LM317T Is there a minimum voltage inputtoget the required output voltage?

RodneyB

Joined Apr 28, 2012
697
Is there a Voltage drop when making a voltage regulator when using the LM317T.

I am trying to regulate the voltage onto a small load to 12 Volts. I have a battery with a voltage of 13.8 volts as the supply but the output never can be set at 12 Volts. I an trying to understand why

ScottWang

Joined Aug 23, 2012
7,398
Voltage drop : VI – VO, Input-to-output differential voltage, 3V
LM317 datasheet -- Please check the page 4.

RodneyB

Joined Apr 28, 2012
697

bertus

Joined Apr 5, 2008
22,276
Hello,

For a lower voltage difference, you will have to use a LDO (low drop out) voltage regulator.
The LD1117 or LM2940 will do the job.

Bertus

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dl324

Joined Mar 30, 2015
16,909
I am trying to regulate the voltage onto a small load to 12 Volts. I have a battery with a voltage of 13.8 volts as the supply but the output never can be set at 12 Volts.
How small of a load and what is the required voltage regulation?

RodneyB

Joined Apr 28, 2012
697
Voltage drop : VI – VO, Input-to-output differential voltage, 3V
LM317 datasheet -- Please check the page 4.
Sorry I don't see where it shows on Page 4 the voltage
How small of a load and what is the required voltage regulation?
The load is 50mA. I would like to keep the voltage at 12 volts

Papabravo

Joined Feb 24, 2006
21,225
Sorry I don't see where it shows on Page 4 the voltage

The load is 50mA. I would like to keep the voltage at 12 volts
On the fourth page, section 7.3, second line item, Vi - Vo Input-to-output differential voltage, MIN(imum) 3V, MAX(imum) 40V.
For 12V out you need at least 15V in.

WBahn

Joined Mar 31, 2012
30,045
Also, even with an LDO regulator, the 13.8 V source that you mention sounds suspiciously like a fully charged lead acid battery. That voltage will drop as the charge drops, so you will probably drop out the regulator before using very much of the battery charge.

dl324

Joined Mar 30, 2015
16,909
The load is 50mA. I would like to keep the voltage at 12 volts
Unnecessary requirements will make for an unnecessarily complex solution. If you're powering from a battery, the voltage will drop; even an LDO regulator will be hard pressed to provide a regulated 12V when the battery voltage is in that vicinity.

WBahn

Joined Mar 31, 2012
30,045
If you really need 12 V from a source that is in the vicinity of 12 V, then you will probably need to use a boost/buck regulator that can handle input voltages both above and below the targeted 12 V output.

Papabravo

Joined Feb 24, 2006
21,225
Also, even with an LDO regulator, the 13.8 V source that you mention sounds suspiciously like a fully charged lead acid battery. That voltage will drop as the charge drops, so you will probably drop out the regulator before using very much of the battery charge.
It is also the standard DC power supply voltage for Amateur Radio equipment.

WBahn

Joined Mar 31, 2012
30,045
It is also the standard DC power supply voltage for Amateur Radio equipment.
Probably because it was historically provided by a lead acid battery.

Note that the TS specified that he is using a battery.

bertus

Joined Apr 5, 2008
22,276
Hello,

The typical dropout voltage of the LM2940 is about 0.1 Volt @ 100 mA:

Bertus

hp1729

Joined Nov 23, 2015
2,304
Is there a Voltage drop when making a voltage regulator when using the LM317T.

I am trying to regulate the voltage onto a small load to 12 Volts. I have a battery with a voltage of 13.8 volts as the supply but the output never can be set at 12 Volts. I an trying to understand why
For 13.8 V in and 12 V out consider just using three rectifiers in series to drop the voltage down.

dl324

Joined Mar 30, 2015
16,909
For 13.8 V in and 12 V out consider just using three rectifiers in series to drop the voltage down.
The OP wanted to supply 12V, which I took to mean precise to at least two decimals, to the load...

hp1729

Joined Nov 23, 2015
2,304
The OP wanted to supply 12V, which I took to mean precise to at least two decimals, to the load...
Yep, then LDO regulators are the way to go.

dl324

Joined Mar 30, 2015
16,909
Yep, then LDO regulators are the way to go.
Unless the OP plans to trim the LDO, it may not give the precision he wants. Datasheet says 11.40-12.60V over the full temperature range. If the circuit will tolerate 5% supply variation, a couple diodes could be sufficient (when the battery is fully charged).

hp1729

Joined Nov 23, 2015
2,304
Unless the OP plans to trim the LDO, it may not give the precision he wants. Datasheet says 11.40-12.60V over the full temperature range. If the circuit will tolerate 5% supply variation, a couple diodes could be sufficient (when the battery is fully charged).
Of course. Always a given.