lm338 regulator programming resistors
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From: Phil Hobbs on 29 Jul 2010 02:14 Grant wrote: > 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. You're no fun anymore. ;) Cheers Phil Hobbs
From: whit3rd on 29 Jul 2010 18:00 On Jul 28, 7:26 pm, Scott <smba...(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. ... 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. So, replace the 120 ohm resistor with a smaller one to make the 2k pot work. The pot is unlikely to be high precision, you'll want to wire it up then use a resistor-substitution box in parallel with the 120 ohm starting-point resistor to find the best R value.
From: David Eather on 29 Jul 2010 19:19 On 29/07/2010 3:13 PM, 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 Change r1 to 82 ohms - the only downside is a slightly higher biasing current - a milliamp or so, which is totally irrelevant.
From: David Eather on 29 Jul 2010 19:22 On 29/07/2010 3:27 PM, Phil Hobbs 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. > > Cheers, > > Phil Hobbs > A serious question. You didn't suggest changing R1. Is there a consideration I am missing?
From: kevin93 on 29 Jul 2010 20:32
On Jul 28, 7:26 pm, Scott <smba...(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. > > Thanks, > Scott This approach has the disadvantage that as you rotate the pot you they will often have momentary disconnects that will result in the max voltage (>30V in your case) at the output. It is much better to have an arrangement where the wiper is not carrying significant current and where any disconnects will cause the output to fall to a low voltage. I would do it with the pot being used as a divider from a reference and feeding the non-inv input of an opamp whose output feeds the LM338 adj pin. Feedback from the output to the inv input of the opamp through a fixed divider will give the appropriate voltage range (together with a compensation capacitor). The LM338 will then act as a protected output stage (overcurrent and thermal protection). A high value resistor and a medium value cap from the non-inv opamp input will ensure that any disconnects as the pot rotates will not cause any disturbances at the output and if they do occur they will be in a safe direction. kevin |