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Understanding LED’sUpdated 2 years ago

LED stands for Light Emitting Diode, and a diode is an electronic device that conducts current in only one direction. The comparison in Fig 6 shows how the water in the pipe gets stopped if the water tries to flow in the other direction. Actually, this is quite a good analogy as you can imagine, if a very small pressure in the pipe tried to open the valve it couldn’t, it needs a little bit of pressure (V) to overcome the inertia of the flap. In the same way, a diode needs a bit of voltage (see Fig 7 below) to do the same – typically between 2 and 3 V. Once overcome, there is nothing to hinder the current except the resistance of the pipe.

 

 

Figure 6 Diode and water valve


 

 


Look very carefully at the LED curve on the left. (fig 7)  What it’s telling us is that if we slowly increase the voltage across the LED, no current flows through it until a certain point beyond 2.5V (the forward voltage), and then it rapidly starts to conduct. If there is nothing to stop it, it will draw all the current it can get from the power supply until it goes up in a blaze of glory.

 

This forward voltage can be considered pretty constant for any given LED and depending on brand and type could be about 2.3 to 3.3V and is what we will call VF

 

Figure 7 LED forward voltage vs current

 


If we add a resistor in series with the LED as a current limiting device, (see Fig 8) and we once again varied the power supply voltage, and we measured the voltage across the LED as well as the voltage across the resistor we would notice the following:


a. The voltage across the LED remains pretty much the same –   and in fact stays at what we expect VF to be – say  2.7V


b. The voltage across the resistor varies with the current

 

 

 

 

Now we know everything we need to start working with our LED:

  1. Because the LED VF is fixed at 2.7V, the voltage across the resistor will ALWAYS be the battery voltage minus VF. If the battery was 9V this is 9-2.7 = 6.3V This is very important to grasp, so think about it until you agree!
  2. Let’s assume the LED manufacturer tells us his LED works best at 40mA (this value could be anything from 10 to 100mA depending on the type of LED)  
  3. Because Ohm’s law tells us that if we have a voltage of 6.3V across a resistor and 40mA flowing through it, the resistor must be R =V/ I    =  6.3 * 1000/40 = 157Ω   [Where did the 1000 come from? Remember the current was in milliamps (40mA) so we had to divide 40 by 1000 and so 1000 now appears in the numerator (or on top)]. If we took a standard resistor value of 150 Ω it would be fine since the current will only increase to 42mA.  


Typical values for these components as used in strip lights are a VF of 3.2V and an operating current of 16mA per LED and a supply voltage of 12V.  

So, if three LEDs are connected in SERIES the resistor would need to be 150 ohms, from


If you look at some LED strips you will see small black things with 151 written on them, these are the resistors we just mentioned, the first 15 is the value and the 1 at the end is the number of zeros ie 150.

(Series means end-to-end, parallel means side by side). There are 3 LED’s (actually each of these LED strips contain 3 LEDs in series that’s why they have 6 legs) per section so the total current per section is 48mA.

 

Figure 9 constant voltage strip

 

 

  

The circuit in Fig 9 is typical of  12V ‘constant voltage’ strips. What this means  is that providing you supply this circuit with a constant voltage of 12V, a current of 15mA will always (and safely) flow in the LED. What they do in practice is connect 3 identical circuits in parallel in each ‘cut-off’ section, and as many as you like in a string, but they are ALL in parallel and each LED only conducts 15mA no matter how many you string together  PROVIDING the power supply stays at a constant voltage. So what we really ought to say that this is not a constant voltage strip (as they term it), but rather a strip designed to be used with a constant voltage power supply.

 

 

There is a small catch we need to be aware of here.  As each LED is taking 16mA, and therefore each  section with 3 LED’s is taking 48mA from the supply, the currents add up quickly, ie 20 sections take almost 1A and you need to be sure the power supply can deliver this comfortably. A good rule would be to allow at least 20% “head room”. So if you need 10A for all your LED’s you should have a 10A+20% = 12A supply.

Now let’s decipher what the manufacturers mean when they talk about a constant current LED string and also a constant current supply.

As we’ve seen previously, LED’s want just the right current through them – one that does not change, so let’s put something other than the simple resistor in series that will make sure the LED current remains the same, no matter what the voltage does – enter the constant current supply! Now be alert here, I’m not talking about the power supply!

 

 We now have a clever little circuit on each section of the actual strip.  See Fig 10.

Figure 10 Constant current strip element

 

 

 


The circuit on the left is a current regulator, don’t worry if you don’t understand how it works, but for those more technical:

The two transistors plus their associated resistors  act as a current regulator. It is a characteristic of transistor Q2 that 0.6V will occur between its terminals 2 and 3 regardless of the supply voltage. Therefore, that same voltage exists across resistor R4. Hence we can calculate the current flowing through R4 as I = 0.6/R4 = 0.6/10 = 60mA. And, this current remains constant, or “regulated”, regardless of the power supply voltage just as long as that voltage is sufficient to drive the circuit. That’s how the current regulator works.

 

 

 

This approach is a big improvement over that in Fig.9. It has the additional advantage that If the power supply voltage goes up and down a bit, or if ‘spikes or switch- on surges’ appear on the supply, the LED’s are buffered from it and will last much longer. This is what they call these strips a ‘constant current LED strip’. In reality, they have 7 LED’s all in series and driven by one constant current transistor pair in each little cut off section. As before, all of these sections are in parallel across the supply and the supply voltage across each one is the same, no matter how many you string together.



From the above you can see that there is more going on in the strip than meets the eye, and for this reason you can’t just cut the strip anywhere but must cut where the maker indicates. The strip on the left (in Fig.11) is a RGB (Red, Green, Blue) type

 

 

 


Figure 11 Where to cut

 

Finally we come to what some suppliers (e.g. Mean Well ) call a ‘constant current’ supply. Actually it is no such thing and no wonder there is confusion! We’ve just learned what a constant current supply is, and if this unit really was a constant current supply -- rated at 11A, as they say on the label -- then no matter what  load we connected to the output, it would try to pump 11A through it but it does no such thing!  

All it is, is a constant voltage supply – ie a regulated supply, with built-in current limiting. In other words it produces a regulated 24V and if you try to draw more than 11A from it, it starts to reduce its voltage so that a constant 11A is never exceeded. It therefore only provides  some? Protection to your circuits and wiring in the event of shorts or overloads, that’s all.

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