# SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

1. ## SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

Each segments use 3-parallel strings and each string is of 6 LEDs.
Datasheet specify Vf=12v (min) ~ 13.5v(max), If=60mA(max)
DP uses 5 LEDs string, and Vf=10v(min), If=20mA(max).
I am choosing 15v SMPS but what current of that SMPS should I choose,
My openion:
for one digit, 60*7 + 20*1= 440mA.
As I need for 6 digit, 440*6 = 2640mA.
Can I consider 15V and little more than 2.6A, (like 3.4A) of SMPS to operate all digit smoothly. OR 18V 3.4A OR will u suggest others way.
Tnx

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2. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

Without knowing more about your devices, I suspect you need to provide current limiting resistors (LEDs are current devices). You will have to size your resistors based on what supply voltage you choose. If you choose a higher voltage, then you're just wasting power across the resistors.

3. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

On consideration of 15v PS unit,
Current limit R=(15-13.5)/0.06=22 OHMS in each segments,
i will use ULN IC for both anode and cathode driver too.
then what should be the current rating of driver smps to drive all six digits.
that i want to buy.
tnx

4. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

1) Do you really want to drive these at their maximum current rating? Don't you want SOME margin? If you design this for 13.5V drop, and the actual drop is 12V, you're going to overcurrent the LEDs.
2) You also need to consider the drop of the drivers in your resistor calculation.
3) You already figured out what worst-case current you need to drive this. Get a supply with a larger rating.

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5. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

for one digit, 60*7 + 20*1= 440mA.
As I need for 6 digit, 440*6 = 2640mA.
It is multiplexed and so only one digit will be ON at a time and so the current draw at any instance is max 440mA.

You should not run LEDs at max ratings. If LED current is max 60mA then it is good to use 40mA.

So, 40mA * 7 segments approx = 280mA

6. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

Originally Posted by baileychic
It is multiplexed and so only one digit will be ON at a time and so the current draw at any instance is max 440mA.

You should not run LEDs at max ratings. If LED current is max 60mA then it is good to use 40mA.

So, 40mA * 7 segments approx = 280mA
Nobody said anything about multiplexing...

7. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

Ok.baileychic..
I will put current limit in each segments with appropriate value to limit currnt in each segments.
is 15V 3.2Amp OR 2.2Amp good selection as power supply unit for driving all 6 digits.
tnx

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8. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

Nobody said anything about multiplexing...
The word hasn't been used yet. But they discussed having anode and cathode drivers, just another word for multiplexed scheme.

9. ## Re: SMPS selection for Big 6-Digit 7-SEGMENTS display as driver

Hi,

Multiplexed or not .... to get equal brightness you need about the same average current.

Example:
With a multiplex rate of 1:3 you need to drive 3 times the current during ON time.

For sure one needs to take care about display specifications.
Usual "20mA" LED s can be driven up to 1A pulsed...With a display I assume it's about the same.

Klaus

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