PAGE 1 of 6 - LED voltage control with limiting resistor
Quick info about LEDs
This is not an LED theory page, therefore i will not go into details about the LEDs. You can read the LED Theory for more extensive info. There are some things though that you need to have in mind:
LEDs are current-depended components. People usually confuse voltage and current, because current depends from the voltage. Same happens with LEDs, the current that an LED will draw depends on the voltage that is applied. But the brightness of the LED depends on the current that it will draw. Ideal LEDs will have the same brightness for the same voltage, since they will draw the same current. But real-life LEDs will draw different current even at same voltages due to slight differences in the P-N materials, so they may appear having slightly different brightness.
LEDs have different forward voltage. We use to say that the LED forward voltage is 3 volts, but this is totally wrong. Different LED colors have different forward voltages and this depends on the chemical that is used. Moreover, LEDs with higher power rates have different supply voltage. So you always need to refer to the manufacturer for the correct voltage.
LEDs have polarity. Just to remind that LEDs have positive and negative, anode and cathode...
The maximum forward voltage is critical. The voltage-to-current characteristic of an LED is similar to a diode. If the maximum forward voltage is exceeded by little, the current is increased logarithmically. So, it may be that the LED is off at 2.5 volts and max brightness at 3.2, but at 3.8 the LED may be in danger. Never exceed maximum forward voltage.
LEDs love being connected in series and hate being connected in parallel. Theoretically, you can connect LEDs both in series and parallel. If you connect them in series, you will need double the voltage to turn them on, and they will draw the same current as one LED. Avoid connecting LEDs in parallel, unless you are absolutely sure that they are completely identical. If not, then all current may go though one LED and it can be damaged! To safely connect LEDs in parallel, then you need to add one resistor for each LED. This is called balancing resistor and balances the current that flows within each LED.
LED voltage control with limiting resistor
This is the most commonly used method to control low-current LEDs, because of the low price (can't get any lower) and the simplicity of the circuit (can't be any simpler either). I'm talking about the limiting resistor method. A resistor is connected in series with an LED. The resistor causes a voltage drop across its leads. With proper calculations the LED gets the proper forward voltage to operate. Here is a typical circuit:
A very common mistake that people do when they try to calculate the resistor R, is that they use the Ohm's law to calculate the R for given V (Vdd) and required I (LED's forward current). This leads to wrong results though. Let's see an example. Suppose that the LED has forward voltage 3 volts and at that voltage it draws 20mA (0.02 Amps). The power supply is 5 volts.
A WRONG calculation is;
R = V / I => R = 5 / 0.02 => R = 250 Ohms [WRONG CALCULATION]
Why is this wrong? As we said above, LEDs are current depended components and they regulate the current themselves from the forward voltage they're provided. You cannot calculate their current directly! Instead, you can calculate the voltage that needs to be dropped across the limiting resistor R, so that the LED will operate to the desired Vf and If (Forward Voltage and Forward Current).
To do this, you first need to calculate the desired voltage drop across the resistor. Simply subtract the LED's forward voltage from the source:
VR = VDD - LEDVf => VR = 5 - 3 = 2V
So, we need to drop 2 volts on the resistor, and the rest will be delivered to the LED. Now we can use the Ohm's law to calculate the resistance of the limiting resistor. We know that we want to drop 2 volts, we know that we want to operate the LED with 3 volts, and we know that the LED draws 20mA at 3Volts. Since the resistor is in series with the LED, the same amount of current will flow through. Here is the formula to calculate the resistor:
R = VR / LEDIf => R = 2 / 0.02 => R = 100 Ohm
The proper resistor value is 100 Ohms. But this is not the end. Resistors have also a maximum power dissipation rate. We say 100 Ohms 1/4 Watt, which means that this resistor can dissipate maximum 250 mWatts. To calculate the dissipated power on the resistor, we will use the I2R formula:
PR = IR2 x R => PR = 0.022 x 100 = 0.04 = 40 mWatts
For this example, a 1/4 watt or 1/8 watt resistor is enough. You need to make sure every time that the resistor wattage is sufficient for the application, otherwise you will end up with a black smoking resistor.
