Hi MrAl,
I would not be an 'electronics addicted' if I was out from the discussion forum. Because you all experience engineers taught me to carefully handle each components. I scrap components from old kits and it's like a magic when these works amazingly in a new circuit again. Such thing became addiction to me so when I feel bore or upset then I start to do electronics. It makes mind so fresh and energetic! Same thing happening currently. (I am trying to make a 'Easy Pulse v1.1' from embedded-lab.com to detect heartbeat from finger tip. It's just a quad OpAmp and few component so felt exciting. I have no exact OpAmp used by designer, will use which I have though.)
If voltage across LEDs depends on its current then why manufacturer rates its voltage like 3.2V white LED or 1.2V red LED or 4.7V zener diodes (zener's regulated voltage is also current depended). Does this mean even a 1N4007 has different forward voltage according to current through it?
Hi,
The voltage rating of an LED is not unlike the voltage rating of most other things we see. If we go to buy a light bulb, we usually look for a given voltage, like 6v, 12v, 24v, or 120v, etc., but there are other things to consider too. LEDs are like that also. When we look for an LED we have to match the current more so then the voltage, but both are important. The LED voltage is actually called the "characteristic voltage" unlike light bulbs where the voltage is really called the "operating voltage". The characteristic voltage is just meant to be used as a quick reference so we can compare different LEDs on the fly, without being too accurate. As you have already seen, the blue LED has a higher forward voltage than most red LEDs for example, so you know that a red LED might work in applications where a blue LED wont work, because the characteristic voltage is higher for blue. A lower voltage available might be able to light a red but not a blue.
But light bulbs too have to be matched in many applications, by specifications beyond their voltage rating. For example, in a particular automobile we usually have to match up the bulb NUMBER, not just the voltage of 12 volts. That number implies not only a given voltage, but also a given base size, bulb glass size, and current rating.
Recall that if you buy a lamp for the house too, you will most likely want to get the right WATTAGE as well as the right voltage. Here in the US we use mostly 120v bulbs, but we choose between wattage ratings of 20 watts, 40 watts, 50 watts, 60 watts, 75 watts, and 100 watts just to name a few. So we need to know that it is for 120 volts, but also what wattage we want to use in that particular location.
So you start to see that a voltage specification isnt really enough for anything when we take a closer look at this, and the LED is no different in this respect. We have to know the characteristic voltage as well as the normal operating current before we know if we can use it in a given circuit. And in the hobby arena we sometimes even go one step further: we sometimes want to know what they can do at other less common current levels like 1ma for example.
Comparing a few bulbs to a few LEDs:
bulb: 120v, 50 watts
bulb: 120v, 100 watts
bulb: 230v, 100 watts
LED: white, 3.5v, 20ma
LED: white, 3.2v, 350ma
LED: blue, 3.6v, 20ma
LED: red, 1.8v, 20ma
Looking at the above we see again that we always have to know something else about either type of device as it is usually not enough to know just the voltage.
Maybe if we went to buy a radio we'd have to know it runs on 12v or 120v, but then we'd also have to know:
1. The type of current, AC or DC
2. The line frequency, like 60 Hz.
so there are almost always other things to consider. The bottom line is that if you went out to buy a bulb for a car you could not just go out looking for a "12 volt" bulb, because that is not enough information even to buy a bulb even though the operating voltage is 12v. Even more to the point, that bulb will sometimes be operating at 14.2 or so volts when the car engine is running because the alternator raises the battery voltage by at least 2 more volts. So it is called a "12v bulb" but it may actually run at something like 11.8v to 14.6v sometimes.
There are however certain LEDs that have a more specific voltage rating. These are LEDs that are made for a particular use such as an indicator lamp. You might find one that says "12v" and that means it runs at pretty close to 12v. But that is really a composite device made up of an LED and resistor, where the designer sized the resistor so the actual LED sees the right current during operation at 12v. So they take some of the guess work out of it. You might still want to know how much current it draws though so you know if your power supply can handle it.