Basics: Picking Resistors for LEDs

5 mm warm white diffused LED

So… you just want to light up an LED. What resistor should you use?

Maybe you know the answer, or maybe everyone already assumes that you should know how to get to the answer.  And in any case, it’s a question that tends to generate more questions before you actually can get an answer: What kind of LED are you using? What power supply? Battery? Plug-in? Part of a larger circuit? Series? Parallel?

Playing with LEDs is supposed to be fun, and figuring out the answers to these questions is actually part of the fun.  There’s a simple formula that you use for figuring it out, Ohm’s Law. That formula is V = I × R, where V is the voltage, I is the current, and R is the resistance. But how do you know what numbers to plug into that formula to get out the right resistor value?

To get the V in our formula, we need to know two things: the voltage of our power supply, and the voltage of our LEDs.

Lets start with a concrete example. Suppose that we are using a 2 × AA battery holder (like this one from our shop), which will provide us with a 3 V of power (with two 1.5 V AA cells in series; we add the voltages), and we’ll plan to hook up a yellow LED (like one of these).

LEDs have a characteristic called “forward voltage” which is often shown on the datasheets as Vf. This forward voltage is the amount of voltage “lost” in the LED when operated at a certain reference current, usually defined to be about 20 milliamps (mA), i.e., 0.020 amps (A).   Vf depends primarily on the color of the LED, but actually varies a bit from LED to LED, sometimes even within the same bag of LEDs. Standard red, orange, yellow and yellow-green LEDs have a Vf of about 1.8 V, while pure-green, blue, white, and UV LEDs have a Vf of about 3.3 V. So, the voltage drop from our yellow LED will be about 1.8 V.

The V in our formula is found by subtracting the LED’s forward voltage from the voltage of the power supply.

3 V (power source) − 1.8 V (LED voltage drop) = 1.2 V

In this case, we’re left with 1.2 V which we’ll plug into our V = I × R formula.

The next thing we need to know is the I, which is current we want to drive the LED at. LEDs have a maximum continuous current rating (often listed as If, or Imax on datasheets). This is often around 25 or 30 mA. What this really means is that a typical current value to aim for with a standard LED is 20 mA to 25 mA—slightly under the maximum current.

Aside: You can always give an LED less current. Running an LED near its rated maximum current gives you maximum brightness, at the cost of power dissipation (heat) and battery life (if you’re running off of batteries, of course.) If you want your batteries to last ten times longer, you can usually just pick a current that is only one tenth of the rated maximum current.

So, 25 mA is the “desired” current— what we’re hoping to get when we pick a resistor, and also the I that we’ll plug into our V = I × R formula.

1.2 V = 25 mA × R

or rephrased:

1.2 V / 25 mA = R

and when we solve that we get:

1.2 V / 25 mA = 1.2 V / 0.025 A  =  48 Ω

Where “48 Ω” is 48 ohms.  (The units are such that 1 V/ 1 A = 1 Ω; one volt divided by one amp equals one ohm.  If you are dealing with current in mA, convert to A by dividing by 1000.)

Our version of the formula now looks like this:

(Power supply voltage − LED voltage) / current (in amps) = desired resistor value (in ohms)

We end up with a resistor value of 48 Ω.  And, that’s a fine starting resistor value for use with a yellow LED and a 3 V source.

Let’s look at resistor values for a moment.  Resistors are usually available in values such as 10 Ω, 12 Ω, 15 Ω, 18 Ω, 22 Ω, 27 Ω, 33 Ω, 39 Ω, 47 Ω, 51 Ω, 56 Ω, 68 Ω, 75 Ω, and 82 Ω (and their multiples, 510 Ω, 5.1K Ω, 51K Ω, etc.), and (unless you specify higher precision while shopping) have a tolerance value of about ±5%.

If you do a lot of electronics projects, you’re likely to have a bunch of resistors lying around. If you’re just getting started, you might want to get an assortment so that you have some handy. Resistors also come rated to handle varying amounts of power— resistors rated for more power (more watts) are able to safely dissipate more heat generated within the resistor. 1/4 watt resistors are probably the most common, and are generally just fine for simple LED circuits like the ones we’re covering here. (We’ve discussed power dissipation previously—look into that when you start to move beyond these basics.)

