Overloading an LM317
in [Design]
|
From: Mino on 7 Aug 2010 11:28 Hello, what happens when too much current is drawn from an LM317 configured for 3.3V output? Does the output voltage decrease?
From: John Larkin on 7 Aug 2010 11:55 On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote: >Hello, what happens when too much current is drawn from an LM317 >configured for 3.3V output? Does the output voltage decrease? It decreases a little bit as you load it, at any current [1]. At some large current you hit either its current limit or its thermal limit. At the current limit, voltage starts to drop steeply, such as to keep the load current constant. If it get too hot before that happens, most 317s will just shut off until they cool down some. Right now, we really need a low-voltage-drop current limiter, to protect a circuit powered by a dc/dc converter. Something like 150 mA would be good. Polyfuses suck. Discrete circuits, like the things James was playing with, would work, but I don't have room for all the parts. We're considering using an adjustable LDO and running it wide-open, so only the current+thermal limits are in use. John [1] I suppose a 317 could have a slight increase in voltage with load, if the thermal situation were right, but most will droop in real life.
From: legg on 7 Aug 2010 12:59 On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote: >Hello, what happens when too much current is drawn from an LM317 >configured for 3.3V output? Does the output voltage decrease? Yes. This part has a data sheet that should answer most of your basic questions. http://www.national.com/ds/LM/LM117.pdf RL
From: Phil Hobbs on 7 Aug 2010 15:18 On 8/7/2010 11:55 AM, John Larkin wrote: > On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote: > >> Hello, what happens when too much current is drawn from an LM317 >> configured for 3.3V output? Does the output voltage decrease? > > It decreases a little bit as you load it, at any current [1]. At some > large current you hit either its current limit or its thermal limit. > At the current limit, voltage starts to drop steeply, such as to keep > the load current constant. If it get too hot before that happens, most > 317s will just shut off until they cool down some. > > Right now, we really need a low-voltage-drop current limiter, to > protect a circuit powered by a dc/dc converter. Something like 150 mA > would be good. Polyfuses suck. Discrete circuits, like the things > James was playing with, would work, but I don't have room for all the > parts. We're considering using an adjustable LDO and running it > wide-open, so only the current+thermal limits are in use. > > > John > > [1] I suppose a 317 could have a slight increase in voltage with load, > if the thermal situation were right, but most will droop in real life. If you tune the input lead length and input bypass cap just right, you can make an AM-band transmitter with one! Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs at electrooptical dot net http://electrooptical.net
From: John Larkin on 7 Aug 2010 15:18
On Sat, 07 Aug 2010 15:18:30 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 8/7/2010 11:55 AM, John Larkin wrote: >> On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote: >> >>> Hello, what happens when too much current is drawn from an LM317 >>> configured for 3.3V output? Does the output voltage decrease? >> >> It decreases a little bit as you load it, at any current [1]. At some >> large current you hit either its current limit or its thermal limit. >> At the current limit, voltage starts to drop steeply, such as to keep >> the load current constant. If it get too hot before that happens, most >> 317s will just shut off until they cool down some. >> >> Right now, we really need a low-voltage-drop current limiter, to >> protect a circuit powered by a dc/dc converter. Something like 150 mA >> would be good. Polyfuses suck. Discrete circuits, like the things >> James was playing with, would work, but I don't have room for all the >> parts. We're considering using an adjustable LDO and running it >> wide-open, so only the current+thermal limits are in use. >> >> >> John >> >> [1] I suppose a 317 could have a slight increase in voltage with load, >> if the thermal situation were right, but most will droop in real life. > >If you tune the input lead length and input bypass cap just right, you >can make an AM-band transmitter with one! > Cool. 317 type regs make nice power amps, too. I think I've seen an audio amp based on one. A 317 and a 337 could make a nice dual-function class AB push-pull amp *and* AM transmitter. John |