LM338 Resistor Wattage
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 4th March 2013, 10:39 PM #1 dsime42   diyAudio Member   Join Date: May 2010 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) ?
 4th March 2013, 10:41 PM #2 dsime42   diyAudio Member   Join Date: May 2010 R1=240 Ohms R2=5126 Ohms Vreg=+28 Volts So, How many watts?
 4th March 2013, 11:11 PM #3 sofaspud   diyAudio Member     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.
 4th March 2013, 11:53 PM #4 dsime42   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?
 5th March 2013, 12:11 AM #5 sofaspud   diyAudio Member     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

 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 Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

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

 New To Site? Need Help?

All times are GMT. The time now is 12:00 AM.