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Topic: Power supply precautions for 8x8x8 Led cube (Read 175 times) previous topic - next topic

Kruze17

Hello there ...
I am having a 7.5v 1a power supply, but when i checked it  with my multi-meter it showed 8.2v 6.6a
I am building a 8x8x8 led cube with the help of instructions given by pyroelectro.com
so my question is it safe to use my power supply 'or' i should but another one 'or' is there any way to tweak my power supply...

Thanks... ;) 

here's the link for instructions: http://www.pyroelectro.com/projects/8x8x8_led_cube/

PS*- I want to power both my arduino uno and led cube driver circuit...

PaulRB

#1
May 18, 2017, 08:07 pm Last Edit: May 18, 2017, 08:09 pm by PaulRB
Click here for some advice that will help you get the answers you need.

How did you measure the 6.6A?

detown

If you look at the parts list for the Jameco kit it shows a 5V 200ma wall wart .

Kruze17

#3
May 19, 2017, 06:31 am Last Edit: May 19, 2017, 06:42 am by Kruze17
Here is my multi-meter's pic with the setting i measured...
Please do tell me if i am use wrong method of measuring amps...
Click here for some advice that will help you get the answers you need.

How did you measure the 6.6A?

PaulRB

#4
May 19, 2017, 07:24 am Last Edit: May 19, 2017, 07:52 am by PaulRB

You said 6.6A. How did you get the multimeter to show 6.6?

Kruze17

Here it is how it shows 6 amps...
when first connected it went 6.6a then gradually decreased remained at 6.1a

Grumpy_Mike

You shorted out the power supply!
You do not measure current like this, it is meaningless number. It can also damage your power supply.

A current meter should be in seriese with a load.

Kruze17

Thanks for the info or i would have damaged my power supply... ;)  :smiley-cry:
As per the instructions given by you(Grumpy_Mike), i also learned how to measure current form Youtube...
Now if connect a pc fan to it, it shows 0.15a and if i connect it to a dc motor the it shows 1.8a...
So does that mean current varies with load??? Please do tell me... And as per the topic can i now use my power supply with my 8x8x8 led cube for powering my driver circuit containing shift registers and the arduino uno...  :smiley-fat: 

Thanks...


PS*-Please do answer all my questions in this post..

      Thanks for showing no hate to me as im new to electronics, i mean not that noob but still
      learning...  ;D

PaulRB

Now if connect a pc fan to it, it shows 0.15a and if i connect it to a dc motor the it shows 1.8a...
What voltage do you measure when those loads are connected? It could be that the PSU shows 8.2V when there is no load, but will show the correct voltage of 7.5V when there is a load.

So does that mean current varies with load???
Yes, of course it does (otherwise the world's largest machine could be powered by the smallest power supply). The load determines the current.

The point of a PSU like the one you are using (we think/hope) is to provide a constant voltage, whatever the load (as long as there is a load and it does not exceed the 1A capacity). If there is no load, or you exceed the capacity, it is not guaranteed to provide the correct voltage.

It would be helpful if you could post a pic of the PSU, showing the label.

Please do tell me... And as per the topic can i now use my power supply with my 8x8x8 led cube for powering my driver circuit containing shift registers and the arduino uno... 
Maybe, we still do not know enough to be sure.

PaulRB

#9
May 19, 2017, 12:40 pm Last Edit: May 19, 2017, 12:44 pm by PaulRB
OK, lets take a look at the schematic for this cube.


First of all, I have to say I don't like it, for various reasons. It's over complicated and overloads the components it uses. It does not in fact use shift registers. It uses 8-bit parallel latches, making the wiring much more complicated.

But we can estimate the current requirements:

Lets assume you are using blue leds and their forward voltage is around 3.2V. The schematic shows 100R series resistors for the columns. So the current flowing through each led in a layer would be around (5-3.2)/100 = 18mA. Each 74hc574 chip will need to source 8 * 18 = 144mA (the maximum for that chip is 70mA). There are 8 of those chips, so the total current would be 1.15A.

Assuming the cube is multiplexed by layer, only one layer would be lit at any instant, so the total demand from all 512 leds would still be 1.15A.

In addition, you want your PSU to power the Arduino, and there will be some small current required by the rest of the circuit.

So if I have estimated correctly (Mike, please check my assumptions & maths), your 1A PSU is not adequate. However, because those '574 chips are overloaded, they may in fact limit the current significantly, by dropping a large voltage and getting hot.

Even if you purchase a 1.5A or 2A 7.5V PSU, you will still have a problem. You will be relying on the 5V regulator built in to the Arduino to regulate the 7.5V down to 5V. That regulator is not rated for more than 1A, so it would overload.

I recommend you buy a 2A 5V regulated/switch-mode PSU. This can power the cube and Arduino, You would power the Arduino through its 5V pin instead of Vin pin or the barrel socket.

PaulRB

If you look at the parts list for the Jameco kit it shows a 5V 200ma wall wart .
That sounds totally inadequate! Have I estimated incorrectly?

Grumpy_Mike

Quote
So if I have estimated correctly (Mike, please check my assumptions & maths)
Look alright to me.

Kruze17

So decided to buy the one u suggested(PaulRB)...
Thanks Grumpy_Mike, PaulRB and detown for all the support...
Lots of Love for Arduino and Open source... :smiley-wink:

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