Power advice needed

[bearpawz]

Ive been working on my little display here, so far so good. With some help from a few people here on the board I got the numeric and bargraph parts to jive with each other.

So here is my current problem (and I swear there was no pun intended there)!

Of course the pic needs a good 5V source, as do the other three chips Im using. Here is whats in the circuit right now:

PIC16F819
MAX7219
Alegro A6276

The max7219 is configured for 4 numeric displays. Rset is configured to deliver 20mA current to the segments

The A6276 is driving 16 5mm LED's. Current for segments on that chip is set for 30mA each per the LED specs.

So doing a little math: 8 segments * 4 = worst case of 32 segments for the numeric display

32 * 20mA = 640
16 * 30mA = 480

So I am using 1120mA (I think thats what.. 1.12 amps?) current to drive those LED's. The 7805 regulator is only rated for 1 amp so needless to say even with the heat sink my 7805 regulator is getting quite toasty warm. (HOT!)

Im thinking about using UDN2981 to interface the LED's to the 12V and just put external current limiting resistors on the LED's. The question I have is: will this be more efficient or is there an even better way to do this? Since this is for an automotive application, and the LED brightness will probably be tied into the stock dimmer anyway, using 12V is probably a better option anyway.

[BH_epuk]

Don't forget in automotive when the engine is running you'll have 13.8V not 12V (This is to charge the battery). Used a DCDC convertor for a car based project a while back, will try to dig out the info in the morning.

[bearpawz]

Yes this is true. I just think of it as 12V for some reason. The power supply I use on my bench is 10Amps 13.8V dc so close enough.

[mackrackit]

If you want to stay with a linear regulator the LM350 is good for 3 amps. It is still a good idea to use a heat sink.

Go with a switching regulator,but the parts count goes up. National Semiconductor has a work-bench thing to help configure a switcher.

[bearpawz]

I am ALMOST done with this part of my project. There are just a few more things to resolve. What I need some help with now is the dimmer circuit. I want to use the stock dimmer switch to adjust the brightness of the LED's. By tieing the whole mess of them to the 12V and giving each individual segment its own current limiting resistor, I have that part working. I can either have the display at 12V+ supply and its full on or use a pot to addjust the brightness. How would I go about using both though? (In other words, get 12V fromt he normal 12V source when the lights are off but get it from the dimmer switch when they are on). I know some are going to recomend PWM but Im not sure how that will effect everything being that the display chip is already using PWM. While Im sure it would be a simple enough task to just throw a sensing pot on an ADC pin and adjust the intensity of the display digitaly, this is only one display of many that will be in this whole mess (yes this is a knight rider project if anyone is curious). So while pwm might work well for this part of the display it may not be a good solution for other parts and I want just one dimmer control to do it all.