voltage regulator, leds

I need to install some LEDs on my dash and wanna put a voltage regulator right before the leds so that they won't burn out in less than one week because of the fluctuating voltage of the car.

I was recommended a LM2937 voltage regulator (https://www.national.com/ds/LM/LM2937.pdf).

'Ideally suited for automotive applications, the LM2937 will
protect itself and any load circuitry from reverse battery
connections, two-battery jumps and up to +60V/−50V load
dump transients. Familiar regulator features such as short
circuit and thermal shutdown protection are also built in.'

The data sheet also recommends a low ESR capacitor (below
3Ω)

My question is, are these two components all I need to make my circuit or there are more?

The LM2937 is a great regulator for automotive use, but is overkill in your situation.  A typical 7805 will work fine and provide years and years of troublefree service in an automotive environment (if used correctly).

What the LM2937 offers that the 7805 doesn't is a low drop out voltage (completely irrelevent to your application) and low quiescent current (again completely irrelevent to your application).

The drop out voltage is how much higher the input has to be then the output for the output to stay regulated.  Typically, in a car, you go with a 5vdc output so as long as you have more then 7vdc on the input (ie battery) the 7805 will be fine.  The quiescent current is how much current the regulator itself needs to operate.  Since you won't be leaving your dash lights on for extended periods when the car isn't running a few extra mA (typically 6-8mA) of current draw isn't going to hurt you.

With any voltage regulator the proper procedure for running it is to select the component (say a National Semiconductor LM2937ET-5.0/NOPB) and then download that specific data sheet (https://www.national.com/ds/LM/LM2937.pdf).  If you look at page 8 of that data sheet it will show you the typical application - follow that diagram and the recomendations closely and you''ll get great results.

The LM2937 has special considerations due to it's low drop out design and the capacitor calculations can be cumbersome (especially in colder environments).  Unlike the 7805 if you don't have the correct capacitance on the LLM2937 the output can oscilate and damage whatever is hooked up to it.  The colder the climate the more capacitance you will need to prevent the oscilation. 

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Kevin Pierson

KPierson wrote:

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The LM2937 is a great regulator for automotive use, but is overkill in your situation.  A typical 7805 will work fine and provide years and years of troublefree service in an automotive environment (if used correctly). 

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Thanks a lot for the heads-up.

I followed your suggestion and found this one: https://www.cd4power.com/data/meters/dms-78xxsr.pdf

I like it because you can switch between 3.3V/5V/12V

Do you think it will work fine for my application?

it's $9.90 in Mouser

https://www.mouser.com/Search/ProductDetail.aspx?qs=35WhQNrE6p3r8osawIMOlg%3d%3d

Are you sure you can "switch" between the voltages?  It looks like there are three part numbers - one for each voltage.

You would be best off just going with a 5vdc regulator and then if you want to switch the brightness use different value resistors to increase/limit the current going to the LEDs.

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Kevin Pierson

What's your reasoning behind using a $9.90 power supply when a $0.40 7805 would work?

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Kevin Pierson

KPierson wrote:

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Are you sure you can "switch" between the voltages?  It looks like there are three part numbers - one for each voltage.

You would be best off just going with a 5vdc regulator and then if you want to switch the brightness use different value resistors to increase/limit the current going to the LEDs.

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Ops.. you are right

7803SR-C
7805SR-C
7812SR-C

So let's say I choose the 7805SR-C. It's ready to use right.. no need for other components.

KPierson wrote:

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What's your reasoning behind using a $9.90 power supply when a $0.40 7805 would work?

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$0.40 ? that's sounds great

u have a link?

Something like this:

https://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM7805CT-ND

How many LEDs are you planning on running?  What kind of current will each one need?

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Kevin Pierson

KPierson wrote:

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Something like this:

https://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM7805CT-ND

How many LEDs are you planning on running?  What kind of current will each one need?

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about five of these https://store.qkits.com/moreinfo.cfm/12VTFR

I don't know with what current they work

do I need a separate voltage regulator for each LED ?

You need to find out what the current draw is.  Since they are designed to run at 12vdc you can assume they have a current limiting resistor built in to them.  I would assume the current would be less then 30mA for each LED.  That would allow you to run ~20 of them off of a 1A 7805 as long as you use a heat sink on the 7805 (or just bolt it to a large metal part of the car).

Why are you going with a flashing LED?  That might look funny when they all flash at different rates due to the fact the flashing is internally controlled.

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Kevin Pierson

cool thanks.. I'll find out what current they work with

only two of them will flash.. the others won't

i'm installing a water injection system.. as well a hydroxy generator

KPierson wrote:

---

The LM2937 is a great regulator for automotive use, but is overkill in your situation.  A typical 7805 will work fine and provide years and years of troublefree service in an automotive environment (if used correctly).

What the LM2937 offers that the 7805 doesn't is a low drop out voltage (completely irrelevent to your application) and low quiescent current (again completely irrelevent to your application).

The drop out voltage is how much higher the input has to be then the output for the output to stay regulated.  Typically, in a car, you go with a 5vdc output so as long as you have more then 7vdc on the input (ie battery) the 7805 will be fine.  The quiescent current is how much current the regulator itself needs to operate.  Since you won't be leaving your dash lights on for extended periods when the car isn't running a few extra mA (typically 6-8mA) of current draw isn't going to hurt you.

With any voltage regulator the proper procedure for running it is to select the component (say a National Semiconductor LM2937ET-5.0/NOPB) and then download that specific data sheet (https://www.national.com/ds/LM/LM2937.pdf).  If you look at page 8 of that data sheet it will show you the typical application - follow that diagram and the recomendations closely and you''ll get great results.

The LM2937 has special considerations due to it's low drop out design and the capacitor calculations can be cumbersome (especially in colder environments).  Unlike the 7805 if you don't have the correct capacitance on the LLM2937 the output can oscilate and damage whatever is hooked up to it.  The colder the climate the more capacitance you will need to prevent the oscilation. 

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Hi Kevin.. first I want to thank you for your great help on previous posts here.. i have leaned a lot. I was looking at https://www.fairchildsemi.com/ds/LM/LM7805.pdf as per your suggestion and on page 22 says to use an input cap of 0.33 mF and an output cap of 0.1 mF

Do you think these capacitors will work for any output voltage I chose? will these capacitor values work fine for 5V or 12V output voltage?

I've chosen to work with 12V instead of 5V output

The most important thing to do when working with voltage regulators is to make sure you follow the instructions in the data sheet for the exact regulator you are using.  Different manufacturers will spec out different components for similar part numbers (ie a Fairchild 7805 may need certain capacitors and a National 7805 may require different caps).

If you are going to go with a 7812 just make sure you order the part that relates to the data sheet and you'll be fine.

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Kevin Pierson

cool. thanks a lot kevin.