red, yellow and green LEDS need about 2.2Volts and 10-20 ma to light up. White ones need about 3.5 volts and 10-20 ma to light up, blue ones something inbetween in voltage but the ma remains about the same.
So if you have a 12 volt toy power pack, take away 2 volts for a red, yellow or green Led and 10 volts must be dropped by the resistor, This is where the 1K resistor does it’s work. V=IR or in this case I=V/R means I (current) = 10 Volt/1000ohms i.e. 0.01Amps or 10 ma. The rheostats in el cheapo power packs are not much more than a 100ohms so changing the rheostat won’t make much difference if you only have one LED. However put 20 LEDS with their own resistor each in parallel and you have 400 ma which, using the same formulae V=IR ie voltage across rheostat of (max) 100ohms X 400ma 10 Volts. It’s not quite that simple but it would work.
The thing to check is that the el cheapo transformer will supply 400ma. I’ve used some mobile (cell) phone chargers for powering LEDs and they work provided you limit the current to their rated charge rate and use resistors to limit the current through the LED to 20ma max.
Of course you can put 2 or 3 LEDS in series, but you still need a resistor although of a lower value. just add the voltage drops accross the LEDS then take the answer away from the supply voltage to determine what resistor value to use. A bonus is that this wastes less power than having them all in parallel.
I’m planning on using mainly yellow and orange LEDS to imitate tungsten bulbs as the white ones are too white and often have a blue tinge. i will use some blue and white ones for special effects, I’m thinking of a flickering white one in the garage to simulate a welder, white one in the store windows, blue ones in the bedrooms.
if your el cheapo power packs do not have a ma rating on them, you
Basic rule of LEDs: Too much current and the magic smoke will escape.
You have to limit the current with a series resistor. If the LED is 2V at 20mA, and the power supply is 12 volts, you will have to lose 10 volts across the series resistor. For that you need 500 ohms in series. (R=V/I)
You can use a variable resistor to alter the brightness, but there are limits to how well that works. Whatever you do, make sure that you won’t burn out the LED at full power.
As to wiring mulitiple LEDs to a PS, as long as you don’t exceed the capacity of the PS to supply current, there shouldn’t be any big problems. Just wire a number of LEDs (with their own series resistor) in parallel.
For example, you could use up to ten 20mA LEDs with a 200mA power supply. Each leg will get 10mA. If you exceed that, your power supply will probably get hot and release it’s magic smoke at some point.
I am making the assumption that the ‘toy power packs’ are for model railroad sets, aka a Like-Like cheapo.
If you use the Variable DC output of the power pack then you need to understand the behavior of an LED. An LED is first and formost a Diode. That being said, an LED will not turn on until it has what is called its Forward Biasing Voltage applied. Aka, if a diode is rated to start conducting at 0.75Vdc, then if you apply 0.5Vdc to the diode, it wont conduct. Once you apply >0.75Vdc then the diode will drop 0.75Vdc constantly. The problem is that the diode, when conducting, has very little resistance. Using ohms law, as you continue to raise the voltage then you will push more and more current through the diode till it fails. Same will be said with LED’s
Calculate for the highest voltage that will be applied to the LED, and if you are using the VariableDC output of a power pack, you had best take a multimeter to it. 90% of 12Vdc powerpacks actually put out 13.6-14.1Vdc at no load. When you load the power pack to 100%, THEN voltage is at 12Vdc.
But for arguements sake, lets say that the highest voltage your LED will see is 14.1Vdc under normal conditions (aka no lighting surges etc). Lets also say we are using part# L03YD from this page. Our LED is rated for 20mA and we can assume the LED will drop 2.25Vdc yeilding us 11.85Vdc left to deal with. As stated above by others, use Ohms law to figure out what resistance we need to limit the current pushed by the remainin 11.85Vdc to 20mA. Calculated out it comes to 592.5 Ohms. 500 Ohms was stated above based on a true 12Vdc circuit (not the 14.1 we are using), so this calculation follows and validates his statement.
SO, we need an (about) 600 Ohm resistor inline with each of our L03YD LED’s in order to keep
Bottom line is, a rheostat pack isn’t going to do diddly with the LEDs. It’s like using an HO rheostat pack on N scale locos - or modern high quality HO models liek my Stewart Baldwin. There’s no control, the loco just leaps into action. The high efficiency motor draws so little current that almost no voltage is dropped by the rheostat.
For LEDs I strongly recomend something that is at least a little regulated. Voltage all over the place will lead to LEDs being damaged. A 12v 1 amp regulated power supply can run 100 LEDs if the reisstors used set their current to 10ma. Or if you have an old computer power supply - between the 5V and 12V outputs you can drive thousands of LEDs.
Now for the resistors. I have had calculus and calculus-physics. But for some reason this simple formula to calculate the what I need for resistance just goes in one ear and out the other. What I’ve ended up doing is just asking people to tell me what I need. I guess it’s time to bite the bullet and work things through.
Obviously, the white and colored LED’s have different needs.
Obviously, I’ll need something different for the 5V and the 12v leads.
Let’s start there. I guess I’m going to reread this stuff and try to learn bone-head electronics.
Just to clarify, Model railroad power packs don’t use rheostats or pots to vary the voltage directly. The throttle you control is one of two possible things:
A pot or rheo that sends a reference signal to a small amplifier
An autotransformer that varies the AC voltage that is fed in to a Rectification stage.
