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April 3[edit]

Feeding a 5W IR LED[edit]

If one has a 5W infrared LED, how much volts of DC does one need to get maximum IR output without blowing it. And please also tell much CURRENT will be sufficient and how does one measure it ? And will using a resistor (how much ohms will make us succeed, BTW has it always to be put on + wiret ? ) And will the resistor lower voltage or current, please. I hope somebody will be kind enough to let me know these things without suggesting me to filter rather confusing waters of Google....124.253.244.183 (talk) 00:37, 3 April 2016 (UTC) [reply]

Our article on Ohm's law covers at least the last part of your question which seems like a homework question. --DHeyward (talk) 02:16, 3 April 2016 (UTC)[reply]

A 5W infrared LED typically (there are 5W versions with other characteristics) has a forward voltage of 2V DC and current rating of 1000 mA (1 Amp). The calculation of the series resistor is by Ohms Law: Supply voltage - 2V / 1 Amp. For example, if the supply voltage is 12 VDC, the voltage to be dropped in the resistor is 10V at 1 Amp, so the resistor should be 10/1 = 10 ohms. The power dissipated in the resistor will be 10V x 1 = 10 Watts. You would normally choose a 10 ohm resistor of at least that rating, example 20W, as a safety margin. The above calculations presume that the power supply is capable of supplying 12V at 1 Amp. If you do the sums correctly, there's probably no need to measure the current, but a 1 Amp (or more) DC ammeter or multimeter set on an appropriate range will allow you to do so. The resistor limits the current that would otherwise flow if the LED were connected directly to the power supply; that current would almost certainly destroy the LED or power supply (in that order of likelihood). Akld guy (talk) 04:15, 3 April 2016 (UTC)[reply]

I don't mean to be snarky but "A 5W infrared LED typically (there are 5W versions with other characteristics) has a forward voltage of 2V DC and current rating of 1000 mA (1 Amp)." seems a little unlikely. I think you mean 5V. In which case rinse and repeat. Guy's advice below seems more robust.Greglocock (talk) 07:34, 3 April 2016 (UTC)[reply]

Yes, the obvious error is 2V @ 1A is only 2W. The question regarding what a resistor will do makes me believe that this is a homework problem and reluctant to provide a complete solution even if they specified an LED. I will say, that I doubt I would ever choose a single 5W IR LED to consume 5W of total power. The spec below has many solutions. --DHeyward (talk) 07:51, 3 April 2016 (UTC)[reply]

@Greglocock: and @DHeyward: You both need to take a look here. The voltage is 2V, the current is 1 Amp for several 5W models. Akld guy (talk) 09:26, 3 April 2016 (UTC)[reply]

@Akld guy: 5W is the power rating, not power consumed. Operating at 2V and 1A will consume 2W with luminous power below that. It's simply not possible to consume 5W with 2V and 1A. It's the same with the resistor you use above. The package/material might be a 5W rated package but it has nothing to do with how much power is actually consumed. You recommended a 20W resistor for margin but you also calculated it only consumed 10W using the same law that says the LED will only consume 2W. The package has margin which usually means the junction temperature is more stable over a variety of operating powers. This margin affects emitter frequency stability over power consumed (i.e. 2W CW is the same peak frequency as the 2W with 50% duty cycle - the same device in the 2W package has more variation). There are other issues that you can see using the curves in the datasheet that make the 5W package more attractive than the 2W package but it's clear that the rating is separate from the consumed power --DHeyward (talk) 10:03, 3 April 2016 (UTC).[reply]

Well, what's your solution to the OP's question. You haven't provided one. Akld guy (talk) 10:25, 3 April 2016 (UTC)[reply]

I think it's a homework problem and we don't do homework problems. He also doesn't provide power supply available or part. This is likely a combination of resistor with parallel and series diodes and part of a beginning course on current, power and the various laws that govern them. --DHeyward (talk) 15:59, 3 April 2016 (UTC)[reply]

The specific LED you are using will have a datasheet with that information. Here is a typical one:[1] --Guy Macon (talk) 06:33, 3 April 2016 (UTC)[reply]

Responsible answers can only be given when the component data sheet is found. A LED has a non-linear current-voltage characteristic, see Light-emitting diode so in the circuit illustrated we can calculate Ohm's law

R={\frac {V}{I}},

only at one (V,I) point for that particular diode. See the article LED circuit. The OP also needs to know that a series resistor reduces both the voltage across the LED and thereby the current (which is the same in both LED and resistor), that one can measure the current either by breaking the circuit to introduce an Ammeter or by measuring the voltage across the resistor R and applying Ohm's law

I={\frac {V}{R}},

and that it makes no difference to the LED whether the resistor is on its + or - wire. Operating a LED near its maximum output makes the manufacturer's data sheet essential and in some cases a different circuit, such as a constant-current driver, may be necessary.

