Electronics 2000

Hi all,

what a great site,

Well straight to the chase, i need to power a temp display in my car (project), but the temp unit is 5 volt & not sure but could be 200ma, I'm going to be hard wiring off the fag lighter which is 12 volts for the supply for this temp unit. Question is what is the easiest way to get my required vltage and ampage etc.. is it just a case of some resistors or a little power supply if so where can i get ths. Much thanks for any help. Colin.

Hi Colin

Thanks for the comments about the site.

A resistor isn't really suitable for this application, since the voltage dropped across it is proportional to the current being drawn, which I presume isn't constant. See the technical data section for info on ohms law.

If you want an off-the-shelf solution you should be able to get a suitable power supply from someone like Maplin (http://www.maplin.co.uk) or Farnell (http://www.farnell.com). These connect to the cigerette lighter and normally allow you to select the output voltage. They are also sometimes called DC-DC converters.

If you are building the meter and want an all in one solution you could look at building a voltage regulator into the circuitry. This could use a 7805 regulator (e.g. http://uk.farnell.com/1102157/semicondu ... R-LM7805CT), you can get the datasheet from this link, this should include a typical application circuit showing the required capacitors. A heatsink will be required too. If you want help with the circuit or size of the required heasink let me know.

7805 cheap as chips in Maplins. remember to put 2 .1ufd capacitors from in and out to ground.