voltage regulator issue

[trackhound]

I am using a digital panel meter to measure the output voltage from a wideband fuel monitor fitted to a car. I need to measure accurately to within 0.2 volts on the panel meter. I am trying to measure a voltage range from 0.7 to 2.0 volts.

I need to ensure the there is a constant 12v power supply to the panel meter in order to ensure an accurate reading.

I am using a 7812 voltage regulator. The voltage on the car's charging circuit varies from 12.7v to 14.1. However the output of the regulator ranges from 11.3-11.7 volts and does not remain constant at 12 volts. I have double checked my wiring and it is correct.

Any ideas/suggestions? Thanks.

 

[KMoffett]

The 7812 will not work for you in this application. It needs an input-output differential of about 2 volts to regulate properly. That means for 12v out, the input must be 14v or higher at all times. I don't have a easy solution at hand, but others will jump in with ideas.

Ken

 

[Boncuk]

If current isn't an issue you might use an LM2940AZ-12 or LM2931AZ-12. Their max output current is 100mA.

They are LDOs requiring normally 500mV above the stabilized output voltage.

Another solution would be using an LM7809 (output voltage 9V)

Boncuk

 

[KMoffett]

trackhound,

Will your digital meter work correctly on a constant 9VDC supply?

Ken

 

[xanadunow]

It may be a long way to get there but you could use a 3 pin regulator to ensure the varying voltage from battery won't change it (9V, 5V - whatever), and the use a DC-DC up converter to bump the voltage to the required 12 Volts supply for your meter, keeping an eye of the current consumption of the meter versus the capability of your up-converter IC.

 

[Roff]

Many panel meters need a power supply which is floating relative to the voltage being measured. Can you provide a link to the datasheet, or provide the part number?

 

[trackhound]

Thanks guys. I bought the panel meter some time ago and no longer have the details or the datasheet. I will try and run off a 9v regulator and see how we go.