# Getting a 2.5v reference voltage.

S

#### Simon

Jan 1, 1970
0
I have a circuit that specified a LM431 coupled with an a resisitor and
potentiometer to give an reference voltage of 2.5v to input in a comparator.
Im unable to get the LM431 from my usual supplier and so have done a google
search which returned a similar circuit using the LM338 for a 2.5v
reference - however once again my usually supplier does not have this part.

I can however get a LM317 - from what I can see it does the same job as both
the LM431 and LM338 - Can anyone explain what the differerence is, with the
aim of getting a 2.5 reference voltage.

Whu is a regulator needed in any case? Could 2.5v not be acheived by a
simple potential divider?

J

#### John Popelish

Jan 1, 1970
0
Simon said:
I have a circuit that specified a LM431 coupled with an a resisitor and
potentiometer to give an reference voltage of 2.5v to input in a comparator.
Im unable to get the LM431 from my usual supplier and so have done a google
search which returned a similar circuit using the LM338 for a 2.5v
reference - however once again my usually supplier does not have this part.

I can however get a LM317 - from what I can see it does the same job as both
the LM431 and LM338 - Can anyone explain what the differerence is, with the
aim of getting a 2.5 reference voltage.

Whu is a regulator needed in any case? Could 2.5v not be acheived by a
simple potential divider?

References generally produce lower noise, have tighter tolerances
(initial and drift with time and temperature) and operate on less
power than regulators. But they share many similarities. A divider
perfectly fixed load current) and does not compensate for any changes
in supply voltage. Otherwise, they are quite useful.

F

#### Fred Bloggs

Jan 1, 1970
0
Simon said:
I have a circuit that specified a LM431 coupled with an a resisitor and
potentiometer to give an reference voltage of 2.5v to input in a comparator.
Im unable to get the LM431 from my usual supplier and so have done a google
search which returned a similar circuit using the LM338 for a 2.5v
reference - however once again my usually supplier does not have this part.

I can however get a LM317 - from what I can see it does the same job as both
the LM431 and LM338 - Can anyone explain what the differerence is, with the
aim of getting a 2.5 reference voltage.

Whu is a regulator needed in any case? Could 2.5v not be acheived by a
simple potential divider?

A more confident answer could depend upon the specifics of the
comparator circuit, but generally a potentiometric divide will do unless
there is some other loading on the 2.5V that pulls the voltage down due
to potentiometer resistance.

S

#### Spehro Pefhany

Jan 1, 1970
0
I have a circuit that specified a LM431 coupled with an a resisitor and
potentiometer to give an reference voltage of 2.5v to input in a comparator.
Im unable to get the LM431 from my usual supplier and so have done a google
search which returned a similar circuit using the LM338 for a 2.5v
reference - however once again my usually supplier does not have this part.

I can however get a LM317 - from what I can see it does the same job as both
the LM431 and LM338 - Can anyone explain what the differerence is, with the
aim of getting a 2.5 reference voltage.

Whu is a regulator needed in any case? Could 2.5v not be acheived by a
simple potential divider?
Just a note- if you can't get an LM431 or exact equivalent from your

They are made by National, Philips, Fairchild, Zetex, Korea
Electronics, TI, etc. A real jellybean part. Last I looked, just about
every PC power supply used one.

Best regards,
Spehro Pefhany

P

#### Paul_Morphy

Jan 1, 1970
0
Simon said:
I have a circuit that specified a LM431 coupled with an a resisitor and
potentiometer to give an reference voltage of 2.5v to input in a comparator.
Im unable to get the LM431 from my usual supplier and so have done a google
search which returned a similar circuit using the LM338 for a 2.5v
reference - however once again my usually supplier does not have this
part.

You don't say where you are, but if you're in the U.S., B.G. Micro
(bgmicro.com) has an IPS338AV which substitutes nicely for the LM338. I
just bought one from them for use in a 1.25-16 V regulated supply. It's a
fat regulator, though, capable of sourcing at least 5 A. Seems overmuch for
a reference-V source. B.G. Micro also has the LM317T for 69 cents. Do a
Google search for "LM317 data" for application info. For small orders, if
envelope, saving you from having to pay their $6 min shipping charge. Regulators are on page 10 of their online (PDF) catalog. "PM" T #### Tim Wescott Jan 1, 1970 0 Simon said: I have a circuit that specified a LM431 coupled with an a resisitor and potentiometer to give an reference voltage of 2.5v to input in a comparator. Im unable to get the LM431 from my usual supplier and so have done a google search which returned a similar circuit using the LM338 for a 2.5v reference - however once again my usually supplier does not have this part. I can however get a LM317 - from what I can see it does the same job as both the LM431 and LM338 - Can anyone explain what the differerence is, with the aim of getting a 2.5 reference voltage. Whu is a regulator needed in any case? Could 2.5v not be acheived by a simple potential divider? Use a voltage reference when you need good absolute accuracy. Conveniently enough if you have a ratiometric sensor and can supply it from your chip supply, the best reference to use for your ADC is also your chip supply. S #### Spehro Pefhany Jan 1, 1970 0 You don't say where you are, but if you're in the U.S., B.G. Micro (bgmicro.com) has an IPS338AV which substitutes nicely for the LM338. I just bought one from them for use in a 1.25-16 V regulated supply. It's a fat regulator, though, capable of sourcing at least 5 A. Seems overmuch for a reference-V source. B.G. Micro also has the LM317T for 69 cents. Do a Google search for "LM317 data" for application info. For small orders, if you ask in advance, B.G. Micro will just send the parts in a padded envelope, saving you from having to pay their$6 min shipping charge.
Regulators are on page 10 of their online (PDF) catalog.

"PM"

Or in the UK, for the TL431, 4 different manufacturers, all in stock:-

http://uk.farnell.com/jsp/search/se...XCFE4AVAAS2IV3?action=0&ImgDisp=Y&QText=TL431

Best regards,
Spehro Pefhany

R

#### R.Lewis

Jan 1, 1970
0
Simon said:
I have a circuit that specified a LM431 coupled with an a resisitor and
potentiometer to give an reference voltage of 2.5v to input in a comparator.
Im unable to get the LM431 from my usual supplier and so have done a google
search which returned a similar circuit using the LM338 for a 2.5v
reference - however once again my usually supplier does not have this part.

I can however get a LM317 - from what I can see it does the same job as both
the LM431 and LM338 - Can anyone explain what the differerence is, with the
aim of getting a 2.5 reference voltage.

Whu is a regulator needed in any case? Could 2.5v not be acheived by a
simple potential divider?

The '431' is such a common part that I find it difficult to believe that any
supplier would be unable to source it.
It is made by a host of companies.
Maybe they know it better as a TL431, or TL431A.

J

#### Joerg

Jan 1, 1970
0
Simon,

As the others have suggested, widen your search parameters for the 431. It is as
widespread as sand on the beaches. A distributor not having any of these sounds
to me like a grocery that doesn't have salt. There is also the LMV431 version
which might work for you.

Regards, Joerg.

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