LED Taillights, Diy

[Matt21lutz]

Originally Posted by ray_mcavoy

Hi Matt,

A few issues I noticed:

(1) This is just a matter of terminology, but PWM stands for Pulse Width Modulation. The regulators you have listed are linear regulators ... they don't use pulse width modulation.

(2) Steady state DC current flow through a capacitor is zero. So the circuit will not work with the .33μF & .1μF capacitors connected as shown. Instead, they should be connected between ground and the regulator's input (& output) terminals respectively.

(3) Linear voltage regulators need a bit of "overhead" voltage to work properly. The NTE1970 you have listed requires a minimum input voltage of 14.8V ... see specs at: http://www.nteinc.com/Web_pgs/Positive.php?a=12 Even their low dropout NTE1954 12V regulator requires a minimum input voltage of 13.6V (http://www.nteinc.com/Web_pgs/pos_lowdo.php?a=12) It's very common for the voltage in a vehicle to drop below that level.

Having the input voltage go below the regulator's minimum requirement won't cause any harm but it will cause the output voltage to sag and therefore your LEDs will be dimmer than normal. So you might want to consider using lower voltage regulators and re-calculating your current limiting resistors to work with the lower voltage. That might also require reducing the number of LEDs in each series string.

Another alternative would be to directly regulate the current through each string of LEDs to the required 20mA. Take a look at the "1A Current Regulator" example on page 16 of the LM317 datasheet (http://www.alldatasheet.com/datashee...NSC/LM317.html) for an idea of how to wire it as a current regulator. You'll obviously have to change the resistor from the 1.2Ω for their 1A example. You can calculate the value using Iout = Vref / R where Iout is the current, Vref is the LM317's 1.25V reference, and R is the resistance. So for Iout = 20mA you'd want 20mA = 1.25V / R. Or R = 1.25V / 20mA = 62.5Ω. The only catch with this method is that you'll need a regulator for each series string of LED's.

uhhhhhhhhhhhhh.. i feel like an idiot now. no idea what almost any of that means... crap. but All I know is i want to limit the voltage to the led's to a max voltage of 12v. of course. each one of my led's run at 2.2v at 20ma each. Really don't want to drop down to a 10v circuit would be to much really, even if I might not even create my own voltage regulator, might just buy one cause this is overly complicated.

[ray_mcavoy]

Originally Posted by Matt21lutz

uhhhhhhhhhhhhh.. i feel like an idiot now. no idea what almost any of that means... crap. but All I know is i want to limit the voltage to the led's to a max voltage of 12v. of course. each one of my led's run at 2.2v at 20ma each. Really don't want to drop down to a 10v circuit would be to much really, even if I might not even create my own voltage regulator, might just buy one cause this is overly complicated.

Okay, a pre-made regulator circuit will probably be your best choice then. But you still have to check the specifications to make sure it'll be suitable for your application. For example, you weren't completely off track with your original circuit using the NTE1970 regulator IC's ... you simply had the capacitors oriented incorrectly and overlooked the regulator's minimum input voltage requirements.

As far as the voltage goes, I agree, you wouldn't want to go much below 12V for your existing arrangement using 5 of those 2.2V LEDs in each series string. To work with a lower voltage, you'd have to reduce the number of LED's in each string. And that would mean adding more strings to keep the total number of LEDs the same.

[Matt21lutz]

Originally Posted by ray_mcavoy

Okay, a pre-made regulator circuit will probably be your best choice then. But you still have to check the specifications to make sure it'll be suitable for your application. For example, you weren't completely off track with your original circuit using the NTE1970 regulator IC's ... you simply had the capacitors oriented incorrectly and overlooked the regulator's minimum input voltage requirements.

As far as the voltage goes, I agree, you wouldn't want to go much below 12V for your existing arrangement using 5 of those 2.2V LEDs in each series string. To work with a lower voltage, you'd have to reduce the number of LED's in each string. And that would mean adding more strings to keep the total number of LEDs the same.