Now, the resistor value we calculated above was 48 Ω, which isn’t one of our common values. But that’s okay, because we’ll be using a resistor with a ±5% tolerance, so it won’t necessarily be exactly that value anyway. To be on the safe side, we generally select the next higher value that we have on hand; 51 Ω in this example.

Lets hook this up:
3 V battery box, 51 Ω resistor, and yellow LED.

circuit

Now, that’s a nice little LED circuit, but how can we do this with more LEDs? Can we just add another resistor and another LED? Well, yes, to a point. Each LED will want 25 mA, so we need to figure out how much current our batteries can source.

Aside: A little digging turns up a helpful technical handbook (pdf) on alkaline batteries from Energizer. It turns out that the harder you drive them, the faster you drain them.  Part of this is obvious: If you continuously draw 1000 mA out of a battery, you would expect the battery to last 1/10 as long as if you draw 100 mA. But there’s actually a second effect, which is that the total energy output the battery (measured in watt-hours) decreases when you approach the limit of how much current the battery can source.  In practice, with AA alkaline batteries, if you drain it at 1000 mA, it will only last about 1/20 as long as it would if you drained it at 100 mA.

 For our single 25 mA LED, AA cells will last a heck of a long time. If we run four LEDs in parallel, requiring 100 mA, we should still get pretty decent battery life. For higher than 500 mA, we should think about plugging into the wall. So, we can add several of our yellow LEDs, each with its own 51 Ω resistor, and drive them happily with a 2xAA battery holder.

circuit

All right, how about a 9 V battery? Let’s stick with our yellow LEDs. If we want to run one LED off of a 9 V battery, that means we have to take up a whopping 7.2 V with our resistor, which would need to be 288 Ω (or the nearest convenient value: 330 Ω, in my workshop).

9 V (power supply) − 1.8 V (yellow LED)  = 7.2 V

7.2 V / 25 mA = 288 Ω (round up to 330 Ω)

Using a resistor for a voltage drop of any size dissipates that energy in the form of heat. That means that we’re just wasting that energy on heat instead of getting more light out of our LED circuit. So can we use multiple LEDs strung together? Yes! Let’s put four of the 1.8 V LEDs in series, adding up to a total of 7.2 V. When we subtract that from our supply voltage of 9 V, we’re left with 1.8 V, requiring only a 72 Ω resistor (or nearest value: 75 Ω).

9 V − (1.8 V × 4) = 9 V − 7.2 V = 1.8 V

1.8 V / 25 mA = 72 Ω (and we then round up to 75 Ω)

Our generalized version of the formula with multiple LEDs in series is:

[Power supply voltage − (LED voltage × number of LEDs)] / current = resistor value

We can even put a couple of these strings of four LEDs plus a resistor in parallel to get more light output, but the more we add, the more we’ll shorten our battery life.

But can we do five in series with a 9 V battery? Well, maybe.  The 1.8 V figure that we’ve been using is a “typical rule of thumb,” only.  If you’re sure the forward voltage is exactly 1.8 V, it will work. But what if it isn’t exactly that?  If the forward voltage is lower, you may overdrive them at a higher current, which can shorten their lifespan (or kill them outright). If the forward voltage is higher, the LEDs may be dim or may not even light. There are some cases where you can drive LEDs in series without a resistor, like in our LED Dining Table Circuit, but in most cases, it’s preferable and safer to use a resistor.

Let’s do one more example, this time with a white LED (you can find some here) and a 3xAA battery box (such as this one). Our power supply voltage is 4.5 V, and our LED Vf is 3.3 V. We’ll still aim for a current of 25 mA.

