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Author Topic: Should I consider transistor saturation voltage when calculating LED's resistor  (Read 1861 times)
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Ukraine
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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.
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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.

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

Thanks for the responses!
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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
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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.
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Options 2 or 3 sound fine, option 1 is not as it will not be very stable.
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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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