Ohhh I thought i had the whole ciruit wrong besides the led wiring setup. But I found something that might work, but i really want to make my own. This is what i found IF i did buy one but not sure, http://www.reuk.co.uk/buy-12-VOLT-RE...-WITH-FUSE.htm. But I continued to look on their site and found this voltage regulator, http://www.ti.com.cn/cn/lit/ds/symlink/lm2940-n.pdf, now it has a .5v dropout and I believe it can handle up 26v. I would like your thoughts because people are saying you know what your talking about and i believe them!

Also if it is and if u have the time could you make a circuit diagram of what i would need to make and o it properly? I would greatly appreciate the help!

[ray_mcavoy]

Originally Posted by Matt21lutz

Ohhh I thought i had the whole ciruit wrong besides the led wiring setup. But I found something that might work, but i really want to make my own. This is what i found IF i did buy one but not sure, http://www.reuk.co.uk/buy-12-VOLT-RE...-WITH-FUSE.htm. But I continued to look on their site and found this voltage regulator, http://www.ti.com.cn/cn/lit/ds/symlink/lm2940-n.pdf, now it has a .5v dropout and I believe it can handle up 26v. I would like your thoughts because people are saying you know what your talking about and i believe them!

From the looks of it, I think the LM2940 regulator IC (integrated circuit) you found in the 2nd link is what they're using on that pre-built circuit board in the 1st link. Specifically, it appears to be the LM2940CT-12 or LM2940T-12 version in a TO220 package.

I took a quick look through the LM2940 datasheet from your 2nd link and it appears as though it would be a good choice for your application. In fact, the description on the first page says it's designed for vehicular applications and has built-in protection against reverse polarity, transient voltage spikes, etc. so that's a plus.

The low 0.5V dropout or "overhead" is also good for your application. That means the regulator only needs a 0.5V difference between the input & output to maintain regulation. In other words, you'll have the full 12V output you're expecting as long as the input stays above 12.5V ... that's much better than the NTE1970's 14.8V min requirement.

As long as your truck's charging system is working properly, you shouldn't have to worry about input voltages lower than that. But as you can see from the "low voltage behavior" (figure 21 on the LM2940 datasheet), it's a fairly linear drop-off once the input goes below about 12V. So a low battery situation would allow the LEDs to dim, but the regulator isn't going to suddenly "switch off" under those conditions.

Originally Posted by Matt21lutz

Also if it is and if u have the time could you make a circuit diagram of what i would need to make and o it properly? I would greatly appreciate the help!

The "typical application" circuit (shown near the bottom of the first page of the LM2940 datasheet) is all you really need. Notice how the capacitors are connected ... that's what I was trying to describe when I said they were connected incorrectly in your original diagram.

So basically, take that "typical application" circuit for the LM2940 and substitute it into your original diagram in place of the NTE1970 & the 2 capacitors and you should be all set. But if that doesn't make any sense, just let me know and I can draw up a diagram for you.

Finally, your original circuit shows 56Ω resistors in series with each LED string. That's a good value to use for a 12V input, but like Dead Parrot pointed out earlier, don't forget about the voltage drop from the diodes being used to isolate the tail from the brake/turn circuits. With silicon diodes (0.7V drop) you'll only have 11.3V available to the LEDs. So you'll need smaller current limiting resistors in order to maintain the full rated 20mA through the LEDs. I'm calculating 15Ω resistors for five 2.2V, 20mA LEDs in series with an 11.3V supply.

[Matt21lutz]

Originally Posted by ray_mcavoy

From the looks of it, I think the LM2940 regulator IC (integrated circuit) you found in the 2nd link is what they're using on that pre-built circuit board in the 1st link. Specifically, it appears to be the LM2940CT-12 or LM2940T-12 version in a TO220 package.