4.5 V − 3.3 V = 1.2 V

1.2 V / 25 mA = 48 Ω (round up to 51 Ω)

So, here are the examples we’ve looked at plus few more with some other common power supply types:

Power Supply Voltage LED Color LED Vf LEDs in series Desired Current Resistor (calculated) Resistor (rounded)
3 V Red, Yellow, or Yellow-Green 1.8 1 25 mA 48 Ω 51 Ω
4.5 V Red, Yellow, or Yellow-Green 1.8 2 25 mA 36 Ω 39 Ω
4.5 V Blue, Green, White, or UV 3.3 1 25 mA 48 Ω 51 Ω
5 V Blue, Green, White, or UV 3.3 1 25 mA 68 Ω 68 Ω
5 V Red, Yellow, or Yellow-Green 1.8 1 25 mA 128 Ω 150 Ω
5 V Red, Yellow, or Yellow-Green 1.8 2 25 mA 56 Ω 56 Ω
9 V Red, Yellow, or Yellow-Green 1.8 4 25 mA 72 Ω 75 Ω
9 V Blue, Green, White, or UV 3.3 2 25 mA 96 Ω 100 Ω

All of these values are based on the same assumptions about forward voltages and desired current that we used in the early examples. You can work those through and check the math, or just use it as a handy table if you think that our assumptions are reasonable. ;)

Now, at some point someone may have told you, “Just use an online LED resistor calculator.”  And indeed there are such things out there— even we have one (well, a printable papercraft version) —so why bother working through all this?   For one thing, it’s much better to understand what and why that calculator is doing what it does.  But it’s also near impossible to use those calculators if you don’t know what variables you’ll need to enter. Hopefully you should now be able to figure out the values you’ll need (power supply voltage, LED voltage and current) to use an LED calculator. But more importantly (1) you can don’t really need one: you can do it yourself and (2)  if you do use one, you can question the underlying assumptions that it may make on your behalf.

Throwies - 08

Hopefully, you’ve also seen that there is much more than just one way to light an LED. And we haven’t even gotten to things like putting LEDs of different values in circuits together! Now, can you go back to sticking LEDs on CR2032 batteries to make LED throwies? Yes, you most definitely can. But you may want to go back and read about when you should add a resistor to even that little circuit!

Finally, let us note that in this article we’ve been talking about your basic through-hole, low-power (though possibly extremely bright) LED. Specialized types like high power LEDs may have somewhat different characteristics and requirements.

Update: corrected the common resistor value list to include more common values.

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67 thoughts on “Basics: Picking Resistors for LEDs

  1. i have 12v (8 aa batteries) to 3 switches, to three LED’s (yellow, red, green). But there seems to be voltage leak in the switches – the LED’s are on, regarless of switch position. I even tried just 2 AA batteries, but still enough power leaked (?) out of the switch to power the LED. How do I fix this?

    • It’s hard to say without knowing the specifics of your circuit. Have you tried taking the switches out of the circuit entirely? You can use two wires as a switch temporarily, holding them together to connect the circuit.

      You might note that 12 V is overkill for a single LED, as we talked about above.

      • I can say that the switches are wired correctly, because when I attached, to experiment, the wiring to a flashlight bulb base, the lightbulb, which i guess requires more watts than LED, the bulb turns on when the switch is turned on, and turns off when switch is off. The reason for the 12v is to also power a switched buzzer, which needs 12v. And for the fun of trying to get the resistors right. But even at 3v (2 aa batteries) the switches don’t seem to keep the LEDs from lighting up.

  2. Small quibble:

    The set of resistor values you listed is almost the E12 series, but not quite. The E12 values are:

    10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82.

    If we include 100 (the first member of the next set up), any value from 1 to 100 is within 10% of an E12 value.

    The values 51 and 75 are from the E24 series, and I’m not sure you can even buy 96 ohm resistors (95.3 and 96.5 are the nearest standard values, from the E96 series).

    That doesn’t change anything you’ve said about calculating resistor values for LEDs, which is all good, useful, and right. Using the E12 scale would just change some of the rounded values a little.

  3. What about the case where I have a single AA battery holder and a ~1.5 V LED? i.e. the power supply and the forward voltage on the LED are the same. Do I still need to put a resistor in the circuit?

    • If your supposedly 1.5 V LED actually ends up being 1.6 V (or higher), then it will be dim or may not light. If it ends up with a lower value, then you could end up with too much current, which can damage your LED.

      If the values match well, then you’re probably fine. But it’s safer to put a small resistor in there anyway, or better yet, use a power supply that is slightly larger than your LED voltage and add a resistor so that you can be sure.