To speed up your calculations, i have prepare an LED resistor calculator. Follow this link:
On-Line LED resistor calculator
ADVANTAGES:
Cheapest method - can't get any cheaper
Simplest method - Can't get any simpler or smller
Perfect for small low-current (<100mA) indicating LEDs on circuits
DISADVANTAGES:
Very bad efficiency factor - Lot of power is dissipated on the resistor as heat
Cannot be used for high-current LEDs due to the high power dissipation
Unstable - A small change on the supply voltage will cause a large change on the LED current
Comments
At 19 October 2015, 14:13:48 user Jakoos wrote: [reply @ Jakoos]There seems to be an inconsistency in your measured values.
The sum of the voltmeter readings should be around 11.3V 0.7 for Rs = 12V for Vdd.
The actual values in the first picture is:
Vdd = 3.36V 8.03V 0.7V = 12.06V Correct.
The value for the second picture is:
Vdd = 3.36V 4.43V 0.7V = 8.49V Which is wrong. It should have been 12V.
Two errors is visible. a) the Vled would be higher in the second instance most probably at 3.5-3.6V while the Vfet should be the difference. You cant loose voltage in a circuit. It must be somewhere and we know it is not on Rs as the transistor will keep it at 0.7V,
At 19 October 2014, 16:40:28 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Charles For 1W led better use a constant current driver, like the single transistor one.
At 18 October 2014, 2:28:07 user Charles wrote: [reply @ Charles]I know most all of your suggestions talk about how the resister is a loss of power. If one was to use a buck converter to step the V down to 3.2, what option would be best to limit current to 1w LED's.
At 14 September 2014, 15:15:54 user Kothandapani wrote: [reply @ Kothandapani]please send some notes of led and led driver then and there
thanks for services
truly yours,
kothandapani
At 3 September 2014, 6:03:09 user duncan wrote: [reply @ duncan]Just wanted to say thanks for sharing. Your presentations are very clear and informative.
At 2 August 2014, 2:56:09 user Ancel wrote: [reply @ Ancel]@Giorgos Lazaridis Hi, I'd like to create a 7 segment constant current driver using these transistors. Is there a way to use a single resistor current reference or will I need one for each segment?
At 6 May 2014, 7:44:48 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@oliver You can connect the output of the arduino at an open-collector transistor circuit. Say a 1K resistor goes to the base of an NPN, the emitter goes to the ground and the collector goes to the input of this circuit. It will provide enough sink current.
At 20 April 2014, 16:23:39 user oliver wrote: [reply @ oliver]@Giorgos Lazaridis
Thanks for the great tutorials. With the transistor-MOSFET driver, when connecting an Arduino PWM pin, only half the current is flowing through the LED, even at 100% duty cycle. I assume this PWM is not sinking current? How to connect an arduino (or any microcontroller PWM output) properly?
At 18 April 2014, 10:51:40 user MaxiD wrote: [reply @ MaxiD]I can put another transistor that controls the gnd bjt transistor for use with pwm?
At 22 March 2014, 16:13:13 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Leon Not quite sure, sorry
At 18 March 2014, 19:55:22 user Leon wrote: [reply @ Leon]can we apply this to operate a laser diode?
At 2 February 2014, 9:34:02 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@karthikeyan check out the datasheet of the LED, this is where you find the info
At 2 February 2014, 9:28:06 user karthikeyan wrote: [reply @ karthikeyan]dear sir can you send to my email about ampere and voltage of one led
At 27 December 2013, 5:21:45 user thiru wrote: [reply @ thiru]VR voltage 230vAc and LED voltage is 15vAc now resistor value? so please send the formula as soon as possible.
At 14 December 2013, 23:58:32 user fabelizer wrote: [reply @ fabelizer]Very helpful! Thanks for all your work!
At 14 October 2013, 19:14:30 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@yony Simply provide PWM pulses to the proper PWM input of the circuits is the easiest way. Foe more sophisticated solutions you have to do the research yourself
At 14 October 2013, 13:44:32 user yony wrote: [reply @ yony]Can you please show how can these circuits be incorporated with a microcontroller?
I'm in a search for an efficient way to drive a 1 watt LED via Arduino (right now I'm using a non-efficient way to do this with a TIP120 transistor and 2 resistors which one of them needs to be at least 1 watt resistor).
Thank you
At 16 June 2013, 8:03:14 user jai ochani wrote: [reply @ jai ochani]please advice me the circuit diagrm for 12v out put, 2Amp led driver.
At 14 February 2013, 17:14:10 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]
At 11 February 2013, 11:46:22 user shamim wrote: [reply @ shamim]we wanted to drive a LED using Mosfet. what Mosfet to use???
