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| When current limiting resistors are needed with LED? |
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| Electro Fan:
--- Quote from: Youkai on April 27, 2018, 05:50:48 am --- --- Quote from: IanB on April 27, 2018, 04:27:14 am ---One way to answer this is to say that a bare LED, all by itself, is never rated for a voltage. Not ever. An LED, by its very nature, is always rated for a current. For example, 20 mA maximum, or if it is a big power LED maybe 300 mA or so (with a heat sink). --- End quote --- Ok so that makes a lot of sense I guess. Looking at this red LED: https://www.superbrightleds.com/moreinfo/through-hole/5mm-red-led-120-degree-viewing-angle-flat-tipped-1200-mcd/279/1206/ the site says it's rated for 30mA. But also for 2.0V. So is it "rated for 2.0V" or is that somehow a misnomer/incorrect way to think about it? EDIT: Oh or is the 2.0V it's forward voltage and it's just poorly labeled? --- End quote --- In addition to being labeled with the Forward Voltage of 2.0 the spec in the link also gives a Maximum Forward Voltage of 2.4, a Peak Forward Current of 100mA, a Power Dissipation of 85mW, and a Total Power Consumption of 0.12 Watts. It’s all under the Specification section. Edit - also from the spec sheet: Continuous Forward Current 30mA |
| Youkai:
Ok so I think knowing that "LED are rated for current not voltage" and that I need to readjust my thinking from "rated for 2V" to "forward voltage of 2V" will both help my greatly. Thanks everyone. |
| IanB:
--- Quote from: Youkai on April 27, 2018, 05:21:20 pm ---Ok so I think knowing that "LED are rated for current not voltage" and that I need to readjust my thinking from "rated for 2V" to "forward voltage of 2V" will both help my greatly. Thanks everyone. --- End quote --- In particular, look back at the data sheet you linked: https://www.superbrightleds.com/moreinfo/through-hole/5mm-red-led-120-degree-viewing-angle-flat-tipped-1200-mcd/279/1206/ Scroll down to the "absolute maximum ratings" section. You must at all time stay within the limits given there. |
| rstofer:
Another thing: There are tolerances in real devices. Just because VF is 2.0V doesn't mean you should just connect it to a 2V power supply with, essentially, infinite current. You want to control current and VF falls out where it does. When you design for, say, 20 mA, you will probably know that you are safe up to, say, 30 mA. As a result, the resistor value isn't super critical and its value can be calculated as I gave above. The current may vary a little bit with tolerances but it won't get to some magic limit. Just remember, it's the current you design for. Many times you will set the design point a good deal lower than the nominal IF because you simply don't need the brightness. And, yes, Ohm's Law is the way the calcs are done as I showed above. You know the current and you get the required voltage (drop) from VSource - VF. Given this voltage drop and current, Ohm's Law gives the resistor value. It will almost never be a standard value so pick the next larger value. One last thing: Ohm's Law is a LAW, not a suggestion! You can't treat it like a speed limit! |
| Vtile:
In the limits of the maximum values, which are typically given in form of Vf and If or Imax there is no difference if you feed the bare led with current or with voltage. What will happen is that the LED will dynamically adjust itself in a way that it pulls or pushes the amount of the missing component (V or I) (Compared to the I/V curve, which is derived this way). Edit. Removed the misleading part. If that is a good design practice is another topic. |
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