Do i need to add capacitor on shift registers?

Forum 2005-2010 (read only) Hardware Interfacing
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hi, do i need to add a capacitor on VDD of a shift register (http://focus.ti.com/lit/ds/symlink/tlc5925.pdf)? i am gonna cascade 98 of them.
i saw in another shift register IC that is is recomended 1 mf, close to VDD pin.

if (answer==yes) {
what kind? mf? quallity?(tantalum,ceramic)? there are thousand of them on mouser electronics...what to choose?

2

So can i choose tantalum?

3

if (answer==yes) {
what kind? mf? quallity?(tantalum,ceramic)? there are thousand of them on mouser electronics...what to choose?

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:slight_smile:

But anyway, with the tantalum caps, be careful not to overvoltage them backwards!!
OMG!!! (btw its an actual video, not a rickroll or anything. Honest!)
P.S. the custom link text is just because I figured out how to do it just now, as opposed to just the url showing.

4

And, if you're going to be using those shift registers to drive LEDs, you should probably add more capacitance to smooth out the Vcc dips from the LEDs turning on.

5

oouch!!!! now i saw tantalum, they are so expensive

any cheap and quallity? recomendations?
and 0,1mf are fine?
yes they are for leds, static led display

6

Ceramic Disc caps...

7

how many pf? these are small usually

8

As mentioned above, probably a 0.1uF across each shift register's posotive (VCC) and negative (GND).

I might have missed it, but I don't beleve you mentioned that shift registers these are...

9

Based on what Richard said, I would add THESE across the power input to each register. As he also said, add a larger cap like a 50uF about every 5 chips. You could probably do main power rail coming from the power supply. Smaller power rails could branch off of the mail rail, with the larger caps on them. Then the small power wires could branch off of them to the shift registers. These small power wires should have the smaller caps on them.

-Jeremy :stuck_out_tongue:

10

Surplus stores like Goldmine, All Electronics, and BG Micro sometimes have really good deals on tantalum caps. Especially if they've already had their leads cut and/or formed for some assembly line that went dark.

If you're really going to have ~1500 LEDs, you may want to use multiple power supplies for them: you're looking at 20-30 Amps total. That takes pretty heavy wires and PCB traces if you connect them all in series.

Also, look into picking a supply close to the nominal voltage of the LEDs, so you have as little waste heat as possible being dissipated by the shift registers.

11

X2 on everythign that Ran Talbott said! I would think that a (Or some) 3.3V power supply would be good if they make them.

12

You want to get yourself some "jellybeans"; cheap commodity SMT 0.1uF ceramic caps for each chip should come in less than $0.01 if you buy a couple thousand, and shouldn't go over $0.05 even for TH components. Then some bigger caps (10uF ceramic, tantalum, or electrolytic) for every 5 or six chips; those should still be pretty cheap...

http://www.ladyada.net/wiki/mdcpickandplace/jellybeans

13

i am gonna cascade 98 of them.

Well one thing no one has pointed out is the fan out. You can't drive the clock line for 98 devices from one arduino output. Best bet would be if you split the shift registers into groups of 10 and drove each group from a buffer like a 74LS04. You can then drive the 10 buffer inputs from the arduino or to save you having to compensate for the buffer inversion in the software, drive them from another gate of a 74LS04.

14

I'm with Mike. Maybe there is a way you can amplify the clock pin? I honestly have no idea... maybe a transistor? Also, maybe you could split the registers into groups, with maybe like 6 data pins and 6 latches or something? That way there is not as long of a cascade (Think of dominoes... The longer the line, the longer the cascading.).

-Jeremy :stuck_out_tongue:

15

hi this is an old thread, but today i deside to continue my project

so i added 0,1uf on each TLC 5925 and many 47uf (1 per 2 TLC5925)

it works but not perfect, i continue to have some noises problems,

my power supply (very powerfull meanwell) is about 4m away from my PCBs and the latch,sin,blank,clk are 4m away from the arduino (i am using flat cable). is it posible that space to couse problems? and if yes what can i do for this?

16

You need more decoupling, probably with bigger capacitors, throw several 100uF in the mix. Did you buffer the clock and latch signals like I said, you can't drive that many outputs from one arduino output.

17

i dont drive 98 TLC5925, i have 2 groups of 49,
i tried to cut the cable data- gap and now the first group works and the other one have problems, when i split that on half it works too

but i would prefer to drive them as i have them (2x49) and not 4x24 becouse i prefer to have less cables.

how can i connect 74LS04? can you make a shinematic for me please?

18

i have 2 groups of 49,

Too many:- about 20 is the maximum.

You need one of these for both the clock and the latch:-

19

my display is splited in 2 rows (2x49 TLC5925) , so i can split it to 4x24.5

i tried and it works but i want to do the best to be very stable,
i'll connect (when i buy) 74LS04 if that will make my application more stable but i have some questions.

  1. why that 74LS04 is needed? to make the signal stronger? could i do that with some transistors?
  2. with the way you told me before, i saw that i will use "common" clock and latchpins, isnt any problem with that?can i send different data one each group with that way?
  3. my application became more stable when i decreased the gap of flat cable (sin,latch,clk), is there any solution/upgrade on that? or my MIC must be always near the shift registers? (can i use any capacitor on that chanels or it just a stupid question?) or it will be solved with 74LS04?
    4)The last Sout of last shift register is on air (nowhere connected) is this problem?
  4. I used a multimeter to see the voltage on my PCBs and i saw that it changes value all time between 3.5(!) and 5 volt, why there is so much drop? (it is not couse the power supply, it is 40A) do i need to add a big capacitor? (piff...my life will be a capacitor!)
20

why that 74LS04 is needed?

to make the signal stronger

could i do that with some transistors

perhaps but transistors would not be as good because you would have to design the circuit properly to be as good as the 74LS04.

isnt any problem with that

How do you know, have you looked at the signals on an oscilloscope?

my application became more stable when i decreased the gap of flat cable (sin,latch,clk), is there any solution/upgrade on that?

Yes put the buffers in between the gaps, that is in front of each bank. Use 74LS14 in place of 74LS04 as they are a bit better coping with noise.

I used a multimeter to see the voltage on my PCBs and i saw that it changes value all time between 3.5(!) and 5 volt, why there is so much drop?

A multimeter is not the best thing for measuring this you need an oscilloscope. It could be that you have a lot of noise on the power supply lines caused by lack of decoupling. Or it could be a poor ground connection. Try running the power and ground from each bank directly back to the arduino instead of chaining them.