      • The specs on the LED state 1.3 V typ., 1.7 V max; while the battery would probably be a rechargeable, so ~1.3 V. I’m guessing I’m okay. I just kept hearing that you have to put a resistor in there, and wasn’t sure why in this case. Thanks.

        • The truth is every power source has an internal resistance anyway. If you calculate that you need a 51 ohm resistor as described in the article, adding .15 ohms internal resistance to that isn’t going to make much of a difference. There is hardly any difference between 51 ohms and 51.15 ohms.

          It’s a difference between 23.53 mA and 23.46 mA of current through the LED. More current=more light.

          However lets say we do all the same math with a 1.9 V power supply. We will find we need a 4 ohm resister to get 25mA of current. However if we add an internal voltage of .15ohms to that for 4.15 ohms our current through the LED drops from 25.00mA to 24.09 mA through the LED I realize that a .91mA change isn’t that much and wouldn’t make much of a difference but when you compare it to the .01mA change in the previous example you start to see that the smaller the numbers involved the more important internal resistance becomes.

          With some of the throwies I’ve seen, they rely entirely upon this internal resistance to regulate the current through the LED.

          Note if the provided voltage drops below the actual forward voltage of the LED, the circuit simply will not conduct current. So if you are providing 1.3V and the actual forward voltage is the “max” forward voltage of 1.7V you are just out of luck. On the other hand if you are providing 1.4 volts and the actual forward voltage is 1.399999 volts. current will flow. How much all depends on the resistance (internal and external) in the circuit.

          • What was left out of the article is the fact that batteries are not stable voltage sources. The stated voltage is merely the average, and it starts higher when full, and goes low to around 1.0 volts when empty.

            Due to this fact, you should never connect a LED directly to a battery without some overhead voltage and a resistor. You see many commercial LED keychains and torches without any resistors, and they end up overdriving the circuit and damaging the LED fairly quickly.

            If you connect a 1.5 Volt LED to a “1.5″ Volt battery, it will initially pass a large current, but the voltage of the battery will drop fairly quickly and the LED will stop shining before the battery is half-empty.

            • It sounds like you’d enjoy the post we linked to at the end of this article about our analysis of those battery + LED circuits.

  4. For most of my quickie calculations, I just use ElectroDroid (not sure of the fruit equivalent). Solid little app for those of us that still have a hard time remembering Ohm’s Law math after 20 years of trying (hey…..not everyone is easily math inclined!). There is a wonderful LED resistance calculator built right in.

  5. This is an amazing post, I thank you greatly for this!

    I am trying to lean electronics and have used many a online calculator but have rarely been satisfied with a simple one led one resistor scenario. I really enjoy the fact you
    A) listed the info as we would read it from a data sheet.
    B) used some creative and not always standard examples (ie using 3 AA batteries, LEDs in a series and parallel
    C) went into some of the design considerations ie how the mA are part of the equation.

    Thank you and please keep awesome post like this coming!

    • The crucial information that was left out of the article was that batteries are not stable voltage sources.

      A standard 1.5 Volt alkaline AA battery will vary from 1.8 V to 1.1 Volts as it empties. You have to take this into account when designing the circuit to never exceed the maximum values for the LED current.

      9 volt batteries are similiarily six 1.5 volt cells in series inside the metal case, so its voltage will vary between 10.8 – 6.6 Volts. Rechargeable cells go from 1.6 to 1.0 volts. Check yours with a multimeter to make sure.

      All the batteries will go all the way down to zero if you drain them that far, but doing so will result in leaking batteries or damaged rechargeable cells.

    • You can use a best guess like we did above: if it is red or yellow, assume around 1.8 V. If it is blue or white, assume around 3.3 V. To be on the safe side, aim for 20 mA, as many older LEDs can’t handle more than that.

    • You can also check LEDs empirically:

      Use three AA batteries for your power supply (4.5v) and a 1000 ohm resistor to limit the current. The most current that can possibly flow through the resistor will be 4.5mA, and will probably be more like 2-3mA.

      Measure the voltage across the LED for that setup, and add a quarter of a volt to get the LED’s forward voltage at 20-25mA.

      LEDs (and all diodes) have an exponential relationship between voltage and current. For every additional quarter volt you put across them, ten times as much current will flow through them.