Either way, its true Voltage control, not current control. Works great for variable light control. Regular bulbs will ride the whole voltage range but LED’s are more limited (but still change in brilliance).
This is the second time someone has mentioned their ‘HO’ power pack not working with their ‘N’ locos. Perhaps there is a very very low quality power pack that does this, but I have run O, HO and N all off the same Tech II without ever having issues. Randy, are you referencing a specific power pack? I am very curious now.
Spacemouse, just get the specs on the LEDs you want to use (aka Voltage and Max Amperage) and describe the application you wish to use them for and I (and lots others too) would be more than willing to help ya out. Including a step by step explanation if you are so inclined.
Lastly, I did a short write up for Basic Electricity if you are interested. Linkage is in my sig. I need more people to give me feed back as to whether its useful as is or if it needs to be editied.
It’s really pretty simple and straightforward. Literally. Forward voltage ont he LED is fixed. It will be stated on the package, or in the specs if you buy bulk from somewhere. That’s not changeable. The LED will drop that much voltage, as long as it’s in working condition. Current for the LED is variable, from some lower threshold that is the minium required to illuminate the LED up to a maximum it can handle. That maximum is what is stated int he specs. Do not exceed that or LED go poof. Mayeb not instantly, if you, say, runt he LED on 40ma when the rating is 30ma. But let the LED have 1 amp whent he rating os 30ma, and it WILL go poof pretty much instantly.
Next, you have your power supply voltage. If using a computer power supply, you have a 12V and a 5V supply (ignoring the -12, and 3.3v if it’s a newer power supply). Basic electronics: 2 loads in series will draw the same current. IE, if 10ma is flowign throw the LED, then the resistor in series with that LED will also see 10ma. Basic electronics #2: two elements in series add their voltages. So if that LED has a 2.1v rating, and you are using a 12v power supply, the voltage across the resistor in series has to be 9.9v. 2.1v from the LED plus 9.9v through the resistor = the 12v power supply.
Third, Ohm’s Law. Ohm’s Law states that E=IR: E (electromotive force - Volts is the commin unit of measure here) = I (current, in amps) times R(resistance, in Ohms). Fill in what we know to calculate the resistor: 9.9v = .010 amps x R. Simple algebra, solve for R, R= 9.9/.010, or 990 ohms. Since they don’t make resistors in every possible numeric value, you pick the closest standard value, in this case 1000 ohms, or 1K.
FOr the 5v supply, it’s the same thing. LED drop of 2.1 volts, leaves 2.9 volts through the resistor. 2.9 volts / .010 amps = 290 ohms
For an LED with a 3v drop instead of 2.1 (the golden white LEDs I use in locos usually are rated at 3v), and a 12v power supply: Vol
Your #1 method is true for more expensive power packs such as the MRC Tech series. And the older (pre-1985, a few still do) toy train AC transformers did use a variable voltage wiper on the transformer coils (but no rectification).
BUT the traditional power packs, and just about all power packs that come with train sets DO use a simple rheostat for train control. HO power packs are/were generally about 50-70 ohms max, and N power packs are/were 70-100 ohms max. As you correctly note, they are very poor voltage control devices, especially given the disparity of current draws of today’s model locomotives. But back in the days of selenium rectifiers, Pittman motors or very similar were almost universal, and were fairly consistent in their power draw at around 0.4 - 0.6 amps, so the scheme was usable. And as Randy correctly points out, the rheostat in a train set power pack will do almost nothing to vary the current or intensity of an LED.
Hrm, I suppose then I have just been lucky as I have only ever had the two types I described. Even my el cheap Life Like one was an autotransformer/rectifier setup. If it is as you two say then the crazy behavior between HO and N makes sense. Also the current throttling capability of a pot for LED application definetly sucks. I am shocked that they would make sucha shoddy product.
Good to know, thanks guys and lets hope my luck continues.
All those great powerpack of the past, like the MRC GOlden Throttlepack, used the rheostat directly on the low-voltage DC side. Prior to the introduction of the Tech II series, there was a Golden Throttlepack 550N for N scale which had a different value rheostat to be able to control N scale locos.
I still have several older units around, a Scintilla Dualpack from who knows when - late 50’s or early 60’s I’d guess, defintely pro-1966 as the company address is given with a zone number not a zip code. It has rheostats in on the DC side of the selenium plate rectifier. A slightly less old MRC dual pack is the same way. The old MRC pack I have with the lever for a speed control is the same way. And ads in the 50’s and 60’s issues of MR I have are FULL of ads for rheostats, botht he Marnold ones with the lever and built-in reversing, and the Ohmite ones that seemed to be the ‘Cadillac’ of controls. Two different values were offered in many makes - but in this case for HO or O scale. The O scale ones would not have good control over a typical HO scale loco.
In fact, the only transformers I’ve seen that work like an autotransformer are the old Lionel ones I have. That WAS a common way for high-end modelers to power their layouts back in the day, using a Variac since this method gives superior control as compared to a rheostat - the voltage drop is not load dependent so you could achieve much smoother starts and stops, and better speed regulation using a Variac power supply.
Well you certainly are talking about gear that is about 20 years older than I am lol. I have 3 power packs back at my mothers house in AZ (Somewhere in the cellar) that are the AC->AutoTransformer->Rectifier type. I will see if I can get her to dig them up and give me the brand names.