voltage regulator, leds

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livehho

Member - Posts: 17

Joined: September 02, 2008

Location: Florida, United States

Posted: May 13, 2009 at 11:38 AM / IP Logged

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 (http://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?

KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: May 13, 2009 at 2:36 PM / IP Logged

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 ( http://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.

Kevin Pierson

livehho

Member - Posts: 17

Joined: September 02, 2008

Location: Florida, United States

Posted: May 13, 2009 at 3:18 PM / IP Logged

KPierson wrote:

Thanks a lot for the heads-up.

I followed your suggestion and found this one: http://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?

livehho

Member - Posts: 17

Joined: September 02, 2008

Location: Florida, United States

Posted: May 13, 2009 at 3:24 PM / IP Logged

it's $9.90 in Mouser

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

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: May 13, 2009 at 3:39 PM / IP Logged

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.

Kevin Pierson

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: May 13, 2009 at 3:41 PM / IP Logged

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

Kevin Pierson

livehho

Member - Posts: 17

Joined: September 02, 2008

Location: Florida, United States

Posted: May 13, 2009 at 3:45 PM / IP Logged

KPierson wrote:

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.

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.

livehho

Member - Posts: 17

Joined: September 02, 2008

Location: Florida, United States

Posted: May 13, 2009 at 3:47 PM / IP Logged

KPierson wrote:

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

$0.40 ? that's sounds great

u have a link?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: May 13, 2009 at 3:57 PM / IP Logged

Something like this:

http://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?

Kevin Pierson

livehho

Member - Posts: 17

Joined: September 02, 2008

Location: Florida, United States

Posted: May 13, 2009 at 4:49 PM / IP Logged

KPierson wrote:

Something like this:

http://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?

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

I don't know with what current they work

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