      • The forward voltage loss of a LED also depends on the current passing through it. For very large currents, the voltage may be 60% more than for very small currents.

        Another empirical way of estimating the Vf of a LED is to connect it to a voltmeter and shining a bright light to it. A LED is technically a reverse solar panel, so it will produce a voltage that is roughly the same as its Vf.

  6. Thank you very much for this article and the rest of the Basics series. For someone who is just getting into electronics, this is some invaluable stuff. PLEASE keep the lessons coming Obi Wan.

  7. Great thread. Quick question. If one of those submersible multicolor changing LED’s takes 2 cr2032 buttons (total 6V), could I replace it with a 4 AA pack and have it last longer without adding in a resistor? Current would be higher buty the voltage would be the same – would it burn it out?

    • I’d say “maybe.” It depends on the particulars of the design. It may be relying on the internal resistance of those coin cells to work correctly.

  8. Can someone please help me with this? On a 9v power source I want to run 4 LEDs, 2 diffused red and 2 warm white. My question is…will it work if the vf and mcd is different between the red and white? Thanks

  9. Hi! I’m new at LEDs, and this has been the most helpful, intelligent article I’ve found so far regarding the basics of LEDs. You mention at the end, “we haven’t even gotten to things like putting LEDs of different values in circuits together.” Do you have an article on that subject, yet? If not, can you refer me to an article or resource that does.

    All I want to know is if there’s anything wrong with having a total of 3 LEDs (2 of one color and 1 of another) on the same power supply and switch. I’d like to learn for myself as well, but can’t seem to find a definitive answer to that question.

    Thanks for a great article!

  10. Great website, helped me a lot.
    I had relight a plexiglass Lowenbrau beer light that was lit with a 6″ fluorescent bulb. I used a 4.5V power block to light 22 water clear 5mm LED’s all in parallel under the sign. This was a battery set of Christmas lights It worked fine for a couple of weeks but burned out all the LED’s. When I checked the power block its V was 8.25V. Is that because there was no load. I have two and they both read the same? Should i use the 8V in the calculator to revise this set up?

  11. Hi all,
    complete novice trying to learn,purchased a kit from bitsbox with breadboard, thought id start out really simple but stumped already, Set up the simplest circuit 9v battery 330 ohm resistor and red led, it was bright for a second or 2 then dimmed quickly, could smell the led burning. I used several online calculators to get the resistor value all said the same.Used the rule of red led 2v. anyone any idea’s what im doing wrong. So want to learn yet so frustrated already.

    • If your LED can handle 25 mA, and your battery is actually giving only 9 V, then 330 ohm should be fine…so long as you don’t have a short circuit. With an LED of unknown provenance, I would go for 20 mA, and 1.8 V for my starting values, which would suggest a slightly higher value (~360 ohm) for the resistor. Why don’t you try with something around 400 ohm? If you’re still having problems, then it is likely that your circuit is hooked up wrong and you’re bypassing your resistor somehow.

      • Thanks, tried 390 ohm direct to led and to battery with out breadboard and works great, maybe i have a problem with the breadboard, Enthusiasm in full flow again now thanks very much.

  12. Hi there, i am running 10 leds from a 9v transformer … After using a calculator it has told me to use a 33ohm resistor. I will be running a parralel circuit. Does each led require a resistor, or can I just use the one 33ohm resistor for the whole circuit?

    • 33R does not sound right for a 9V supply unless you have a high current LED (or somehow have a higher fV, which is not very likely). You would get about 170 mA through a normal white LED and waste about 60% of your power over the resistor (which would have to handle an absolute minimum of 1 Watt of power, so a normal 1/4W resistor would just not do).

      In theory you could use a single resistor for your LEDs but then you would have to calculate for the total current going through all LEDs. That would have you running them all off a 3.3R, 10 Watt resistor. That’s a pretty good way to warm your room.

      You should be able to run 10 white or blue LEDs as 5 pairs (5 parallel strings of 2 in series) with a much smaller level of power wastage. If they are orange, yellow or red you could probably have them in threes.

  13. if fixed power suply of 10 volt is avalaible and we need 5 volt then how can we get 5 volt from the avalaibe power suply?

