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Topic: Should I consider transistor saturation voltage when calculating LED's resistor (Read 1 time) previous topic - next topic

artvolk

Good day!

I've 1W power LED which is connected via ULN2003A to PWM pin. ULN2003A has saturation voltage about 1V. My power source is 5V, so should I calculate resistor for 5V or for 5V-1V=4V (LED current is 320mA)?

P.S. It seems that there is no LED driver, except switching regulators which work with so low voltage drop, so I'll use a resistor.

CrossRoads

Yes!

What's the voltage drop of the LED?
V=IR, or V/I = R

(5 - Vled - VULN)/0.32 = R

Calculate the R power rating also

P=I^2*R, so 0.32*0.32*R, then use something rated higher than that power rating.

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.
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Terry King

The ULN2003A spec sheet should have a curve of saturation voltage VS current so you can get an accurate idea of the voltage drop.

Yes you should use 5.0 - Chip voltage drop if you want an accurate current to flow.

You also need to know the forward voltage VS current for the LED: that is also a curve not a fixed value.

THEN it's just simple subtraction and Ohm's Law  :)
Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

artvolk

Thanks for the reply, the datasheet indeed has such curves, so it is easy to calculate!

Thanks for the responses!

retrolefty

Quote
so should I calculate resistor for 5V or for 5V-1V=4V (LED current is 320mA)?


Yes you should include it, however you also must include the Vf drop of the specific led you are using.
For your example lets assume a 3vdc Vf drop for the led assembly as I don't know the spec for your led.

R = (5v - (1v + 3v)) / .320 = 3.1 ohms

By the way, those high current led assemblies almost always recommend not to try and use a simple series current limiting resistor to maintain proper current control. They demand a constant current driver to operate safely.

Lefty

artvolk


By the way, those high current led assemblies almost always recommend not to try and use a simple series current limiting resistor to maintain proper current control. They demand a constant current driver to operate safely.

The problem is that I ended up with three possible options for LED drivers:
1. 5V power supply + resistors (it seems to work in this project: http://tobe.nimio.info/diy/ikea/led/mood/lamp), not sure how reliable it is.

2. 5V power supply FET-based constant current driver http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/step2/Specs-Function/ (0.5V dropout, seems it will work for +5V), but FETs are more expensive than LM317T from next option

3. At least 7V power supply and simple constant current driver using LM317T (http://users.telenet.be/davshomepage/current-source.htm) -- requires at least 3V dropout and will dissipate at least 3V * 0,32A = 1W.

Grumpy_Mike

Options 2 or 3 sound fine, option 1 is not as it will not be very stable.

artvolk

Thanks, Mike!

I've assembled one driver on LM317T and it seems to be produce quite stable current, regardless of input voltage changes.

Thanks!

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