shift register questions

Hi-

I haven't worked with shift registers before but have looked through the tutorials and get the gist of it. I have a couple of questions though-

I'm planning on using the TPIC6B595: http://www.sparkfun.com/commerce/product_info.php?products_id=734

  1. is there a practical limit on how many shift registers can be daisy-chained together? 3? 4? 50? ;-)

  2. if I want the shift register to source higher voltage/current than the arduino can handle, is it just a matter of powering the register with my 2nd v source (but having common ground with the arduino source)?

tx,

--roy

There is no limit to how many you can use. The only limit is how fast you can update all of them.

Check the datasheet to see the max voltage it can handle. You'll probably need transistors.

The datasheet says that each drain on the register can sink 150 mA continously and 500 mA peak for at short time.

What are you going to control with the shiftregister ?

This device is serially controlled from arduino, so it will not draw very much current from the Arduino board.

You could power the register with 5v from Arduino to the Vcc pin, but just remember that this is only the supply violtage for the logic in the IC, NOT for the things you are driving with it, they need a seperate powersupply.

Look at the figure in page 5 of the datasheet.

thanks! I missed this circuit. I am wanting to control an array of electromagnets. My plan is to use H-bridges but then I saw this chip and thought it might handle the current itself.

I am not fully following this circuit though - perhaps you (or anyone) can spot me as I walk through it:

  1. 24 v is tied through a 235 ohm resistor to both output & drain and tied to ground through a 30 pF cap.

a. the resistor limits the total flow of current to output & drain
b. the cap builds up charge and blocks current to ground [??]
c. when data is high, drain is on and 24v flows to output & drain
d. output has it’s own connection to ground [??]

I think I am not understanding the current flow here: when drain is off, current is still available to output- unless (I guess) it has a higher resistance load than the cap.

Am I even close in working this through?

tx,

–Roy

That's a test circuit to verify operation.

This might help: http://www.nalanda.nitc.ac.in/industry/datasheets/Texas_analog/SEM4_1.pdf