its a lm2917 based tacho with LM3914 as led driver.i know lm2917 is built for higher voltages but i want to feed it exactly 12v so that it will live long.u know automotive conditions are very rough and rapid.and another issue is i have only one LM2917.i got after searching a lot(in my area they stop importing this chip,and online purchase is not possible for me) so i don't want to spoil it.
and ultimately the whole circuit is designed to work with 12v supply.

 

The LM2917 has a built-in 7.5V zener diode regulator. The datasheet shows how it is powered through a 470 ohm resistor then the LM2917 circuit operates from the regulated 7.5V from the zener diode.
Then you don't need a separate circuit for 12V.

The LM3914 operates from 3V to 25V and has a built-in voltage reference and all the LED outputs are current regulated so you don't need a 5V regulator, just a resistor feeding the LEDs will get hot instead of the IC or the 5V regulator getting hot.

__________________
Uncle $crooge

 

It sounds like the whole circuit will work from anywhere inbetween 10V to 25V so trying to run it from exactly 12V is a totaly waste of time. You don't need a DC-DC converter!

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

Well, of course not. But neither will a 7805. They are both Linear Regulators. And I thought that we were talking about the 5V output. But even with a 12V LDO (Low Drop-Out) linear regulator, the input voltage has to be higher than the output voltage, by at least the drop-out spec (and a little higher is usually better), for it to be able to maintain the output at 12V.

You'd need a switchmode power supply, to get 12V output for input ranging from 10.5V to 15V. I suggest that you either download LTspice (aka SwitcherCad), at linear.com, and use the "File" - "Switch Selector Guide" automatic SMPS designer menu-option, or, go to national.com and use their on-line automatic SMPS designer. Then maybe also look for appnotes about snubbers for SMPSs. You might also want an LC filter on the output. And you'll probably want a 12V LDO linear regulator after the output.

If you meant to say "5V" instead of "12V", then yes, the 7805 and LM2937 "should" both be able to do it, although they might dissipate a fair amount of heat, especially at the higher end of the input voltage range. And the 7805, at least, might not be very happy about it, and might tend to fail. The LM2937 is a little more robust.

At any rate, it sounds like you should listen to what audioguru and Hero999 have recently posted, about your ICs' voltage requirements, and avoid some wasted time and resources.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

-

 

thanks for the info guys.i'll do that with the 2917 tacho design.
and why asked for both 5v and 12v bcz i had another design in my mind.which uses 5v input
http://www.edn.com/article/CA6399101...dustryid=44217

and it seems instead of wasting my time on only psu unit i think i shd concentrate more on tacho working.
thanks once again for sharing knowledge with me.