For you electronic types - resistor values?

[BLKMGK]

Been awhile since I've done the RoyGBiv stuff and my memories of electronics are pretty dim so I've got to humble myself and ask - If I've got an LED that can accept a max input of 3.7 and a supply voltage of say 14volts - what size resistor must I use? The same goes for 2.1volts.

 

I visited Radio Shack today looking for LEDs for my turn signals, brake light, and high beams. Naturally they were out of stock on quite a few things but I was able to pickup some gree, some red, and surprise surprise a BLUE LED! The blue one has a much higher output than the others so hitting the brights shoudl be interesting. It MIGHT even be suitable for gauge illumination - we'll see. Right now I've got to get a resistor in line before I can try it. I bought a jumo assorted pack of carbon film 1/2watt resistors that ought to do the trick. Mind you I've forgotten how to read the color bands on the resistors and which leg is anode and if it should be ground or not. (ahem) 13 years of not having to use this knowledge has created some serious rust. Help on this would be appreciated and I'll be sure to post results for others!

 

Now then - mount them on top of the dash, on top of the steering cover, or pierce the crappy cracked dashpad near the gauge mounts to insert them from the side? I'm leaning towards the steering cover for ease of access. I've figured out which is the gauge lighting (redblue stripe) and which is brakehigh beam - hint, the highbeam harness won't reach the brake hole on the gauge. Now I've just got to puzzle out left and right blinker lights (lol). 5050 chance I'll get it right unless some wiring diagram I've got calls it out...

 

P.S. Mini shift light is going where the VIN tag is - no holes needed.

[Guest kc6wfs]

Here is the color code.

I think the best way to drop the voltage would be get a step down transformer.

RS has them cheep.

The resistor would get nice and hot all the time disapating that much voltage.

 

Dave

 

 

Resistor Color Codes

First 3 bands

Black

0

Brown

1

Red

2

Orange

3

Yellow

4

Green

5

Blue

6

Violet

7

Gray

8

White

9

Gold

*.1

Silver

*.01

Fourth Band

Gold

5%

Silver

10%

[pparaska]

I suggest you read:

http://webhome.idirect.com/~jadams/electronics/faq.htm#volt_drop

 

You need to know what current it draws, then us ohms law to find the resistance you need.

 

It can be done, but voltage regulator chips are cheap and easy to use. BTW, Radio Shack sells LEDs with dropping resistors embedded.

 

The color codes, etc. are explained here:

http://webhome.idirect.com/~jadams/electronics/resistor_codes.htm

 

Or you could try to remember this sentence and what it really stands for to give you the color codes .

Big Brown Rabbits Often Yield Great Big Vocal Groans When Gingerly Slapped

 

[Guest Anonymous]

Shhhh, we have to be verwy verwy quiet, weir hunting wabits...

 

 

Lone

[Andrew Bayley]

OK Pete... I've never heard that one before. Even now, I'm still learning something every day.

 

Jim, what size resistors did you get? The quickest, easiest and cheapest way to do this would be to use a simple load resistor for each LED, then run all the LED's in parallel. Voltage regulators can do the trick, but there is more involved. Besides, voltage supply in a car is fairly consistent and LED's are very tolerant of voltage fluctuations.

 

For current calculations, assume the vehicle is running at 13.6 volts (typical GM alternator output). Next, subtract the 0.7 voltage loss across a silicon LED (only 0.4 volt for germanium). With 12.9 volts remaining, divide this number by a common sized resistor. Using 1K ohms, I came up with a current draw of about 13mA. This is a little low for most LED's. Most of them like to run around 25mA. In order to run 25mA, a 500 ohm resistor would be needed.

 

Of course, my calculations are given on a very old knowledge of LED's. Some of the newer units can probably tolerate much more current draw. As for power dissipation, 25mA will only draw about 1/3 of a watt. Getting close to the ½ watt limit of those resistors, but nothing I'd be too concerned about. I've seen many automotive products get released drawing many many many watts through ½ watt resistors. I wont say who, but it's a lot more common than you think.

 

Anyway, good luck

 

-Andy

[BLKMGK]

Color code is actually on the back of the package, what I need is the value of the resistor to drop the voltage. I realize that they sell assemblies wih this but my Rat Shack only had ONE green and ONE red assembly - no blue etc. I bought bare LEDs inending to add the dropping resistors myself. Current draw is also on the package so I'll check that ou and refresh myself on Ohm's law.

