# LM2940 12V 1A Low Dropout Regulator

# LM2940 12V 1A Low Dropout Regulator

## Find out how to use an LM2940 to efficiently regulate voltage

**home**> electric circuit | storage | general | lighting

Many 12V-rated devices can be damaged by being exposed to voltages in excess of

**12 Volts**- particularly LED lights which have a maximum working voltage typically of

**13.5 Volts**.

Nominally

**12 Volt Lead acid batteries**typically operate in the range of 12.6V (40% charged) to 13.6 V (100% charged). When under charge, the voltage can easily get over 14V - sufficient to cause damage to sensitive 12V equipment.

## Voltage Regulators

A**voltage regulator**is a device which takes an input voltage and outputs a stable fixed voltage. Variable voltage regulators are available (such as the very useful

**LM317T**) which the user can set to output any required voltage (1.25 to 30V). For the more commonly required voltages of 3.3, 5, and 12 Volts there are

**fixed voltage regulators**which can be used to output just one voltage (such as the

**L7812**12V fixed regulator).

Unfortunately the regulators mentioned above have quite a high

**voltage drop**- typically up to

**3 Volts**. Therefore, to obtain a reliable 12 Volt output voltage, an input of 15V+ is required - more than a

*12V*battery produces even when under charge. To get around this problem

**low dropout regulators**were developed.

A

**low dropout regulator**supplies a fixed output voltage with a typical

*voltage drop*of just 0.5 Volts. This makes them perfect for use with 12 Volt lead acid batteries. Ideally a lead acid battery should never be allowed to get below 12.6 Volts (40% charge), and so since 12.6 - 0.5 is still over 12 Volts, a low dropout 12V regulator will be able to reliably supply a 12 Volt output whenever the battery has sufficient charge.

## LM2940CT 12 Volt 1 Amp Low Dropout Regulator

The

**LM2940CT-12**is a 12V low dropout voltage regulator integrated circuit (pictured above). It can output up to

**1 Amp**at a fixed 12 Volts DC with a dropout of just

**0.5 Volts**. It can accept input voltages from

**5.45V to 26V DC**. It can therefore be used to provide a fixed 12 Volt DC supply to a string of up to ten 12V 1 Watt LED spotlights guaranteeing their longevity.

NEW The

**LM2940CT-12**is now available for sale in the REUK Shop.

When the input voltage is below 12.5 Volts - as will happen if you let your battery drain below 40% charge, the output voltage will be approximately equal to the input voltage minus a

*voltage dropout*of 0.1 to 0.5 Volts. With a current draw of 0.4 Amps, the input-output voltage differential is 0.25V, and it is 0.5V at 1 Amp, and just 0.1V at 0.1 Amps.

If high voltages, or high currents (between 0.5 and 1 Amps) are to pass through the regulator, then a suitable

**heatsink*** (such as the one pictured above from the REUK Shop) should be bolted onto the regulator to help any generated heat dissipate.

*

*When using a lead acid battery for the input voltage, the maximum power to be dissipated by the regulator is (15-12)Volts * 1 Amp (according to*

**Ohm's Law**) = 3 Watts, and in most applications, the power dissipated will be far lower.The

**LM2940CT**range has internal short circuit and thermal overload protection, so you need to make a special effort to damage one. They are available in 5V (LM2940CT-5), 12V (LM2940CT-12), and 15V (LM2940CT-15) specifications.

Click here to view the

**LM2940 Specification Sheet**(PDF) for more technical information.

## Using the LM2940

Below is a diagram showing the three*legs*of the LM2940 - one for the input voltage, one for the output voltage, and one for the ground:

In order to maintain a

**stable**12 Volt output, an external

**capacitor**(of at least 22uF) must be used to smooth off the output from the integrated circuit (IC), with the capacitor located as close to the IC as possible.

Therefore, a typical working

**1 Amp 12 Volt regulator**using the LM2940VT-12 would look like this:

## Buy a 12 Volt Regulator

NEW We are now selling UK-built complete**12 Volt Regulators**in the REUK Shop based around the circuit shown above.

NEW - If you need a 12 Volt voltage regulator to supply more than 1 Amp of current to power an LCD television, laptop, or large amount of lighting, then take a look at our article on the Amperor ADP-90DC-12.

## Adjustable Low Dropout Voltage Regulators

If you need a**low dropout voltage regulator**which can be adjusted to give a non-standard voltage (in the range 5 to 15 Volts DC), click here to read our new article

**LM2941 1 Amp Low Dropout Adjustable Regulator**.

Article Last Modified: 22:21, 24th Sep 2014

## Comment on this Article

If you have any comments on this article, please email them to neil@reuk.co.uk.I am setting up a CCTV system with a DVR recorder with built in 250GB hard drive. It runs on 12 Volts and with 1 camera the drain will be about 2 Amps max. It may help to let others know there is another low drop regulator (LT1084CP-12) this is rated at 5 amps and may be an idea for higher loads. There is also a 3 amp and 7.5 amp version.This is the 12 Volt one I put together. The device to the right is a blocking diode (40 PIV 10 Amp Schottky rectifier with 0.2V dropout) to give protection if the battery is connected the wrong way.The unit will be on float charge with a 85 Ah battery so being able to have battery backup, but I hope this extra information on the above regulators is of help. low drop out regulators now)Peter, 10th September 2009 Update January 2012: Click here now to find out more about or to buy either LT1083 (7.5 Amp), LT1084 (5 Amp), or LT1085 (3 Amp)* Click to view the LT1083/84/85 Datasheet for further information on this range of low dropout voltage regulators from Linear Technology. |

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**relevant topics**:

**electric circuit, storage, general, lighting**.