to multiply by the voltage across R2 (which is Vout - 1.24V). That gives the wattage dissipated in R2.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

Last edited by sofaspud; 4th March 2013 at 11:13 PM.
  Reply With Quote
Old 4th March 2013, 11:53 PM   #4
dsime42 is offline dsime42  United States
diyAudio Member
 
Join Date: May 2010
Thanks
(R1) I=1.25/240 = .0052 A

(R1) W=(.0052)^2 * 240 = 0.0065 A
(R2) W=(.0052)^2 * 5136 = 0.1393 A

Looks like 1/4 watt will be fine.

Any idea why I see people specing these resistors @1 Watt or 2Watt?
  Reply With Quote
Old 5th March 2013, 12:11 AM   #5
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
Not at common output voltages.
For high voltage supplies, or high levels of R1 current, it may be necessary.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
HELP : what wattage for resistor ? fredericasse Analog Line Level 7 25th December 2012 11:21 AM
Resistor wattage anyone? serengetiplains Parts 2 3rd October 2011 11:53 PM
Resistor wattage? losacco Tubes / Valves 4 23rd March 2010 07:33 AM
resistor wattage? jarthel Pass Labs 1 27th June 2007 04:14 AM
zobel resistor wattage saramac Multi-Way 1 14th April 2007 09:17 PM


New To Site? Need Help?

All times are GMT. The time now is 01:25 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2