 

I do realize that the resistor will get hot - however remember what these are being used for. Blinkers, High Beam indicator, and Brake light indicator. None of these should ever be on for extended periods of time except maybe the brake. If I were to use a blue LED for illumination then certainly a dropping resistor might not be a great idea. I'm going to see how the blue colored bulbs work first - I might hate it

 

I do have one slight other concern. Blinkers function based on resistance in the circuit heating up an element. Replacing tail lights with LEDs can cause the blinkers ot NOT function - I'm hoping hat doing the same to the blinker indicator bulb doesn't foul things up. I'm pretty sure it won't since a burnt out bulb would kill the signals but... Guess we'll see!

 

Now, which is ground, Anode or the other? Looking at these LEDs it's not even real clear which is which! Both legs are nearly the same length, there are no markings that I can see, and the package mentions nothing about polarity. Grr!

[pparaska]

Andy - I got that cute little sentence off one of those pages I referenced - I'd never heard it either. I used to know the color code by heart (I was a little electronics geek as a kid).

 

Oh yeah, please email me who designs auto electronics with HOT resistor design - love to hear about that!

 

Jim, To figure out the anode/ cathode thing, use and ohm meter. If it's a FLUKE or something, it will have a diode test function. Anyway, you could put the required voltage across the diode both ways to snoop out the anode, it won't hurt it.

 

Oh, the LED for auto/motorcycle places on the web (don't have any URLs handy, but search the LED tail light threads here) have special blinker modules that don't depend on current draw for the flasher to work for just this reason (LEDs).

[BLKMGK]

I knew about the flasher module but that's yet another hassle I hoped to avoid. Had forgotten about the Diode test on most meters, naturally mine's gone missing - I've needed it for several things now and it's been a no-show. Grr! Perhaps it's time for yet another one from Radio Shack, I lose them too often to buy a FLuke. Diodes are supposed to be marked for Anode, either with a shorter leg, a flat on the case, or some other mark. I've not pulled these from the package yet but so far I see nothing to mark them. Not even a blob of metal on one leg or the other. Oh well, for the price I won't whine if it works.

 

It did really irk me when they admitted to not having LED holders in stock. I don't mind using bare LEDs but it makes little sense to sell LEDs, have holders in the catalog, and not stock them. Sheesh! Their stock on LEDs that had mounts and resistors was pathetic too. Having Blue LEDs made up for most of that though, I was shocked to see them. In several brightness values too!

 

P.S. Thanks Andrew,that helps! I was actually going to figure on 14volts though just in case!

 

[ May 22, 2001: Message edited by: BLKMGK ]

[BLKMGK]

Hrm, try this on for size... Speaking to a friend who fiddles with this stuff constantly I got a slightly different answer..

 

Take input voltage, say 14volts, subtract desired voltage of device (3.7 blue 2.1 other), then divide with Ohm's law to get power draw. That sound right?

 

For the blue LED this is actually pretty straightforward and you were very close Andrew. 3.7 volts device, draws 20ma, I'm figuring a 680 Ohm resistor will be good to go. A little low probably but it puts out 2600mcd - quite bright! I was told to put a megaohm resistor across the legs too as blue LEDs are supposed to be static sensitive. Oh, Math on this (rounded) comes to about "0.015" which I think would be 15ma?!

 

Here's the cute one - the other LEDs take 2.1volts which is lower but the amperage is only 2ma! Output is a tiny 5.5mcd (!) so these won't be blindingly bright. (ahem) IF I'm doing the math right I think a 4.7K resistor will yield "0.0025" for the other LEDs.

 

My head hurts. Serves me right for not keeping this stuff fresh in my memory. I've nto breadboarded or soldered together anything more complicated than two pieces of wire in years. Shessh! I'm going out to the garage tonight to look for suitable mounting spots for these, the column cover being high on the list (I've got a spare). Would appreciate a double check on my figures before I actually install.

 

 

Update: It's done. Not fully wired into the car yet but wired into the column cover with leads. A few bullet connectors and I should be ready to roll. Tested with a 9volt battery, lights were fine but a touch dim - this is good. Blue LED is BRIGHT I'll take some pics when I've got a moment and post them. I think this is a decent solution for the indicator problem...

 

[ May 22, 2001: Message edited by: BLKMGK ]

[Guest MM_280Z]

Only a handful of ideas will get you through all your problems here:

 

1. Ohm's Law: V = I * R for a resistor.

2. The sum of the voltages around any circuit (current loop) is zero.

3. LEDs (diodes in general) clamp the voltage across them (3.7 V for your blue LEDs). If you try to drive them harder the voltage across them will not rise much but the current through them will skyrocket (until you blow it out).

