Author Topic: 7805 or something else?  (Read 2495 times)

stirfoo

7805 or something else?
« on: March 04, 2013, 03:08:42 PM »
My circuit consists of:

 1 ATMega328p microcontroller
 3 MCP23017 port expanders
 2 ULN2003 drivers
14 9vdc non-latching relays
24 T1 LEDs
 1 T1 3/4 RGB LED
 1 Seven segment


The power will come from a Boss 9vdc adaptor rated @ 2A. The relays will be energized through the UNL2003's from the 9v rail.

Everything else needs 5vdc. So I was thinking a 7805 would be okay. If all the LEDs including the 7 seg draw 20mA each, that should be approximately 680mA total if all are lit. That would be a rare instance but it could happen.

Another unknown for me is when multiple relays energize. That's going to put a load on the Boss adaptor (which will also be feeding the regulator). Will this affect the consistency of the 5vdc output? Each relay coil is listed @ 580 ohms. So they should draw approx 15mA each. 15 x 14 = 210mA total if all are energized. Another rare instance. But, the current will probably spike when a relay is energized, yes?

The average 7805 will allow 1A max, correct? I was looking on Jameco for a regulator but there are many listed. If all this looks okay, which regulator should I use?

G. Hoffman

Re: 7805 or something else?
« Reply #1 on: March 04, 2013, 06:12:28 PM »
Well, just for the LEDs, you've got an occasional 680mA x 4V drop across the 7805, so about 2.5W of heat to disperse - but presumably that is a momentary load...  Add up the load of everything on the 5V rail, and figure out how much extra power you need to get rid of.

What I'm saying is, put a heat sink on your 7805, or leave a big chunk of copper under your 7805 and lay it down.  But yes, you are probably fine.  You shouldn't need a massive heat sink, but it would be good to give the 7805 a way to dissipate the excess energy.  If you can get a double sided board made, you can lay down the 7805 and via stitch the top plane/heat sink to another on the bottom, but you probably don't need to go to that much trouble.


Gabriel

G. Hoffman

Re: 7805 or something else?
« Reply #2 on: March 04, 2013, 06:28:53 PM »
Oh yeah, and you could probably throw a 1k to 2K resistor on those LEDs, draw a LOT less current through them, and still have them plenty bright.  Just because they CAN draw 20mA doesn't mean they need to, and if they are super bright LEDs they are a lot brighter than they probably need to be at 20 mA.


Gabriel

stirfoo

Re: 7805 or something else?
« Reply #3 on: March 04, 2013, 08:35:32 PM »
So heat sink and up the LED resistors. Sounds good.

Thanks Gabriel. I think I'm almost ready to put a stomp-able prototype together.

G. Hoffman

Re: 7805 or something else?
« Reply #4 on: March 04, 2013, 10:11:50 PM »
So heat sink and up the LED resistors. Sounds good.

Thanks Gabriel. I think I'm almost ready to put a stomp-able prototype together.


Do the math.  Breadboard it.  Experiment.  I'd probably just lay the 7805 down on its back, bolt it to the PCB, and make sure there is some copper under it to help soak up the heat.  That doesn't work so well on a single sided PCB, though, so you can go to rat shack and buy some of those little sheet metal heat sinks, but make sure you get some isolation washers while you are at it, since the heat spreader is attached to one of the pins (I can't recall which one).


Gabriel