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converting a light bar

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dcsbass

New Member
I'm a cop. I have a old directional bar. It used old 12V bulbs and a controller to show, which direction for cars to go. From the rear of cruiser! I have purchased LED's to replace the bulbs. Also a new control head for directions. The LED's are 20mA, 3.5 to 4.0 v max. Bought at RadioS 10mm White 276-0005, for info.The old light bar has 8 bulbs and was 12VDC. I need to reduce 12VDC to 3.5VDC ! It has only one power wire in 12VDC, and 8 wires out, to ground. This is a neg ground system controller, so is new one. New one is also strictly for LED's, power is rated for LED's.
Power wire does this inside bar: --- Power, X plug, G to control head

___XG___XG__XG___XG__XG__XG___XG__XG__XG
----X----X----X----X----X----X----X----X----X

:confused: Not great but hope fully you get idea. Any help would be appreciated. :eek:
 

Mike_2545

Super Moderator
Some 175 ohm resistors (1 watt or higher) will effectively take 12 v to 3.5v at 20 ma. The resistors will get warm so go with BIG ones. One resistor for each LED.

There are several ways to do this. This suggestion is but one.

By the way I count 9 XG combination in your "drawing"
 
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Sceadwian

Banned
If you wire the LED's in series/parallel combination you don't have to drop so much through a resistor. Three LED's in series will need roughly a 150 ohm resistor but only 1/8th of a watt.
 

dcsbass

New Member
convert a light bar

mike2545: I just put them up there. Didn't count right.
Sceadwian: So you say run three, three, and what size for the last two.
I've never played with LED's before.
It has one hot wire that goes from plug to plug and so on. It is in series! Just cut and add new wire and resistiors? Is there a formula to work it out?

:eek:Thanks for Help:confused:
 
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Sceadwian

Banned
If you're using car voltage they would have to be done in three's.
Connect three in series with the 150ohm resistor and pretend that that whole 'unit' is a 12 volt device now. You'd have to do everything in three's though. Keep in mind diodes only work in one direction so if you don't hook them up right you wont' get anything out of it.
The anode is the longer of the leads on the LEDs and the cathodes is the shorter and usually has a flat spot on the package. So you hook them up anode to cathode. Doesn't matter which side you put the resistor on but it goes in series like the LED's. Cathode side goes to +, and Anode side goes to - (I might have that backwards I tend to mix up anode and cathode but if it doesn't work just reverse it)
 
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dcsbass

New Member
convert a light bar

Sceadwian : It is car voltage. So 150 ohm will do it ! That will work . Three wires, three solder joints ,and three resistors. that is a piece of cake. Any idea on cost of resistors? And could it or would it hurt to go a little bigger? On resistors? Package says 3.5 VDC to 4.0 VDC? It would be smaller on resistors actually Right? Instead correct?
:confused:Thanks Again:eek:
 

Sceadwian

Banned
Umm the exact resistor you should be using should really be calculated from the measured voltage drop of the three diodess at their rated current, but that might be a bit much for you to do if you're not comfortable with a multimeter and have a constant current source to use =) 150ohms is what I'd recommend generically and would try first. 1/4 150ohm resistors should be easy to get in bulk for almost nothing. 100ohm's might over current the LED's too much and 200 may make the LED's too dim, but you can try them if you want.
 
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Andy1845c

Active Member
Hi dcsbass - I made an LED traffic advosor a while back - Take a look here
http://www.electro-tech-online.com/threads/finished-traffic-advisor-project.33618/

Each light is 36 5mm LEDs running at ~50mA. Thats 288 LEDs total to make it that bright. I spendt many hours soldering them all.

I buildt my own controler and it regulates the voltage at 9 volts. This allowed me to use 4 leds in series with a resistor. The voltage is always 9 volts, weather the car is running or not. If I had not regulated the voltage, the voltage would swing from 12 volts or less when the engine is off, to over 14 volts when running. This really becomes a problem the more LEDs you have in series. If you do the math, you can make everything perfect at 12v, but redo the math for 14.8 volts and the LEDs will get too much current and burn out. Redo everything for 14.8 volts and then they will only glow very dim with the engine off. It really kind of sucks. There is no perfect answer.

Also, there are large voltage spikes in the electrical system of a car. These can be over 100 volts. Some here say not to worry about them with LED circuits, as they are don't last long, but it is something to think about.

Bottom line is it is alot easier to buy a LED bar then build one.

I have been working on version 2.0 of the one I buildt. But only because I still have a sack of amber LEDs and because I want something a little different then the standard size and mount. For it I am planning to set each light head up with a automotive rated 12v low drop out regulator. I have not exparamented with the idea yet to see if it will still work with the truck not running.
 

dcsbass

New Member
Hey Andy1845c, I like it ! It looks good. How bright is it out side at night? Just curious?

Hey Sceadwian, I got some 150ohms today at work. turns out our repair shop had some that haven't been used. A surplus of all kinds, and ranges. Mine are Brown-Green-Brown-Gold-Yellow. I'm can use a multimeter with no problem. have three. My best is a Fluke. Use to be a ham years ago. Had to build my first set. If that says any thing! I do have a question for you now. I found a guy, who has a formula posted about what resistor to use. It is the following:



--------------------------------------------------------------------------------

Here is the formula. You will need to know what drive current the leds will require(20mA is a safe bet usually).

