Pages: [1]   Go Down
Author Topic: LED strip question  (Read 274 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 46
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I have some questions regarding LED strips

Consider the one below;
http://www.ebay.com/itm/Cool-White-Non-Waterproof-5M-3528-Led-SMD-600-Lights-Flexible-Strip-Light-DC-12V-/131135597329?pt=US_Car_Lighting&hash=item1e884a6b11

Length: 5 meter
Working voltage is 12 VDC
Working Current/meter: 0.35 - 0.4 A

First of all, do all those LED strips on the market have their own resistor ? So, do I need to add a current limiting resistor ?

If I decide to use 1 meter, then I will still be applying 12 VDC. However, this time I need to limit the current to 0.35 - 0.4 A with a resistor, am I correct ? Or can I drop the voltage and do not use any resistor ?

I am kinda confused. Appreciate if you clarify it.

Thank you.

Logged

Global Moderator
Boston area, metrowest
Offline Offline
Brattain Member
*****
Karma: 538
Posts: 27145
Author of "Arduino for Teens". Available for Design & Build services. Now with Unlimited Eagle board sizes!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

For Dumb LED strips: Every 3 LEDs will have its own current limit resistor; power & gnd are passed along in parallel to the next group of 3. Each group of 3 will draw ~20mA from 12V supply. A shorter length will thus draw less current.  PWM/transistor can be used to control brightness.

For Smart LED strips, a control chip will control each LED/group of LEDs. Current draw will be similar. Software controls the brightness.

No external resistor is needed either way.

As an example, here is 74HC595 controlling AOI518 N-channel MOSFETs to fade strips with 9 LEDs (3 groups of 3) up & down.
http://www.crossroadsfencing.com/BobuinoRev17/
http://youtu.be/hAVf1D1L-7c
« Last Edit: July 10, 2014, 01:38:38 pm by CrossRoads » Logged

Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Offline Offline
Newbie
*
Karma: 0
Posts: 46
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

For Dumb LED strips: Every 3 LEDs will have its own current limit resistor; power & gnd are passed along in parallel to the next group of 3. Each group of 3 will draw ~20mA from 12V supply. A shorter length will thus draw less current.  PWM/transistor can be used to control brightness.

For Smart LED strips, a control chip will control each LED/group of LEDs. Current draw will be similar. Software controls the brightness.

No external resistor is needed either way.

As an example, here is 74HC595 controlling AOI518 N-channel MOSFETs to fade strips with 9 LEDs (3 groups of 3) up & down.
http://www.crossroadsfencing.com/BobuinoRev17/
http://youtu.be/hAVf1D1L-7c


Okay, so regardless of the brand and length of the LED strip, I will power it with 12 VDC without any resistors.
Thank you (:
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 46
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

For Dumb LED strips: Every 3 LEDs will have its own current limit resistor; power & gnd are passed along in parallel to the next group of 3. Each group of 3 will draw ~20mA from 12V supply. A shorter length will thus draw less current.  PWM/transistor can be used to control brightness.

For Smart LED strips, a control chip will control each LED/group of LEDs. Current draw will be similar. Software controls the brightness.

No external resistor is needed either way.

As an example, here is 74HC595 controlling AOI518 N-channel MOSFETs to fade strips with 9 LEDs (3 groups of 3) up & down.
http://www.crossroadsfencing.com/BobuinoRev17/
http://youtu.be/hAVf1D1L-7c


One more question if you don't mind;

To drive one of these LED strips, I need to use a NPN transistor. However, I am not sure which transistor to choose and the resistor to connect its base.

I reckon the resistor should be something like that for 5 Meters;

5* Hfe / (1.3 * 2)

where 5 is the switching voltage from digitalOutput and 2 is the load current drawn by LED strip
« Last Edit: July 10, 2014, 02:35:44 pm by zoom » Logged

Global Moderator
Boston area, metrowest
Offline Offline
Brattain Member
*****
Karma: 538
Posts: 27145
Author of "Arduino for Teens". Available for Design & Build services. Now with Unlimited Eagle board sizes!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Use an N-channel MOSFET, AOI518, 55 cents from digikey.com
The board at my youtube link above has 32 of them, turned on/off by 74HC595, so '328P output will not have any issue driving them either.
Use 10K resistor from gate to Gnd to keep it off when the IO pin is not being driven (i.e. during reset conditions).
Logged

Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Valencia, Spain
Offline Offline
Faraday Member
**
Karma: 146
Posts: 5535
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

For Dumb LED strips: Every 3 LEDs will have its own current limit resistor;

If it's a 12V strip then it will have resistors.

One more question if you don't mind;

To drive one of these LED strips, I need to use a NPN transistor. However, I am not sure which transistor to choose and the resistor to connect its base.

5M of LEDs needs about 2A so you should really use MOSFETs for this.

I reckon the resistor should be something like that for 5 Meters;

5* Hfe / (1.3 * 2)

where 5 is the switching voltage from digitalOutput and 2 is the load current drawn by LED strip


There's no need to mess around with Hfe calculations if all you want is to switch stuff on/off with a BJT. Just use 220 or 330 Ohms, whatever's handy. That will fully turn on just about anything.

Logged

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Pages: [1]   Go Up
Jump to: