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sccoupe
11-16-2009, 10:36 AM
I have been using a straight voltage regulator with no caps in an automotive environment for some time now and wonder if my odd problems are because of this. Is the attached schematic sufficient for a clean 5vdc source from a dirty 13.8vdc automotive system?

Thanks

Giuseppe
11-16-2009, 11:04 AM
Well when I started to use caps in my circuits, it definitely solved some of my "weird" issues in projects. For example, I made a door strike system with an LCD display and whenever the doorstrike was energized, the back emf temporarily turned the LCD off. Once those caps were hooked up, all that stopped. Here is what I usually use in my circuits http://www.sminntech.com/images/7805datasheet.gif . The datasheet that comes with the voltage regulator of choice usually gives suggestion circuits for regular use.

Mike Green
11-16-2009, 11:21 AM
Some voltage regulators require a minimum output capacitance for proper operation. Check the datasheet for the NCP1117 for specifics for that regulator. Most regulators also require at least a 0.1uF input capacitor if the regulator is any distance from the power source (like more than a few inches). Given the amount of noise in any automotive electrical system, I'd strongly suggest a lot of input filtering, at least what you show in your diagram, possibly an input choke as well.

sccoupe
11-16-2009, 12:07 PM
Would a choke this the one below do the trick? I'm not really sure how to pick the correct one. A nice small smd type like this would work physically in my application. This would just go in series with the 13.8vdc input?

http://mouser.com/ProductDetail/TDK/HF50ACC322513-T/?qs=sGAEpiMZZMtEZV4pKngjOdzADnFclkmY

Mike Green
11-16-2009, 12:36 PM
I don't know whether the choke you showed will "do the trick". It's mostly for attenuating high frequency noise (> 100MHz) which can help, but I was thinking about a larger choke (1uH or more). It would indeed go in series with the 13.8V input.

mouser.com/ProductDetail/Murata-Power-Solutions/11R102C/?qs=sGAEpiMZZMsg%252by3WlYCkU%252b0DGp0rEzrkadS26O eULxY%3d (http://mouser.com/ProductDetail/Murata-Power-Solutions/11R102C/?qs=sGAEpiMZZMsg%252by3WlYCkU%252b0DGp0rEzrkadS26O eULxY%3d)

Franklin
11-16-2009, 12:43 PM
www.seetron.com/an_vpwr1.htm (http://www.seetron.com/an_vpwr1.htm)

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- Stephen

Phil Pilgrim (PhiPi)
11-16-2009, 12:45 PM
The '1117 is not designed for automotive use. The LM2940 is, and I would recommend switching parts. In addition to the usual short-circit and thermal overload protection, the LM2940 can sustain a reverse battery connection and is protected against excessive input voltage, as can occur duing two-battery jump starts.

-Phil

Phil Pilgrim (PhiPi)
11-16-2009, 12:45 PM
The '1117 is not designed for automotive use. The LM2940 is, and I would recommend switching parts. In addition to the usual short-circuit and thermal overload protection, the LM2940 can sustain a reverse battery connection and is protected against excessive input voltage, as can occur duing two-battery jump starts.

-Phil