R(Ohms)= (Vin-Vforward) / Current in Amps

for for an LED with a forward voltage of say 2.2 Volts and a drive current of 20mA running of 12 Volts it would be:

R= (12-2.2) / 0.02
R= 9.8 / 0.02
R= 490 Ohms

Also had this:
so for 5 20ma LEDs each with a forward voltage of 2.0 Volts it would be.

R= (Vin - V(total)forward) / Current
R= (12-(5*2.0)) / 0.02
R= (12-10) / 0.02
R= 2 / 0.02
R= 100 Ohms
He was explaining a LED problem to some guy on a car. The guy was wanting to put in LEDs in the third eye brake light, on a spoiler.

Using his first formula I came up with 425 ohms,for one. On his second one using the three. I came up with 75 ohms, for the three you said use. Am I missing something with his formula? Like I said curious. I am going with 150 ohm. I'm not doubting you what so ever. Like I said I've haven't worked with LED's before !!!!!
I just curious were he got this? I was bored this morning about 3 am,when I found it.

:confused:Thanks For All Input !!!!:eek:
 

Sceadwian

Banned
150 ohms is for the 3.5 volt you listed in your first post using three in series. 3.5 * 3 is 10.5 volts. Simple subtraction shows you have 3.5 volts left to get rid of at 14 volts 150 ohms will give you 23ma's at 14 volts which should be fine, the actual value you need for exactly 20ma with that voltage drop per LED is 175 but I don't think they make 175ohm resistors in standard sizes and there's no point worrying over a couple ma's 3.5 volts at 20ma's sounds about right for a typical white LED. You use three in series otherwise you have to drop more voltage across the resistor (wasted as heat)
You have to determine the optimum number to run in series for each individual color of LED you're using because the forward voltage is going to be different for each unless you have a more sophisticated driver circuit for them. The calculations are pretty simple, just doing the math I've hooked over 80 white LED's in two strings of 40 hooked up in anti parallel directly to a 120volt outlet and ran it for over an hour with no problems or blowouts, gotta be doing something right =) Flicker was too much though and I didn't feel like making a full wave rectifier for it, besides that was a lot of LED's =\
 
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dcsbass

New Member
Like I said: I'm going with yours. Was it right on colors, and ever thing. Had to dig that chart back out. I still a little foggy on the color code and percentage, and heat.
My Electrician at work ,said they have some 160's, and I though 170's.I'll get him to look . Just to see.
Not doubting your word what so ever. I just want it to work.
Thanks for info on leads. I didn't know about that at all. so I have come to the right place.
Any place to buy LED lights, for inside car Red and Blue? Bright and cheap?


:confused:Thank You Again Very Much :eek:
 

Sceadwian

Banned
170's would be ideal. You may want to put a smallish electrolytic cap in parallel with your LED's to buffer the power from noise and mute short voltage spikes, I'm not sure what to expect from voltage spikes on a car though so someone else here may be better at recommending surge protection. I suck at reading resistor codes I always use a meter to verify the resistance. Make sure you calculate the power requirements of the resistor too, in this case it's negligible, only about 80mws so even a 1/8th watt resistor will work. The more voltage you drop on the resistor the more power it uses the beefier the resistor you need, which is the major reason to use as many in series as you practically can.
E-bay is a good place to buy LED's cheap but I'd recommend posting any auction you're thinking of buying on here just so we can comment on it.
 

Sceadwian

Banned
Mike 175 ohms is for three in series. You said 175 ohms would drop 12 volts to 3.5 which it most definitely won't. It'll drop it BY 3.5 not TO 3.5

For a single LED at 3.6 volts on a 12 volt supply you'd need a 525ohm 1watt (it would dissipate about 210mw) the 3 diodes in series with 175 ohms will only dissipate 80mw's

If you used at 175ohm resistor on a 12 volt supply with a diode that dropped 3.5 volts the diode would get 60ma's, probably burn out inside a minute.
 
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Sceadwian

Banned
Search the forums, I probably make red faced posts at least twice as often as you =)
 

dcsbass

New Member
convert a light bar

Mr Sceadwian, what size a smallish electrolytic cap in parallel with your LED's ? How would you figure it out? OHM's Law? Should I post this in different section? Or keep going?
I didn't get to look for resistors today, may be tomorrow.
Thank You All for Advice....
Great place will recommend you to ever body that I know. That plays, or tinkers like I do. With electric projects..

:confused:Thank You Again:eek:


PS: Reason for bold is respect, and I don't give that out lightly!!!
 

Sceadwian

Banned
A small one, maybe 10u electrolytic, just to buffer the power a little and suck up small surges. You may want to start a new thread if you want more advice on the cap though, as I'm not real sure what good guidelines are for automotive use. With a 10u cap connected in parallel to the 3 LED's in series it's going to take around 2-4ms minimum for the LED's to turn off after power has been removed. Some kind of cap is definitely recommended though to avoid noise/voltage spikes from frying the LED's. A smaller one if you want a sharper turn off. A car's electrical system is usually a 'worst case scenario' as far as noise/pulse prevention goes what with all the ground loops and inductive loads around every corner.
 
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