Pages: [1]   Go Down
Author Topic: 2x74HC595N with 16 LEDs and only 2 resistors  (Read 910 times)
0 Members and 1 Guest are viewing this topic.
0
Offline Offline
Jr. Member
**
Karma: 0
Posts: 51
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

i am running 16LEDs with 2 74HC595N shift registers. looking through the datasheet, i realized that the recommended operational voltage for the shift register is between 2-7V. great, i think, since the output voltage the 74HC595N gives is more or less the same as the supply voltage, i can save a lot of resistors by simply applying a 220ohm resistor to the supply voltage.  smiley-grin



proudly showing this to a friend, he became pale, telling me i might burn LEDs / the IC / the resistor with this setup.

so far nothing has burned, i've had my LEDs pulsing all day now, but since i am designing this for a PCB i would like to be sure.

anyone have any objections to this trick?
Logged

Norway@Oslo
Offline Offline
Edison Member
*
Karma: 13
Posts: 2033
loveArduino(true);
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

My instinct tells me that it should be OK as long as each of the 595 only drives one LED at a time.

But then again... I'm a codemonkey, not a hardware wizard.
Logged

0
Offline Offline
Jr. Member
**
Karma: 0
Posts: 51
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

well, i would like to drive all LEDs at once
doing it right now
nothing burned.... yet smiley-wink
Logged

0
Offline Offline
Full Member
***
Karma: 0
Posts: 112
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

LEDs can have varying forward voltages, even from within the same batch.  Sometimes the difference is slight, sometimes it is significant.  Due to the nature of diodes, placing two diodes in parallel with different Vf will cause current to be unbalanced between them.  If the difference is significant enough, one or more LEDs may entirely not light, and/or enough current may wind up flowing through an LED to cause damage.  It's possible that you got lucky and all of your LEDs are very close in Vf, and you won't have either of those issues.  But even then there's a significant shortcoming: the more current you pull through the resistor, the more voltage it will drop, which means that the more LEDs are on at a given time, the dimmer they will be.
Logged

0
Offline Offline
Jr. Member
**
Karma: 0
Posts: 51
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

i had the feeling there was something dodgy going on... guess i will add separate resistors now...
thanks for the thorough explanation!
/j
Logged

Spokane, Washington
Offline Offline
God Member
*****
Karma: 1
Posts: 686
My name is Bob, and I'm an addict.
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

I'm still fairly new to electronics, but what about if you connected all the grounds of the leds (before connecting to main ground), then placing a resistor from all the grounds of the leds to the ground of the supply (of course calculating the value needed for all the leds)?

But then again, I'm assuming that same value could be too little for just driving one led at a time?

I would experiment, but I <3 my LEDs! smiley-grin Questions save me money, generally.

Any help understanding the concept, and how hard it would fail would be appreciated. smiley
Logged

London, England
Offline Offline
Edison Member
*
Karma: 4
Posts: 1026
Go! Go! Arduinoooo !!!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

People will tell you that this is a big no no. However, I have taken the risk with 595's running RGB Dot Matrix's directly, with no resistors. They have been running for months now without any problem at all.
Logged

Niagara on the Lake, Ontario
Offline Offline
Full Member
***
Karma: 0
Posts: 144
Arduino rocks!
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I'm still fairly new to electronics, but what about if you connected all the grounds of the leds (before connecting to main ground), then placing a resistor from all the grounds of the leds to the ground of the supply (of course calculating the value needed for all the leds)?

But then again, I'm assuming that same value could be too little for just driving one led at a time?

I would experiment, but I <3 my LEDs! Cheesy Questions save me money, generally.

Any help understanding the concept, and how hard it would fail would be appreciated.
Again, the problem with using a single resistor is, unless there is going to be a set amount of LEDs on at any given time, the resistance you will need varies.

For example, lets say you have a 2v, 20mA LED, and youre driving it with 5v. If you have one LED on, you need a 150ohm resistor. however, if you turn two LEDs on, and run them in parallel, with one resistor, you now need a 75ohm resistor. Turn three on and you need 50 ohms. Its possible to use a single resistor, but you will notice a difference in brightness as you light up more or less LEDs
Logged

Pages: [1]   Go Up
Jump to: