lm338 regulator programming resistors
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From: Scott on 28 Jul 2010 22:26 The typical LM338 power supply circuit in the application notes uses a fixed 120 ohm resistor and a variable 2k pot. The LM338 resistance formula is Vout = 1.25 * (1+ R2/R1). With R2 as 2k and R1 as 120 ohms, this yields an adjustment range from 1.2 to 22 volts. I'm building a power supply and want to increase my range to around 30V or so. To get the full range I would need a 2.7k pot. Unfortunately, all I have on hand are 2k and 5k. Using a 5k pot means the voltage will be maxxed out somewhere just over half the range of the pot. So, what I'm wondering is what happens if we substitute a larger resistor for R1. a 5K pot with a 220 ohm resistor would be just about right. I've read the datasheet, but I haven't quite figured out what effect this would have. I've seen some web articles that mention using resistors as high as 240 ohms, but they aren't specific about what side effect this may have. Thanks, Scott
From: Grant on 29 Jul 2010 01:13 On Wed, 28 Jul 2010 19:26:10 -0700 (PDT), Scott <smbaker(a)gmail.com> wrote: >The typical LM338 power supply circuit in the application notes uses a >fixed 120 ohm resistor and a variable 2k pot. The LM338 resistance >formula is Vout = 1.25 * (1+ R2/R1). With R2 as 2k and R1 as 120 ohms, >this yields an adjustment range from 1.2 to 22 volts. I'm building a >power supply and want to increase my range to around 30V or so. > >To get the full range I would need a 2.7k pot. Unfortunately, all I >have on hand are 2k and 5k. Using a 5k pot means the voltage will be >maxxed out somewhere just over half the range of the pot. > >So, what I'm wondering is what happens if we substitute a larger >resistor for R1. a 5K pot with a 220 ohm resistor would be just about >right. I've read the datasheet, but I haven't quite figured out what >effect this would have. I've seen some web articles that mention using >resistors as high as 240 ohms, but they aren't specific about what >side effect this may have. You could place a resistor in parallel with the 5k pot to bring it down to 2.7k too. Always more than one solution. The 120 Ohm set resistor has the advantage of soaking up the bias current from the regulator. You can up that, provided you have a minimum load for regulation. Add an output LED or something to eat the bias or quiescent current. Grant. > >Thanks, >Scott
From: Scott on 29 Jul 2010 01:23 > You could place a resistor in parallel with the 5k pot to bring it > down to 2.7k too. Always more than one solution. I thought about that, but I ran the numbers through a parallel resistance calculator, and it looks like it messed up the linearity of the pot. Possibly not bad enough to make it unusable for a power supply, so that may be an option. Thanks for the tip on the LED; that might just do it.
From: Phil Hobbs on 29 Jul 2010 01:27 Grant wrote: > On Wed, 28 Jul 2010 19:26:10 -0700 (PDT), Scott <smbaker(a)gmail.com> wrote: > >> The typical LM338 power supply circuit in the application notes uses a >> fixed 120 ohm resistor and a variable 2k pot. The LM338 resistance >> formula is Vout = 1.25 * (1+ R2/R1). With R2 as 2k and R1 as 120 ohms, >> this yields an adjustment range from 1.2 to 22 volts. I'm building a >> power supply and want to increase my range to around 30V or so. >> >> To get the full range I would need a 2.7k pot. Unfortunately, all I >> have on hand are 2k and 5k. Using a 5k pot means the voltage will be >> maxxed out somewhere just over half the range of the pot. >> >> So, what I'm wondering is what happens if we substitute a larger >> resistor for R1. a 5K pot with a 220 ohm resistor would be just about >> right. I've read the datasheet, but I haven't quite figured out what >> effect this would have. I've seen some web articles that mention using >> resistors as high as 240 ohms, but they aren't specific about what >> side effect this may have. > > You could place a resistor in parallel with the 5k pot to bring it > down to 2.7k too. Always more than one solution. > > The 120 Ohm set resistor has the advantage of soaking up the bias > current from the regulator. You can up that, provided you have a > minimum load for regulation. Add an output LED or something to > eat the bias or quiescent current. > > Grant. >> Thanks, >> Scott If you add an op amp, you can make the resistances anything convenient. Alternatively, if you don't mind a bit of nonlinearity, you can load down the pot with a 5.90k resistor in parallel. Or, of course, use a 2k pot and an 86.6 ohm resistor from output to feedback. Cheers, Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Grant on 29 Jul 2010 02:06
On Thu, 29 Jul 2010 01:27:32 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >Grant wrote: >> On Wed, 28 Jul 2010 19:26:10 -0700 (PDT), Scott <smbaker(a)gmail.com> wrote: >> >>> The typical LM338 power supply circuit in the application notes uses a >>> fixed 120 ohm resistor and a variable 2k pot. The LM338 resistance >>> formula is Vout = 1.25 * (1+ R2/R1). With R2 as 2k and R1 as 120 ohms, >>> this yields an adjustment range from 1.2 to 22 volts. I'm building a >>> power supply and want to increase my range to around 30V or so. >>> >>> To get the full range I would need a 2.7k pot. Unfortunately, all I >>> have on hand are 2k and 5k. Using a 5k pot means the voltage will be >>> maxxed out somewhere just over half the range of the pot. >>> >>> So, what I'm wondering is what happens if we substitute a larger >>> resistor for R1. a 5K pot with a 220 ohm resistor would be just about >>> right. I've read the datasheet, but I haven't quite figured out what >>> effect this would have. I've seen some web articles that mention using >>> resistors as high as 240 ohms, but they aren't specific about what >>> side effect this may have. >> >> You could place a resistor in parallel with the 5k pot to bring it >> down to 2.7k too. Always more than one solution. >> >> The 120 Ohm set resistor has the advantage of soaking up the bias >> current from the regulator. You can up that, provided you have a >> minimum load for regulation. Add an output LED or something to >> eat the bias or quiescent current. >> >> Grant. >>> Thanks, >>> Scott > >If you add an op amp, you can make the resistances anything convenient. > Alternatively, if you don't mind a bit of nonlinearity, you can load >down the pot with a 5.90k resistor in parallel. Or, of course, use a >2k pot and an 86.6 ohm resistor from output to feedback. Or 91 + 1800 in E24 series :) Grant. > >Cheers, > >Phil Hobbs |