  14. what would be the answer here?

    One 10-ohm resistor is in series with another resistor and a 20V battery. If the current in the circuit is 2/3 of an amp, what is the value of the second resistor?

  15. I am a complete newb to all this, but I wanted to wire up 2 white 3mm leds to a 12v molex from a computer power source in series. I have some resistors that I believe are all 1/4w resistors but some rate at I think 100ohms and some that rate at 420ohms. The Leds rating is:
    • Forward current: 20 mA continous, 50 mA peak
    • Forward voltage: 3.0 to 3.4 VDC
    • Viewing angle: 20° to 25°
    • Luminous intensity: 4000 mcd
    When imputing to calculate what kind of resistors I needed, I input 20 ma
    for forward current and 3 for forward voltage I resulted in needing a 320ohm resistor. So my question are:
    1. I noticed you mentioned that you upped the 20ma for forward current to hit a middle mark between 20ma and 30ma so would you in this case aim for a middle mark of 35 for mine being 20ma to 50ma peek?
    2. I saw that you also loosely at one point mentioned that a 1/4w resistor would be fine in your example. Are the ohms on a resistor the most important thing to follow or is there a general wattage or voltage range that 1/4w resistors can handle?
    3. In my predicament, using what I have, would you use the one 420ohm resistor or would you use 4 100ohm resistors in series to cut down on the amount of overhead making it closer to 320ohms = 400ohms insted of 420ohms?
    4. Does it matter what size resistor you use as long as it is over the amount you need? Meaning if you only need say 320ohm resistor could I hypothetically use a 1000ohm resistor and not have a problem with lighting the leds or do you need to get a resistor that is as close as possible to what you need because the resistor would then start drawing all the power and the leds would be left with out power?
    5. Last question (side question) I was also interested in putting in some RGB led tape that has a remote receiver into the same computer. The Led tape claims to only use 12v to run, now if I wanted to do away with the led tapes power block and somehow wire a molex plug (powering from the computer power block) to the remote receiver for the tape which has a female dc plug input only would you have any ideas on how one might do that?
    Thank you in advance for any help you can give me.

    • 1. No, that’s not right. For an LED with a 30 mA limit, we suggested aiming for 25 mA forward current.
      2. A 1/4 W resistor is, by definition, one rated to handle up to 1/4 W of power. You need to use a resistor of the right resistance for your application, that is also rated for AT LEAST as much power as will be generated. There is not one or the other that is “more important.”
      3. You can always use a resistor with HIGHER resistance than the value that you calculate. However, the higher it is above the minimum, the dimmer the LED will run.
      4. See answer 3.
      5. Not sure; check with the manufacturer.

      • Thank you very much for reply. I was questioning though for a little clerification my leds as stated above seams to have a continuos current of 20ma but a peak of 50ma so what would you advise i should aim for?

        As for the 1/4 w vs ohms question, I was just curious, do you know why labels state that a resistor is a 1/4w resistors while also at the sametime on the same label stating different ohm ratings?
        Thank you again for any help.

  16. is there anyway someone could post a actual picture of multiple LEDs connected with the resistors and stuff, this is my 1st electronics project and i wanna make sure i get it right, I do best with visuals lol

  17. I’m sorry to ask this again but I am currently doing a project and I’m stuck with this problem. I’m trying to figure out what resistors I will need for my led setup and I’m trying to do the calculations. The question I have is I have led’s that have a forward voltage of 20ma and a peak of 50ma would the typical current value to aim for be 35ma? If anyone could help me I would really appreciate it. Thank you in advance for any info you can give me.

  18. I am a total newb. Here is my problem. I have a 20w LED, 32V, 600mA. Is it possible to power this LED directly from a 32v transformer, 1000mA (AC to DC) without any resistor? Would be any damage to LED? Is it better to use a 32v transformer with only 400mA?
    Any help will be highly appreciated.

    • If you’re new at this, starting with expensive, difficult to control, high power devices is probably *not* the best place to begin. Start with the little guys and learn what you’re doing before you try to tackle that monster!

      • thank you for taking your time to offer a reply but advices like this have the same value to me as some advices about a healthy lifestyle. Not relevant. I have asked a clear question and I I need a clear answer or nothing. thank you though.

