Power supply and LED voltage
Moderators: Sparky, Moderators
Power supply and LED voltage
Probably an already answered question but I'm lighting a model with about 20 5mm led's hooked up to F.O's. What voltage or spec LED's do I need if I just want to plug the model when finished into a 12v regulated wallwart without any circuit boards or voltage controllers, or is that even possible. Electrical isn't my thing and just want to keep it as simple. Thanks, Ken
He's asking us to provide that...Chacal wrote:You gotta know (and tell us) the forward current of the LEDs.
kenny: imple question: what color LED are you going to use? That's an easy way to get a feel for the sort of power characteristics the LED is likely to have.
Assuming it's white, that's usually a 3V drop (approximately) with up to 20mA running through it. You could hook four of 'em together in series to drop 12V. Then you could take five such series-chains, hook them up in parallel to still drop 12V, but bring the total number of LEDs up to twenty. But each one of those individual chains has 20mA running through it, so your power supply has to be able to output at least 100mA.
LEDs are very sensitive to voltage changes: a small increase in voltage means a big increase in power, it doesn't take much to burn out an LED. So if you use that kind of circuit you need to be very sure you never drive it at higher than 12V. You could make the circuit a bit more stable over different voltages by adding resistors to drop some of the voltage - I generally recommend this as it gives you at least a little safety margin in case you hook up the wrong voltage... But any power dissipated by a resistor is converted to heat and wasted, so it's good to keep from using too many as well...
A few fundamental bits of info on electricity that you should take the time to understand:
Voltage is a measure of electrical "potential". It's like altitude, you always have to measure it relative to something. Usually in circuits there's a reference point called "ground" - usually for single-voltage power sources this corresponds to the negative terminal of the source. If you think about the voltage of a point in the circuit, this is relative to "ground" - but you can also measure the difference between two points in the circuit: if you were measuring across an LED that's operating, you'd see a drop (usually between 1.5-3.0V) in potential that represents the work that LED is doing. If you add together all the voltage drops in a loop, the sum is always zero.
Current is the rate at which charge is moving through your circuit. The thing to remember about current is that it always has to go somewhere - the amount of current going into any point on the circuit always equals the current going out. Therefore, if a circuit is a simple loop, the current through it will be the same anywhere on the loop. If the circuit branches, then the current into that point will equal the sum of the current on the two branches.
---GEC (三面図流の初段)
There are no rats.
The skulls eat them.
There are no rats.
The skulls eat them.
Thanks tetsujin. So will using a regulated 12v max wallwart be suitable? They are all white LED's with just a momentary on switch. And resistors basically drop the voltage being supplied to the LED? And if I have the four LED's wired to drop the 12v, what would the resistor need to be rated at? Thanks again for the description.
The LED series/parallel array wizard should give you the resistors and wiring diagram that you need. It looks like you'll need two 270 ohm resistors for the four LEDs... but you're dissipating 480mW -almost half a watt of heat.
That helps, but heat will be a problem. If I use a different voltage wallwart and rearange the led's in series can I lower the heat output by using less resistors?macfrank wrote:The LED series/parallel array wizard should give you the resistors and wiring diagram that you need. It looks like you'll need two 270 ohm resistors for the four LEDs... but you're dissipating 480mW -almost half a watt of heat.
That's correct. There are advantages to matching your load to your voltage source - wasted energy is converted to heat, so if you reduce waste, you reduce heat. But waste can have advantages as well - I elaborate on this below.kenny77 wrote:If I use a different voltage wallwart and rearange the led's in series can I lower the heat output by using less resistors?
Yes. Unregulated would not be suitable, however. An unregulated 12V power supply will probably deliver more than 12V.kenny77 wrote:Thanks tetsujin. So will using a regulated 12v max wallwart be suitable? They are all white LED's with just a momentary on switch. And resistors basically drop the voltage being supplied to the LED?
Basically, power supplies are imperfect: if you measure the voltage across one when there's nothing attached to it, you'll get some voltage higher than its supposed output. If you start drawing a whole bunch of current from it, however, the voltage will drop... Kind of like bleeding air from a tank while a compressor is simultaneously re-filling it. The pressure drops when you bleed air from it.
An "unregulated 12V power supply" generally means one of a few different things. Either the power supply will provide 12V when you're drawing a certain load from it, or else it will provide more than 12V, enough for something with a 12V regulator to operate.
