Hi, as title I need a a 12v (10%) regulated supply from my car but not sure the best method to do it. It only needs to supply 200mA. I have several linear regulators (lm317's) but not sure what happens when they drop out - do they turn off, or just stop regulating? thanks
Below about 14V input the output voltage becomes the input less about 2V; a better solution is a quality LDO regulator such as Micrel's MIC2940 with drop-out voltage as low as 40mV at your light load. If you need 12V out when the ingoing is less than 12V (eg, a run-down battery or when starting the engine) then there may be a DC-DC converter that would do.
Anything you do, make sure its up for automotive duty... load dumps up to 70+ volts, momentary voltage reversals, etc. A car electrical system isnt a nice environment.
Thanks. I bought a couple of lm2941's LDO's that are rated for automotive use. Small problem is that I now need upto 18w. I did plan to have this on all the time but that would likely drain the battery very fast if the engine is not on (listening to radio for about 2 hours is enough to drain the battery) any ideas? since car batterys seem quite expensive, and I would also rather not damage my current one by draining it every couple of days. Anyway the plan is to have a hard disk recorder, a camera, and a small lcd in the car.
those are both far too expensive, and potentially use far too much power (160w.. I only need 20!) Crank protection would be nice, but not needed.. a nice large capacitor (+ diode to prevent reverse discharge) should do for that (i think? just guessing here) Id really like to know if I could use the lm2941 with a transistor.. to boost the output current (whilst still keeping all the protections that this chip provides if possible). There are circuit diagrams for this in lm317 datasheets but the lm2941 only has two diagrams (one is regulator the other is a 1A switch).
Simple solution? Get a 1amp 12 regulator (lest then £1) connect to support electronics and place on a 12v alarm battery (put battery after regulator circuit) Total cost (excluding alarm battry) £1-2. Car battery run 7>15v in uk cars.
If you read up, i already said all of that.. except the alarm battery bit - which wouldnt work becuase it's need to be huge. 65p...
what is your point? nothing on that site is within budget, far overrated, and probably also too big. Ive done quite a few searches on google now, not found any useful schematics for the 2941.. any ideas wheather this could work if i swap the 317 for 2941? since i dont know enough about LDO regulators. also I will need a 5v line aswell. Planning to use a lm338T for this but need to find out if I can use extra components to make it safe for in car use.
For in-car use, I would add a series diode, (protect against reversals) A low ohm resistor in series (1ohm or so for this, only 20W being drawn...) and a large 25-30v zener diode reverse biased from the + to the - after the diode and resistor to clamp any large spikes. The 1 ohm resistor keep too much current from being drawn though the zener when a spike occurs.
thanks, thats perfect. That gives me an excuse for a 20 mile drive tomorrow, to the nearest maplin shop theres only one thing missing, which is a way to put the recorder into standby (to turn off the hdd) if the battery gets low. ideal use for a pic micro would be to copy the remote control code for it. ill have to look for some usable code sometime I expect this is something that has been done before (i think its a bit beyond my coding skills, anything that needs to be timed,i find very difficult)
Fair play, your choice at the end of the day. I know this guys has made his own PSU for in car use. Dont know if the links helps, but just thought i would post it. You can find him on http://www.digital-car.co.uk/ There is always this. Good luck.
More expensive but much simpler, MIC29502BT is a 5A adjustable LDO, £5.50+VAT at Rapid. Damn, Rapid have dropped it. They still have the 3A version.