4. LEDs conduct current in one direction only (unlike light bulbs). If you connect them backwards they will not work.

 

 

#2 is why your friend said to subtract the voltages and divide by your desired LED current to get the resistor value.

 

Re-arrange Ohm's law R = V/I = (14.4 - 3.7)/0.020 = 535 Ohms. Pick a resistor value close to that and you'll be fine.

 

14.4 - 3.7 = Voltage the resistor must drop. 14.4 is usually used as the nominal voltage for car electronics.

I = current through resistor = current through diode because you wire them in series.

 

Also a good idea to check whether you are exceeding your resistor power rating:

 

Power = V * I = (14.4 - 3.7) * 0.020 = 0.214 Watts, which is less than your 1/2 watt rating on you resistors so you are OK. (could even get away with 1/4 watt resistors).

 

Rinse and repeat for your other LEDs.

 

BTW, you mentioned using LEDs for taillights. My friend bought some LED clusters from from JCWhitney. They have all necessary electronics inside a sealed unit, and even have a brighter/darker effect for stop/taillighting. They plug right into his Altima's wiring harness.

 

I just realized I never properly introduced myself. I am an Electrical Engineering senior at UCSD

 

Good luck with your project. Let us know how it goes... I was considering using blue LEDs for guage illumination myself!

 

 

Update:

Ah, I should have read all the posts before replying. Here's some more stuff:

 

Anode is positive side (+), Cathode is negative side (-). The schematic symbol for a diode is an arrow pointing from anode to cathode (the direction of current flow).

 

Remember that if you make a cluster of LEDs, wire the LEDs in parallel (all anodes together, all cathodes together). You can use a single resistor in series with this cluster, though it must be beefier since it is now carrying more current. I.e., is you have a 5 LED cluster at 20 mA per LED, then you have 100 mA through the single inline resistor, and must calculate it's value accordingly with 100 mA in your Ohm's law calculation. Definitely check the power on the resistor though! You can distribute the power across several resistors to stay within the power rating.

 

Resistor values in series simply add, resistors in parallel combine according to

 

R_total = 1/(1/R1 + 1/R2 + 1/R3 + ...)

 

 

Oh yea, my friend's Altima also flashes the blinkers really fast. Turns out this is an error code; the computer is saying a bulb is burned out! Not sure what'll happen on the Z. Assuming the flasher is expecting a particular current draw, you can "fool" by placing a (very beefy) resistor across the terminals bulb. Not exactly the best practice but it may do the trick. If you can, get that special flasher. I did the calculations and a 20 Ohm resistor should about mimic a 10 Watt bulb. Measure the current at the bulb (get a Fluke! ) to get a better estimate. Use the power equation:

 

P = V*I = R * I^2

 

Note that incanescent bulbs are also quite an inductive load (not just resistive) so I'm not sure how well this would work. It may also drop the voltage on the line enough that you LEDs will run dim as well.

 

Here is a site which lists places to find the brightest LEDs of all colors: http://www.misty.com/people/don/led.html

 

[ May 25, 2001: Message edited by: MM_280Z ]

[Oltmann]

If this site hasn't been posted before.

 

For the Love of LEDs

 

You should be able to get your fill of photons.

[BLKMGK]

Good stuff guys, thanks!

 

However - this is for indicator lights not tailights Not sure if I'll have the fast flash, I'm betting not. This blue LED is REAL bright so if you consider them for lighting check out RatShack first. Me, I colored my bulbs with a marker - we'll see how that works out.

 

I knew about anodecathode (I was trained as an electronics tech 13 years ago but haven't used it in over 10) however these LEDs were NOT mrked. One lead was a HAIR shorter than the other - no other indications! So, I spoke to a friend who works with this stuff often - he told me that the NEG side oculd be figured out by looking inside the LED. Bigger side is grounded, so far it seems he was right

 

Lastly, for anyone following in my footsteps - per Pete - the brake and high beam indicators are WEIRD! They have two leads and cannot be wired to a common ground. You either have to use a set of relays or wire them differently than you do the turn signal indicators. Yuck! Had I known this BEFORE I got this far and soldered everything to as common ground I'd have been MUCH better off! Don't make my mistake please. Thanks goto Pete who noticed this and explained to me why it is.

 

Hope to have my stuff wired by the end of this long holiday. I'll put upa WEB page on it when I've got time - it's NOT hard to do if you want to keep these lights and run aftermarket gauges.