A regulated power supply is a power supply that has a voltage regulator built in. What this means is that it will use various techniques to keep the supply voltage at exactly 12V no matter how much (or little) current is being drawn. A regulator adds to the cost of the power supply, and if you don't need your power supply regulated then the regulator is wasting a certain amount of your power for no good reason...
If the four LEDs drop 12V, you don't need a resistor. If you want a resistor, then you don't want the LEDs to drop 12V...And if I have the four LED's wired to drop the 12v, what would the resistor need to be rated at? Thanks again for the description.
Picture a circuit loop: battery, LED, LED, LED, LED, back to battery. Also suppose the four LEDs are identical, and the battery is perfect and delivers exactly 12V.
Now, there's two fundamental laws that are going to determine how the circuit behaves: these are called Kirchoff's Current Law and Kirchoff's Voltage Law. The current law just says that all the current going into a point has to equal all the current going out of a point - in the case of a simple loop-circuit like this, it also means that the current through any device on the loop will be the same. The voltage law says that if you measure the voltage drops on all the devices on the loop (treating the battery as a negative "drop") they will add to zero. This means that the four LEDs in this circuit will drop 12V.
Now, the four LEDs are identical, and they have a curve that defines the relationship between the voltage across them and the current through them. Most of the time I don't worry about the whole curve, I just look for a single, safe operating point. For white LEDs it's usually about 3V at 20mA. So from that you can see the solution: each LED will drop 3V, and have 20mA going through it, and the four together will drop the 12V from the battery. The LEDs won't draw more than 20mA, because that would make their voltage go above 12V - so as long as your source is a stable 12V, you're safe.
Now, if you were suddenly driving current through the circuit with 15V, that would be (for four identical LEDs) 3.75V per LED - that would correspond to another point on the voltage/current curve for the LED - possibly something over 100mA, or five times the safe maximum for the device... This is where a resistor comes in. The useful characteristic of a resistor is that its voltage/current relationship is linear. You double the voltage across it, and the current through it will double - not blow up exponentially like a LED. But to take advantage of this, you need a fair amount of voltage drop across the resistor.
If you were driving three LEDs in series on that 12V, the three LEDs would have to drop only about 9V to stay in their safe operating range, so what you want is a resistor that will drop the last three volts while passing the amount of current you want to go through your LEDs (20mA) - 3V/20mA = 150 Ohms (That's Ohm's law, V=IR, or V/I=R or other equivalent expressions... Basically, it's a linear relationship - if the amount of current stays the same and the resistance to current doubles, then it takes twice as much voltage to push that current through.)
Now, suppose you drove that circuit - three white LEDs and a 120 Ohm resistor, on 15V. How do you know what it'll do? Basically you'd need to find a value for the current through the circuit at which the Voltage law is satisfied. If the current went up to 30mA, the drop across the resistor would go up to 4.5V, but the voltage across each LED would probably only go up by about a tenth of a volt. Probably the current through the circuit would go up to nearly 40mA - still outside the safe range of the LEDs, probably, but not as bad as 100mA...
Using a regulated source matched to your LEDs is safe. If you wanted the circuit to be protected against mistakes like accidentally connecting too much voltage to it, you'd want to have a regulator integrated into your circuit instead of into the power supply... You can use a voltage regulator (and, if necessary, resistors) - but a somewhat better option for LEDs is a current regulator... A current regulator adjusts its output voltage to try to maintain a constant level of output current. This makes them very easy to use with LEDs - just hook a few LEDs in series on a current driver and you're in business. If you want to try a current driver, there's a Madman Lighting board you might want to look into...
I think it's worth being a little bit cautions, to guard against that kind of mishap - after all, lights are usually sealed inside a model without a good way to get at 'em... It all depends on how confident you are that you won't connect the wrong power supply in the future. XD
---GEC (三面図流の初段)
There are no rats.
The skulls eat them.
There are no rats.
The skulls eat them.
With 4 white LEDs at 3.3V and 20ma each, your absolute minimum is going to be slightly higher than 1/4 watt (264mW) - either driving all four from a 3.3V power supply in parallel or all for from a 12V regulated powr supply in series.kenny77 wrote: That helps, but heat will be a problem. If I use a different voltage wallwart and rearange the led's in series can I lower the heat output by using less resistors?
Also, white LEDs, unlike single color LEDs will get dimmer with time - the white light comes from a phosphor coating that is excited by a UV LED underneath. That coating will degrade with time, depending on how long you drive the LEDs at full power. In most model applications, this isn't too significant, but if you leave it on for hours each night, it will be.