At 16 January 2013, 15:39:39 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Ian for 90% efficiency you will need other type of circuits, not a linear driver. An SMPS instead
At 14 January 2013, 13:22:32 user Ian wrote: [reply @ Ian]Hi Giorgos,
I'm wishing to use currents between 10mA and 1A depending upon the type of LED connected. I'd like the efficiency to be 90% plus if possible. I've seen Rds(on) values as low as 0.021 ohm, which I'd guess would translate into less power losses?
I'm guessing that from a 5v PIC micro that a logic level mosfet is the only way to go as opposed to a normal mosfet?
Thank you, Ian
At 14 January 2013, 9:17:48 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Ian depends on the current and your efficiency requirements. Typically, on resistance of mosfets is very low. In many cases it is lower than 1Ohm (not high current mosfets). What current requirements you have? More than 1 amp?
At 11 January 2013, 17:49:19 user Ian wrote: [reply @ Ian]Hi Giorgos, Thank you for your help with current measurement. Can you also tell me in the selection of a power MOSFET if Rds(on) should be as low as possible or doesn't it matter? I'm presuming the MOSFET is operating in saturation mode or would be if PWM was applied to the gate? Thank you, Ian
At 7 January 2013, 5:34:43 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Ian I put the scale of the multimeter to amperes and connected it in series with the LED
At 6 January 2013, 22:01:19 user Ian wrote: [reply @ Ian]Hello Giorgos, Could you tell me how you measured the current through the LEDs as shown in your photos please? Thank you, Ian
At 3 January 2013, 8:41:11 user adithyan wrote: [reply @ adithyan]Circuits are plenty for pwm control of led brightness. It is agreed that there cannot be a linear control of led brightness. Assuming that the pwm controls applied and the led current exceeds the 20ma limit in case of 4mm and 5mm Leds and 350ma in one watt, how a feed back control either shuts off the led or keeps the led current at maximum is yet to be published
At 2 October 2012, 15:17:29 user Ajay wrote: [reply @ Ajay]Pls. give detail related to LED Driver Design.
Regards
Ajay
At 28 September 2012, 18:38:22 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@kwstas there is no "specific" number. I would not go above 3.3 volts though, just to maintain the power dissipation on the emitter resistor low.
At 27 September 2012, 7:44:27 user kwstas wrote: [reply @ kwstas]@Giorgos Lazaridis Thank you very much but i'm asking about the zener diodes..
At 26 September 2012, 17:48:15 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@kwstas same diodes (1n4001 if i remember well or similar)
At 26 September 2012, 17:15:07 user kwstas wrote: [reply @ kwstas]I would like to tell me what diodes did you use for the 1w Led.
Thank you
At 9 March 2012, 8:53:16 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Ghlargh although i plan to use such drivers, i will not go into details.
At 9 March 2012, 8:46:17 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Alvie it is the most stable of all. i tried it with 10w led. i prepare video for this.
At 9 March 2012, 7:46:04 user Ghlargh wrote: [reply @ Ghlargh]You should, if possible, explain how multiple constant current drivers with a single reference resistor work. Such as the Allegro A6282
At 8 March 2012, 22:13:28 user Alvie wrote: [reply @ Alvie]Regarding the BJT MosFET circuit:
Can you prove it's stability ? Just by looking at the diagram I think it might oscillate, eventually at high frequencies, and eventually amplify the thermal noise.
At 7 March 2012, 20:10:23 user Daniel wrote: [reply @ Daniel]hey you should take a look at how leds could be used as sensors(ldrs)
http://www.instructables.com/id/Light-Sensing-LEDs/
At 26 February 2012, 6:32:30 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]@Lupin Thank you very much for pinpointing those mistakes. You are right for both. The first one is already corrected, as for the second i will recompile the video. Thanks for noticing it soon.
At 25 February 2012, 22:54:05 user Lupin wrote: [reply @ Lupin]Just wanted to mention, tha in the example it seems you forgot to subtract V_Z in the calculation of I_RB_MAX.
On the other hand, I got an issue on how you calculated P_RB in the video (around 5:50). You added the base current to I_RB_MAX. Isn't that already included?
Still many thanks for all the explanations and videos! I'm teaching a beginners electronics course (diodes, transistors, opamps) next semester at uni. Teaching applications, when you got to do theory first always falls short. I guess I'll refer some students to your page if they want to know more. |
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