12V power from car battery

Other Hardware General Electronics
1

Hi lads,
I'm planning to build a scrolling LED sign for my car and I wish to use a bunch of MAX7129 to drive a series of 8x8 LED matrix.

Now, MAX7129 have an input voltage range from 0.3V to 5.5V and the LED matrix is a common cathode with 1.8V Vf and I plan to daisy chain 5 or 6 of those, meaning the current will be something like 800-960mA.

Yesterday night had this sick idea to use a voltage divider to split the 12V into 3 lines 4V each to power the drivers and LEDs instead of using an LM317 to reduce the 12V to the ordinary 5V (that would have to dissipate 7V at 1A!).
Obviously the data lines will not be at the same potential the Arduino is, which will mean I'll not be able to drive them directly and I was thinking I might be able to use some optocouplers I've laying around... like the TLP521...

Is this idea so sick?

I can then use 8V to power my Arduino

2

I am truely interested in how you intend to do all this splitting. A circuit would be excellent to see!

I think it will not be as simple or sick as may think now.

Weedpharma

3

Using voltage divider for powering will not work. Voltage will change depending on the current drawn.
Use a switching regulator.

4

What happens when one string has all LEDs off and the another all ON? The voltages will be all over the place.

Why not buy a cheap switching regulator from eBay.

Rob

5

Mr. Rob is right

A LM2576 is cheap and will give you 3A with very low heat. All you need is one inductor and one diode to make it work.

Regards

6

:smiley: I knew there was something wrong in my nighttime idea...

You are right, the voltage divider will not provide stable output because the variability of the equivalent resistance of the circuits in parallel will influence the voltage levels.

And on top of that I believe the optocouplers will probably cause some troubles with the data lines in terms of switching speed and edge sharpness.

I've already ordered one of those step down buck regulators.

Thanks everybody for the lesson :wink:

7

others parts are required for the buck regulator: Low ESR capacitors (105°C). use the datasheet to find the right values.
Don't use standard capacitors (85°C).
another solution is a point of load (POL) module they are now very cheap compared to the price of the regulator + diode + caps + PCB
I'm using some POL from TI and they can give 6Amps from a 1" by 1.5" module! (synchronous rectification)

8
9

Yes, I'm using the PTN78020W model in some of my designs (inductrial touchpanel PCs)
very reliable but you must add capacitors (I'm using solid polymer capacitors because my system must work @ -30°C)

10

Hi, MAX7129?????????
I don't think so, can you look again at the spec sheet for it?

Tom....... :slight_smile:

11

TomGeorge:
Hi, MAX7129?????????

Typo :smiley:

I inverted the two central digits... they are MAX7219 :sweat_smile: