Newbie!! 12v DC regulator help please

[Adwfocus]

Hi im new to this forum and im after help with a 12v dc regulator basically im trying to make a basic 12v regulator for a electronic oil pressure gauge for a car the problem is that these gauges are voltage sensitive and the voltage can fluctuate quite a lot in a car 12v-14v approx and this causes the gauge to read incorrectly so was looking at using a LM7812 regulator but i have read that the LM78XX series needs at least +3v on the input compared to the output?? thus making the LM7812 not usable with my input voltage of 12v-14v??
an advise would be great.

Cheers

[mariush]

On a car, you're going to have voltages varying from 10 to 14 volts, depending on what you do with the car. When you start the car for example, the process will "stress" the battery and the voltage will drop down quite a lot under 12 volts.  When the alternator charges the battery, the voltage provided can be around 13-14v.

A 7812 is a linear regulator, and linear regulators need to have the input voltage above the output voltage by some amount. In the case of 7812, that value is about 1.5-2 volts.  There is no exact value, it varies with how much current your device uses, how hot (or cold) the linear regulator is, and how well the linear regulator was actually produced.

So a 7812 is not a good solution for your problem. In fact, a linear regulator is not really a good solution. 

Ideally, you would use a SEPIC regulator ( a regulator that can reduce or increase the incoming voltage to a particular output voltage) but I guess that's a bit over your head. 

Alternatively, you could use a linear regulator like 7809  or 1117 (adjustable to output 9v)  to reduce the voltage to something like 9v for example, and then use another boost regulator to raise that 9v to exactly 12v.   You can find such ready made boost regulators on eBay.

[SeanB]

If it is designed for a car it likely will be very happy with 9-10V, which is a voltage that is available off the shelf. You also get a voltage regulator designed for use in cars, which is going to survive what the electric system gives it and still work.

http://www.infineon.com/cms/en/product/power/supply-voltage-regulator/linear-voltage-regulators-for-automotive-applications/channel.html?channel=ff80808112ab681d0112ab69bfb202ef

http://bob_skelly.home.comcast.net/~bob_skelly/voltageStabilizer/voltageStabilizer.html

[Adwfocus]

Thanks for the advise guys the problem isnt that it will hurt the gauge the varying voltages make the gauge change reading what isnt what I want I just want the pressure sensor to influence the gauge, people normally just live with it but I was under the impression that it is possible to make a constant voltage regulator to regulate the voltage at 12v? I been doing a bit of digging and found this on ebay i think this will is what im looking for??

http://www.ebay.co.uk/itm/NEW-DEOK-DC-Boost-Buck-Converter-Step-Up-and-Down-Voltage-Regulator-3-5-28V-to-1/301316727599?_trksid=p2046732.c100040.m2060&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20140107095009%26meid%3De7ad3d0cb0c8476daf5c3e5d1f2751c0%26pid%3D100040%26prg%3D20140107095009%26rk%3D2%26rkt%3D4%26sd%3D291106567043

[Seekonk]

Look for a 2596 switching regulator module on ebay for about $2.  You shouldn't need the boost function.   That should power it nicely.

[Paul Price]

http://www.newark.com/xp-power/ie0512s/converter-dc-to-dc-12v-1w/dp/38K2324

The above will convert 5V to 12V with enough power to for your oil pressure gauge with excellent 12V regulation.

To power this tiny module, use a 7805 regulator on a small metal heat sink. Connect the 7805 input pin  to the car +12 V
and use the 5V output to power the DC-DC converter shown above. The 12V input pin is on the top left, the middle pin of the 7805 connects to ground and the output pin connects to the converter module. Small .1 uf bypass capacitors on the input and output of the 7805 pins may or may not be needed.

The idea here is to use a 7805 to convert 12V battery unregulated down to +5V and use this regulated 5V to power  the converter module shown above.

[sleemanj]

You don't say what country you are in, but search eBay for "Auto Step Up Step Down", the devices you want have 2 inductors on the board, generally LM2577 based, put your input voltage in one end, turn the pot until you find the 12v output you are looking for on the other end, once set, the input voltage can vary both above and below the output and the output will stay at the set point.

[AmmoJammo]

Thanks for the advise guys the problem isnt that it will hurt the gauge the varying voltages make the gauge change reading what isnt what I want I just want the pressure sensor to influence the gauge, people normally just live with it but I was under the impression that it is possible to make a constant voltage regulator to regulate the voltage at 12v? I been doing a bit of digging and found this on ebay i think this will is what im looking for??

http://www.ebay.co.uk/itm/NEW-DEOK-DC-Boost-Buck-Converter-Step-Up-and-Down-Voltage-Regulator-3-5-28V-to-1/301316727599?_trksid=p2046732.c100040.m2060&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20140107095009%26meid%3De7ad3d0cb0c8476daf5c3e5d1f2751c0%26pid%3D100040%26prg%3D20140107095009%26rk%3D2%26rkt%3D4%26sd%3D291106567043

Yes, that should do what you need. Being buck/boost, it won't matter if the voltage is below, or above the 12volt output you need, it will still give you a regulated 12volts

You could most likely also get away with a very low drop out 12volt regulator, as you'll only need the regulated 12volts when the engine is running, so should always have at least 12.5volts (enough for a low drop out regulator to regulate properly) unless your alternator has died.

[Adwfocus]

Thanks for all the advice im going to try one of them 2596 regulators and see how I get on and im from the UK

[Seekonk]

I think you should step back for a second.  Could it just be that only you are experiencing this problem?  Seems like a car gauge should be designed for a car environment.  Many gauges will go crazy if there is not a good ground strap from the engine to the vehicle frame.  That can easily develop almost a volt between the engine and the body.  The engine is electrically isolated except through the bearing path in the drivetrain.  I've seen this many times drive temp gauges crazy when the headlights or heater is turned on.