resistor value for LM317

[Snaggy]

usually a fixed resistor value of 220 ohm is used, with a 5k trimmer.
Is there any problem if I use a 450 ohm resistor with a 10k trimmer?
Maybe the current on the regulation pin will be too low? Any other
possible problem?

thanks

 

[Rich Webb]

On Wed, 26 May 2010 10:12:18 -0700 (PDT), Snaggy <l.cioria@gmail.com>
wrote:

usually a fixed resistor value of 220 ohm is used, with a 5k trimmer.
Is there any problem if I use a 450 ohm resistor with a 10k trimmer?
Maybe the current on the regulation pin will be too low? Any other
possible problem?

The necessary minimum operating (e.g., load) current varies with the
delta-V across the regulator (Vin-Vout) and with temperature. The
guaranteed largest required minimum seems to be about three times as
large as typical: 10 mA vs 3.5 mA at a 40 V differential and 25C.

If your dV is only around 5 V then the typical minimum necessary load is
only about 1.5 mA, but 5 mA would be safer.

Grind through the datasheet; lots of good information there.

--
Rich Webb Norfolk, VA

 

[whit3rd]

On May 26, 10:12 am, Snaggy <l.cio...@gmail.com> wrote:

usually a fixed resistor value of 220 ohm is used, with a 5k trimmer.
Is there any problem if I use a 450 ohm resistor with a 10k trimmer?
Maybe the current on the regulation pin will be too low?

Not just 'on the regulation pin'; the OUTPUT current of an LM317
must be over 4 mA for proper bias, and if the recommended
resistances are used, that happens without any load.
So, is your load going to reliably sink current? How much?

 

[Snaggy]

On May 26, 8:52 pm, whit3rd <whit...@gmail.com> wrote:

On May 26, 10:12 am, Snaggy <l.cio...@gmail.com> wrote:

usually a fixed resistor value of 220 ohm is used, with a 5k trimmer.
Is there any problem if I use a 450 ohm resistor with a 10k trimmer?
Maybe the current on the regulation pin will be too low?

Not just 'on the regulation pin'; the OUTPUT current of an LM317
must be over 4 mA for proper bias, and if the recommended
resistances are  used, that happens without any load.
So, is your load going to reliably sink current?  How much?

I understand.. is it ok if I add a load (led in series with 2k
resistor -> 2,5mA at 5 volts and more than 10mA at 30 volts)?
It should be more than enough, right?

 

[Don Klipstein]

In <2857c2a6-a35d-4587-b6c9-13afca22d111@i31g2000vbt.googlegroups.com>,
Snaggy wrote:

On May 26, 8:52 pm, whit3rd <whit...@gmail.com> wrote:
On May 26, 10:12 am, Snaggy <l.cio...@gmail.com> wrote:

usually a fixed resistor value of 220 ohm is used, with a 5k trimmer.
Is there any problem if I use a 450 ohm resistor with a 10k trimmer?
Maybe the current on the regulation pin will be too low?

Not just 'on the regulation pin'; the OUTPUT current of an LM317
must be over 4 mA for proper bias, and if the recommended
resistances are  used, that happens without any load.
So, is your load going to reliably sink current?  How much?

I understand.. is it ok if I add a load (led in series with 2k
resistor -> 2,5mA at 5 volts and more than 10mA at 30 volts)?
It should be more than enough, right?

There is a simpler way to make a LM317 into a current regulator.

From the regulation pin to ground, put a resistor of such value that
voltage across it is 1.2 volts at the desired amount of current plus the
amount of current that the regulation pin sources. For 10 mA, this is 120
ohms if the regulation pin sources only negligible current, 85.7 ohms if
the regulation pin sources 4 milliamps. 82 ohms is a standard value that
results in at least 10.6 to approaching 14.6 mA, and 100 ohms is a standard
value that results in at least 8 to approaching 12 mA, and 120 ohms is a
standard value that results in at least 6 to approaching 10 mA.

56 ohms results in at least 17.4 to approaching 21.4 mA. 47 ohms
results in at least 21.5 to approaching 25.5 mA. Apply to these pairs of
figures the resistor tolerance percentage of the upper figure in each pair
(as in +/- whatever percentage of 1.2V divided by resistor value.)

These figures are "on paper", and I did not actually measure them,
although I have used this arrangement as a 20 mA regulated current "LED
driver", probably with 56 ohms.

Connect the load from the ouput pin to the regulation pin that has the
above resistor going to "ground" AKA "common negative".

I hope this helps at least someone,

--
- Don Klipstein (don@misty.com)