# MAX7221 driving 9V 7-segment and RSet value

Hi all!

I have a problem to count the RSet resistor value for MAX7221 IC. I also have problem how should I drive 7-segment displays with that same IC with 9 volts.

Displays that I soon have are Kingbright's SC23-12SRWA(sorry I can't post a link because this is my first post here in this awesome forum :) ). Those displays forward voltage are 7.2 volts but I think I drive them with 9 volts. How should I connect these to my MAX7221? The IC can feed enough current, 30mA per segment.

What should be the resistor for RSet?

Thanks!

• visualblack

I think this problem is so hard to solve but for me is it. Please give some guidance if you can.

One thing that I found out is that I could use MAX394 with MAX7221 but this is also hard for me to understand how that should work. Should I use some basic Shift Refisters or something?

• visualblack

calculate the RSet resistor value for MAX7221 IC

It is all on page 10 of the data sheet:-

The current per segment is approximately 100 times
the current in ISET. To select RSET, see Table 11. The
MAX7219/MAX7221’s maximum recommended segment
current is 40mA. For segment current levels
above these levels, external digit drivers will be needed.
In this application, the MAX7219/MAX7221 serve
only as controllers for other high-current drivers or transistors.
Therefore, to conserve power, use RSET = 47k[ch937]
when using external current sources as segment drivers.

how should I drive 7-segment displays with that same IC with 9 volts

You can’t use this IC to drive a matrix at 9V it is only designed for a 5V system. So you can not use this chip with this display.

One thing that I found out is that I could use MAX394 with MAX7221

You found out wrong this chip is of no use to you in this application.

Thanks Grumpy_Mike for respond.

How could I drive that 7-segment? Maybe common shift register and couple of transistors?

• visualblack

What you need is to have a source and sink control on the display and multiplex it in software.

The sink is easy, just use some ULN2803 darlington drivers.

For the source you need a high sided driver like the VN750-E. Then you will probably need some output expanders to drive those switches, this is where the shift registers will come in.