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About MOSFET switch problem

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The led brightens is exactly the same when it has the same current no matter how it is placed.
Your previous test was with 40mA which was shared in the two leds that were in parallel so how do you compare the brightness of that to the brightness of 80mA for one led
 

alexan_e said:
The led brightens is exactly the same when it has the same current no matter how it is placed.
Your previous test was with 40mA which was shared in the two leds that were in parallel so how do you compare the brightness of that to the brightness of 80mA for one led
Click to expand...
No. As what I post on thread #118, I measured 80mA for 14 leds. But the brightness is quite dim. Do you know why?
 

Maybe i can help.
post 118 show u getting 80ma...right ?..now if u know ohms law the total 80ma is divided in 2 leds led 13 and led 14
so each led gets approximately 40 ma.
now on post # 120 you put 80ma in one led and got more brightness so now u should understand whats your problem.
 
dselec said:
Maybe i can help.
post 118 show u getting 80ma...right ?..now if u know ohms law the total 80ma is divided in 2 leds led 13 and led 14
so each led gets approximately 40 ma.
now on post # 120 you put 80ma in one led and got more brightness so now u should understand whats your problem.
Click to expand...
Noted.
I have no datasheet about my led. How do I determine its maximum current? I mean, how bright it can get?
 

Hmm ...i guess try diff currents and see when it burns out .. i would apply 100ma for 5 leds at room temp and see if they last for a long time... remember heat dissipation and ambient temp is extremely important..if 80ma is good enough for u let it stay so . led current.JPG

if u want to get even higher-brightness u can give it pulse current 200ma -500ma.

and IMPORTANT U NEED TO ADD NPN TRANSISTOR TO DRIVE THE LEDS NO DIRECT CONNECTION TO MCU.
THIS SITE WILL HELP U https://www.electronics-tutorials.ws/index.html
 
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Actually, I'm planning to use the tpic6c595 to drive the leds. The mcu just send serial data, clock and latch signal to it. What do u guys think?
 

tpic6c595 is good edit your post with new device so other people will get the right diagram.
did u understand my diagram showing how current works ?
 

Yes. I understand our diagram dselec.

By the way, what is the difference between this design:
LED1.png
with this design?
LE.PNG
Which is better?
 

The first is better because in some cases if one led goes "lazy" in dimmer the other in parallel may compensate for it allowing a slight rise in its voltage . in the lower diag. a faulty led may reduce the overall voltage for all the rest in series.
this matter needs further research .
 

dselec said:
The first is better because in some cases if one led goes "lazy" in dimmer the other in parallel may compensate for it allowing a slight rise in its voltage . in the lower diag. a faulty led may reduce the overall voltage for all the rest in series.
this matter needs further research .
Click to expand...

LED3.png
For my above design, how do you calculate the total current it need? I mean if I measure using Ammeter I can get the value, but how do we calculate it? Currently I'm putting 30 ohm of current limiting resistor R1 for it and my ammeter get 80mA. How do we calculate 80mA out?
 

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There another thing I want to ask, I know that we can adjust the brightness of the led using PWM. But if we connect the mcu to TPIC6C595 to drive the led, then how do we input the PWM signal? There's no PWM pin on the TPIC6C595.

---------- Post added at 15:29 ---------- Previous post was at 13:55 ----------

I just found on the internet that you could input the PWM signal on the /G (pin 8) of TPIC6C595 to control the brightness. IS this true?
 

TPIC6C595 to control the brightness. IS this true ?
yes
 

yes start from 50% -100%
remind me why u using shift register... why not use the mcu port directly or use simple uln series drivers?
 
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dselec said:
yes start from 50% -100%
remind me why u using shift register... why not use the mcu port directly or use simple uln series drivers?
Click to expand...

This is because I already have an existing design on using a shift register. Don't think I want to change so much on the firmware.
And plus I'm planning to control more than one digit of the 7 segment LED. If I got 5 digits, direct connect to mcu port will result in using 5x8=40 pins. I do not have that much io pins.
 

Post your design its interesting why they use shift...
 

Each digit of 7 Segment is connected to a TPIC6C595. So, if I got four digit, then I got four TPIC6C595. From the mcu side, I just use 3 pins to send signal to TPIC6C595. No matter how many digit of the 7 segment LED there is, I just use 3 pins from mcu.
 

For level shifting and interfacing of microcontroller to the outside/industrial field you might find here useful hits: https://www.ichaus.biz/mcu_interface .

Enjoy your design work!
 
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