Questions regarding TPIC6B595N, external PSU and resistors

Other Hardware LEDs and Multiplexing
1

Hi everyone!

I try to control 24 x LED's with 3 x TPIC6B595N 8-bit shift registers, driving each LED with max. current (3.3V @ ~18 mA) with the help of an external PSU. I designed already a KiCad schematic, but I have some questions regarding the external PSU and the needed resistors:

  1. I want to use a step-down voltage regulator to adjust the input voltage of the used external PSU (to get an appropriate output voltage for the circuit). However, how is the current of the external PSU effected? Does it stay the same or is it changing due to the voltage regulator?

  2. Because I only plan to use 3 x TPIC6B595N shift registers, a PSU which delivers (3.3V @) 450 mA would be sufficient, right? But what do I have to do when an external power supply supports a higher current (for example 3.3V @ 1A)?

  3. In the schematic each LED is connected to a separate resistor. Is that the correct way to guaranty an equal illuminating power of each LED, independent of how many of them are on HIGH?

  4. I am unsure which resistors I need? Is R=V/I=3.3V/0.020A= 165 ohm, right?

Any help and suggestions regarding the questions/schematic are welcome! Thanks and a happy new year ! Cheers, :slight_smile:

Circuit.png
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2

Looks like you got just about everything backwards!

mfry:

  1. I want to use a step-down voltage regulator to adjust the input voltage of the used external PSU (to get an appropriate output voltage for the circuit). However, how is the current of the external PSU effected? Does it stay the same or is it changing due to the voltage regulator?

That all depends on what you mean by "a step-down voltage regulator". If you propose to use a"linear" regulator, then the answer is - don't! It would be a total waste of time.

It has perhaps not occurred to you that in your diagram, there is no place for your external LED supply to connect. This is because the TPIC6B595N is actually a "current sink" - it can pull cathodes of LEDs down to ground, but not supply any positive voltage to them.

So what you actually need to do, is to supply your voltage to the top rail you presently label "GND" and have all the LEDs point the other way.

mfry:
2. Because I only plan to use 3 x TPIC6B595N shift registers, a PSU which delivers (3.3V @) 450 mA would be sufficient, right?

No. Too close to the working voltage of white LEDs, might be OK for red, but I am betting you do not want to use red LEDs.

mfry:
But what do I have to do when an external power supply supports a higher current (for example 3.3V @ 1A)?

That would be very appropriate. It is reasonable design practice to use a power supply which is capable of supplying a fair bit more than you require. Two times will do quite nicely.

mfry:
3. In the schematic each LED is connected to a separate resistor. Is that the correct way to guaranty an equal illuminating power of each LED, independent of how many of them are on HIGH?

Certainly is!

mfry:
4. I am unsure which resistors I need? Is R=V/I=3.3V/0.020A= 165 ohm, right?

No, the resistors will not "see" 3.3V, they will see whatever is the difference between whatever power supply voltage you use, and the voltage drop of the LED, minus a little (albeit not much) in the TPIC6B595. The voltage drop of the LED could be 1.4V for red LEDs, about 3V for blue or white.

But what are you going to use for the actual power supply? If you use a switchmode regulated 5V supply, then for white LEDs, the voltage drop would be about 2V and a 100 ohm resistor would be appropriate. If on the other hand, you are using a 12V power supply, then 9V/20 mA is 470 ohms.

So, what power supply is it going to be?

3

Hi Paul_B,

thanks for the fast reply!

Looks like you got just about everything backwards!

Ah, alright :o . I changed the the schematic as you suggested (see new png file)!

That all depends on what you mean by "a step-down voltage regulator".

The idea was to use an external and adjustable Voltage Regulator, so that I can later on use it for different projects :
http://www.hobbytronics.co.uk/s18v20f12-12v-regulator?keyword=voltage%20regulator

Would that work?

So, what power supply is it going to be?

I have an unused regulated power supply adapter with 12V 1.2A DC output, which I would combine with the external voltage regulator? Or should I use something else?

Cheers, :slight_smile:

Circuit.png
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4

As I explained before, if you have a 12V supply, then you connect it to Vin on the Arduino board, which has its own regulator.

You use 470 ohm resistors from the 12V supply to each of the white LEDs.

Vcc to the TPIC6B595 ICs comes from the 5V line on the Arduino. They use negligible current (less than 1 mA) in themselves so if no other devices are drawing current from the Arduino regulator, there should be no problem.

That's it. Done!

The switchmode regulator you cited is not in any way relevant to this project; it is for an entirely different function. It is to generate a fixed 12V from a widely varying or unstable input voltage.

You could use an appropriate switchmode regulator to convert your 12V to 5V to power the LEDs more efficiently, if that was actually important, but there is absolutely no advantage in using a linear regulator for the LEDs.

5

Hi Paul_B,

thanks a lot for the reply's, they helped me quiet a lot!

Just a further question: If I would use an external 5V/1A power supply (instead of the 12V/1.2A), could I connect it directly to the circuit (as seen in the schematic)? I am thinking to design a simple PCB board which gets the power directly from the external power supply, rather than through the Arduino board as you explained.

Cheers, :slight_smile:

P.S. Sorry for my late answer, but I had some busy last weeks...

6

If I would use an external 5V/1A power supply (instead of the 12V/1.2A), could I connect it directly to the circuit (as seen in the schematic)?

Yes but you could connect 12V directly to all the resistors feeding the LEDs and then have 5V connected to the Vcc of your chips if you want.

7

Thanks for the reply Grumpy_Mike!

Yes but you could connect 12V directly to all the resistors feeding the LEDs and then have 5V connected to the Vcc of your chips if you want.

Ah, OK. So the chips get the 5V from the Arduino board, and the LEDs get the 12V from the additional power supply. And then I should only connect both GND together, right?

Cheers, :slight_smile:

8

That's about the size of it.