The responses above have linked to two different LED data sheets. From Akid guy, Type LZ1-00R400 quotes typical V_F 1.9V @ 1A, so we may conclude that a 5V supply and 3.1 ohm resistor will suit. The power dissipations are 1.9W in the LED and 3.1W in the resistor; that implies a heatsink for the LED and preferably at least a 5W rated resistor. But look further in the data sheet: individual LEDs can have V_F @ 1A that vary from 1.7 to 2.7 V and this can have the consequence that resistors must be selected to match the brightnesses of an array of LEDs. From Guy Macon, Type LZ4-00R408 quotes V_F @ 700mA that vary from 6.8 to 10.8 V, and in this case I_F = 1A is its absolute maximum. One should realize that this device is actually specified as 4 LEDs connected in series and the series resistor circuit is less suitable (and power wasteful). A constant current supply circuit and a proper heat sink are needed for safe operation of this device. AllBestFaith (talk) 13:47, 3 April 2016 (UTC)[reply]

Actually Guy Macon's are not connected at all. The datasheet assumes that configuration for publishing tables and it would popular if you have two lithium ion 18650 batteries (popular torch rechargeable). if you only had 1 battery, it can be configured as 2 parallel and 2 series LEDs and the voltage would 3.6V instead of the nominal 7.2V with the same power delivery. --DHeyward (talk) 16:33, 3 April 2016 (UTC)[reply]

Well noticed! The LZ4-00R408 is a 4-diode package which may have external series connections, but doesn't have to. The data sheet mentions that the diode forward voltages match within ± 0.16V which suggests the diodes also work well when connected in parallel; then implicitly V_F @ 2.8 A is 1.7 to 2.7 V. AllBestFaith (talk) 19:54, 3 April 2016 (UTC)[reply]

You can not really perform this calculation unless you know 3 things: what is your supply voltage? what is your LED forward voltage? and what is your LED forward current? You can make an educated guess as to the last 2, but once you have those 3 values the rest is easy! or as stated above you can use a constant current source, which is probably the better and safer way to drive a high power LED. Vespine (talk) 02:19, 4 April 2016 (UTC)[reply]

First, for high power LEDs, there are special designed stepdown ~~boost converters~~ buck converter avail. Second, IR-transmitter LEDs are designed for a higher pulse and thermal recovering periode. Else, if You are using the LED as light for IR camera, You need either synconize the pulse to the captured picture or use a linear constant current source what might be fed from a buffered stepdown converter. As CPUs needs to be aware from brownout, see how stepdown converters on computer mainboards for CPUs are designed and what might be useful for Your application. --Hans Haase (有问题吗) 09:15, 7 April 2016 (UTC)[reply]

Downloading the Science Wiki[edit]

Hello, is there any way to download the Science (Chemistry, Biology, Physics) Wiki so I can use for my studies even when I'm offline? — Preceding unsigned comment added by 1.132.96.62 (talk) 11:30, 3 April 2016 (UTC)[reply]

Could you clarify what you're asking for? If you're referring to Wikipedia articles, you might be interested in our Book creator page, though I'm afraid that it explains that the program is mostly on hiatus. The existing science books are listed here. You can also download individual articles as PDFs (there will be a link on the left side of the screen, just below the option to create a book). Matt Deres (talk) 12:41, 3 April 2016 (UTC)[reply]

The answer depends on what sort of device you are using and the capacity it has. See : Wikipedia:Database download. Haven't used it myself but one should (I think) be able to download articles by just category. --Aspro (talk) 16:51, 3 April 2016 (UTC)[reply]

Are you talking about pages on Wikipedia, or some other wiki? "Wiki" and "Wikipedia" do not have the same meaning. --71.110.8.102 (talk) 21:48, 3 April 2016 (UTC)[reply]

→xowa --Hans Haase (有问题吗) 12:02, 7 April 2016 (UTC)[reply]