74hc595 Question

Gorilla

Hi, I am designing a PCB for 12 74HC595's daisy chained together. Since pins 16 and 10 are connected to 5v can I run a trace and connect all of the shift registers 5v inputs together? Can I do this with all the grounds?

Mike Green

Yes, that's standard practice. Remember to use a 0.1uF capacitor at each 74HC595 between the 5V and ground traces.

RobotWorkshop

One other thing to keep in mind when you start using a large # of chips like that is the common clock and latch lines that connect to all the 74HC595 chips. What are you driving that with? It's always good to check the data sheet to see if it will drive that many inputs. Some references are:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=DIG4503

http://www.interfacebus.com/Logic_Design_IC_Fanout.html

http://www2.ogu.edu.tr/~redizkan/IC-Logic.pdf

http://www.interfacebus.com/Design_Translation.html

http://verticalhorizons.in/difference-between-fan-in-and-fan-out-in-digital-electronics/

Duane Degn

I saw a video of Ben Heck building his pinball machine where he had a buch of 595 chips stacked on top of one another to drive a bunch of LEDs. I thought it was really cool to see so many chips stacked on top of each other.

I gave it a try myself with a 12 x 10 LED array.



The clock, latch and other appropriate pins are all soldered together on 16 595 chips.

BTW, my LED array will display images (black and red) from a video camera.

jeevanreddy

sir even iam also facing the same problem.im building a project called 3d led cube in which 512 rgb leds are used along with 25(74hc595) shiftregisters.i have added 0.1uf capacitors at each shiftregister but cant decide how much current should be supplied at pin 10 and 16.please help me.im using pc smps to supply 5v to the leds.the power supply is rated with 5v@20amps,does it damage my ic's if i connect this powersupply dorectly to mu ics????iam attaching the circuit which i ahve soldered!
please help me
thankyou

Mike Green

The amount of current you need depends on the LEDs and how much current they draw. Typically you have a resistor in series with the LED to set the amount of current. A typical maximum current for most LEDs is 20mA, but the datasheet for the LED will give the actual value. A common current for a high brightness LED would be 10mA. At 8 LEDs per 74HC595, that's 80mA if all are lit plus a little for the 74HC595. The 74HC595 datasheet gives the current drain with no loads attached. It will also give the maximum current for the 74HC595 which includes all the loads (LEDs). The NXP 74HC595 has a limit of 70mA total current, so you'd have to limit the LEDs to maybe 8mA for a total current under 70mA.

A well regulated 5V supply can certainly be attached directly to the Vdd/Vss pins of a 74HC595. You should have a 0.1uF capacitor attached to the Vdd/Vss pins at each IC. You should probably also have an electrolytic capacitor across the Vdd/Vss supply at the board level, something in the range of 10uF to 500uF or more depending on how fast you're going to change the LEDs. The higher the speed and the more LEDs may change at a time, the larger the capacitor.

Duane Degn

I looked at the schematic. Wow, that's a lot of soldering.

The schematic shows current limiting resistors to each LED (I assume, I couldn't see the actual LEDs on the schematic). These resistors will limit the current for you. As Mike said, you can connect the 74HC595 directly to your 5V source. You don't need to limit the current to the chip since it will only draw what it needs.

If the person who designed the schematic did it correctly, the LEDs shouldn't draw too much current from the chips.

You'll need a good power source. I think the cube will draw too much current to power from a USB connection. Let us know if you need help selecting a power supply. Edit: It looks like you already have a good one.

I hope you post of video of the complete cube. It sounds really cool.

jeevanreddy

thankyou Mike Green sir and Duane Degn sir for your valuable suggestions.
If u keenly observe my schematic each output of shift register is connected to base of transistor with 1k resistor in between.so only 5ma current is sourced from single output of shiftregister.
actually i thought that if i connect the smps powersupply which has max ratings like 5V@35a to shiftregisters directly it whould fry my chips but now i came know the reality by your clear explanations.tomorrow im gona connect the powersupply and test the output and revertback soon.
thankyou once again!