  19. What if I tell you that I am not a newb anymore. I have read a lot in the mean time, I have tested my rig and I have done lots of measurements. Are you going to give me a different answer. Or maybe you have no answer but you have plenty of time on your hands….

    • This article is about getting started with low power LEDs, so it’s not a good spot to ask for specific help on a different topic. You could post a message in our “ask an Evil Mad Scientist” forum, but your best bet may be going straight to the manufacturer with your question. The datasheet should also include information on how to drive them.

  20. Want to run one LED off a 1.5 volt power supply?Trying to find a coil(small) or a iC chip to increase the power out put to around 5 volts to run the LED.I’m trying to put all of this in a model train.Can you help? thank you Kirt

  21. Hey this was really helpful. I have a question though. I need to run about 25 LEDs in a parallel circuit. Do you think that a 6 V power supple will be enough?

    • For a parallel circuit, you just need to determine if 6 V is enough to supply one LED (hint: it usually is). But, you also need to determine if your power supply has enough current-sourcing capability to drive all 25 LEDs. For example, if each LED is run at 1 mA, then the power supply needs to be able to source 25 mA.

      • ok. I am using 2 AA battery packs to run it. How do i find out if my power supple is able to source 25 mA?

        • You need to know what current your LEDs can take.

          If they are the normal small plastic-encapsulated LEDs then about 20mA is typical. However, I have obviously never even seen your LEDs so that is only a guess. If you have a datasheet or similar, consult that.

          AA batteries are generally rated at a constant current of 500mA. That is conveniently 25 x 20mA so 25 normal small LEDs should run off a AA battery pack.

          You need to find a datasheet for your LEDs or measure their forward voltage so that you can work out what resistance you need to avoid burning them out. This article then tells you what you need to know to calculate that.

          If you can’t find the forward voltage then you could start with a 330 ohm resistor and one LED and measure the current drawn from new batteries. Check that it’s less than 20mA (at 6V it should be). It it’s too low and you want brighter LEDs then gradually drop the resistance until you near 20mA. Then use the value you select for each LED.

  22. Ok i know this Isn’t really the same topic to what you have here but i figured maybe you can help me. I have car audio and i want to hook up LEDs around my subwoofers and I’d like to run it off my amp so they phase with the music, what i was wondering is if it can be ran out of the speaker power in watts? (i have no clue when it comes to this stuff) or if it has to be volts? if i hooked up resistors would it change the ohm load per channel? because that would change how much power my amp puts out or would this not be possible?

    • You’re right, this is off-topic. I recommend you use the “ask an evil mad scientist” section of our forums. Or your favorite automotive forum.

  23. Okay, I don’t really understand this. Lmao. But I’m trying to wire 4 lights to a truck. 2 in the front (regular) 2 in the back (red). And I did the front the other day with 2 random resistors and when we put in a new 9v it fried the lights. I don’t want that happening this time. So I tried looking it up and saw this site. Could you help me? I want to make this proffesional. I don’t know what resistors i would need to keep it powered. Also do I use a resistor for each light? All four lights are going to be on the same battery. Thanks for anything you can help me with!

    • For automotive applications, I would strongly recommend that you talk with your local mechanic. Random resistors are definitely NOT the right way to go.

      • It’s for an rc car. Sorry, should have mentioned that. Lol. I’m just wondering what I would need to wire 4 lights. The light bulbs are
        1.8V
        20mA
        120mcd
        I want to have 2 light bulbs in the front and then it traveling through the body of the car to the back. On the same battery. Is there a certain way I need to do it? Or is there a certain amount of resistors I need?
        I have a 9v battery. And 4 leds pushing 1.8v.
        9 – 7.2= 1.8 / 20 = 0.09 so how many resistors would I need? And they’re 90ohm. Correct?

        • Yep. For four 1.8 V LEDs in series running off of a 9 V battery you need a 90 ohm resistor to maintain 20 mA of current. But that assumes you have all four LEDs wired in series with the resistor, which sounds different from what you tried the first time.

          Also, you mentioned that the two in front are “regular” but the two in back are red. I don’t know what regular means in this context, but if they are white, they are not likely to be 1.8 V.

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