Power Supply for a LM317
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Posted by goodfella on September 18, 2008, 9:24 am
Please log in for more thread options I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power supply. The LM317 can only handle 1.5 amps, but the circuits that I will be testing are rated at less than .5 amps. Will the 4 amp overdrive the LM317 even though I will be using less than an amp? Thanks | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Rheilly Phoull on September 18, 2008, 9:48 am
Please log in for more thread options goodfella wrote: Rheilly P (Dont forget to read books) | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Dave, I can't do that on September 18, 2008, 10:12 am
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> > overdrive the LM317 even though I will be using less than an amp?
>
> YUP !! Er, I don't know much about electronics, but isn't current the actual current being drawn and not necessarily the current that is available ? RP would be correct only if the device being powered suddenly drew more than 1.5A. Since you are only drawing 0.5A I can't see any problem, however depending on what you are testing there might be an inrush that could exceed 1.5A and then the 4A would come into play. A DC motor comes to mind where at start up current for a fraction of a second can be many times the run current. I would think you could use a current limiting resistor in the 4A line and keep your hands warm this Winter. <g>
I am NOT an expert on this so I suggest you wait for more definitive answers. Dave | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Ted Pavlic on September 18, 2008, 10:58 am
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> I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
> supply. =A0The LM317 can only handle 1.5 amps, but the circuits that I > will be testing are rated at less than .5 =A0amps. =A0Will the 4 amp > overdrive the LM317 even though I will be using less than an amp? > Thanks You will have no problem using the 16V wall wart with the LM317. The current rating on the wall wart tells you want it is CAPABLE of. In other words, it tells you the maximum power output of the device. In your case, you'll never need that much. Of course, the LM317 will draw a SMALL amount of extra power, and so you want a power supply that can provide GREATER than 0.5A. So you're in good shape with your supply and your IC. *) On a related note, it is easy to increase the current carrying ability of the LM317. If you look at the application examples at the end of the datasheet, you'll see how you can add external pass transistors around the LM317. As the LM317 draws more current, it will signal to the external power transistors that they should provide that current. So the LM317 becomes a control circuit driving pass transistors that actually carry most of the current. *) On another related note, if you look at the LM317 schematic, you'll find that internally it uses a Darlington pair as a set of pass transistors that do exactly the same as what I just mentioned. It has a bandgap reference to hold the 1.25V, and that reference drives a buffer which drives the base of the internal pass transistor which delivers all of the power to the output. --Ted | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Robert Monsen on September 18, 2008, 12:14 pm
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On Thu, 18 Sep 2008 06:24:08 -0700 (PDT), goodfella >I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
>supply. The LM317 can only handle 1.5 amps, but the circuits that I >will be testing are rated at less than .5 amps. Will the 4 amp >overdrive the LM317 even though I will be using less than an amp? >Thanks No, it isn't a problem. The LM317 will limit the current. Regards, Bob Monsen | |||||||||||||||||||||||||||||||||||||||||||
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> supply. The LM317 can only handle 1.5 amps, but the circuits that I
> will be testing are rated at less than .5 amps. Will the 4 amp
> overdrive the LM317 even though I will be using less than an amp?
> Thanks