I took a quick look through the LM2940 datasheet from your 2nd link and it appears as though it would be a good choice for your application. In fact, the description on the first page says it's designed for vehicular applications and has built-in protection against reverse polarity, transient voltage spikes, etc. so that's a plus.

The low 0.5V dropout or "overhead" is also good for your application. That means the regulator only needs a 0.5V difference between the input & output to maintain regulation. In other words, you'll have the full 12V output you're expecting as long as the input stays above 12.5V ... that's much better than the NTE1970's 14.8V min requirement.

As long as your truck's charging system is working properly, you shouldn't have to worry about input voltages lower than that. But as you can see from the "low voltage behavior" (figure 21 on the LM2940 datasheet), it's a fairly linear drop-off once the input goes below about 12V. So a low battery situation would allow the LEDs to dim, but the regulator isn't going to suddenly "switch off" under those conditions.

The "typical application" circuit (shown near the bottom of the first page of the LM2940 datasheet) is all you really need. Notice how the capacitors are connected ... that's what I was trying to describe when I said they were connected incorrectly in your original diagram.

So basically, take that "typical application" circuit for the LM2940 and substitute it into your original diagram in place of the NTE1970 & the 2 capacitors and you should be all set. But if that doesn't make any sense, just let me know and I can draw up a diagram for you.

Finally, your original circuit shows 56Ω resistors in series with each LED string. That's a good value to use for a 12V input, but like Dead Parrot pointed out earlier, don't forget about the voltage drop from the diodes being used to isolate the tail from the brake/turn circuits. With silicon diodes (0.7V drop) you'll only have 11.3V available to the LEDs. So you'll need smaller current limiting resistors in order to maintain the full rated 20mA through the LEDs. I'm calculating 15Ω resistors for five 2.2V, 20mA LEDs in series with an 11.3V supply.

awesome, I did see that it said for vehicular use on that just wanted to make sure. and as far as the diodes actually taking away some voltage.. i would have never thought about that unfortunately when i ordered all my resistors i got 56 ohm as the smallest ones, i could always buy more but then im paying triple just for shipping a few dollars worth of resistors, but i might. The only thing would that would be different is the 56 ohm would make the led's dimmer , just not sure how much actually... we'll find out! I'll probably make a few tester first then make the final product.

But i would like to get your opinion on something else that i thought about, what do you feel is the best way to dim the led's? with a 1/2 watt resistor or to use a voltage regulator with a lower output voltage? for instance a 10v output.

but thank you for all the help so far as well! i greatly appreciate it!

[ray_mcavoy]

Originally Posted by Matt21lutz

awesome, I did see that it said for vehicular use on that just wanted to make sure. and as far as the diodes actually taking away some voltage.. i would have never thought about that unfortunately when i ordered all my resistors i got 56 ohm as the smallest ones, i could always buy more but then im paying triple just for shipping a few dollars worth of resistors, but i might. The only thing would that would be different is the 56 ohm would make the led's dimmer , just not sure how much actually... we'll find out! I'll probably make a few tester first then make the final product.

You're right ... they'll definitely be dimmer than normal with 56Ω resistors on a 12V supply after you add in the diode drop. And without some additional data on the LEDs you're using, it's hard to say just how dim they'll be.

You can put some of the 56Ω resistors you already have in parallel to obtain lower resistances. To calculate the total resistance, add the reciprocals and take the reciprocal of the sum. Rtotal = 1 / ((1/R1) + (1/R2) + ... + (1/Rn)). So 3 of those 56Ω resistors in parallel = 18.7Ω ... and that's pretty close to the 15Ω I calculated earlier. Using 3 parallel resistors for each LED string obviously isn't something you'd want to do for your final design (it'd be a waste of resistors & board space). But I thought I'd mention it because it's something you could do for preliminary testing on a string or two of LEDs without having to buy more resistors.

Originally Posted by Matt21lutz

But i would like to get your opinion on something else that i thought about, what do you feel is the best way to dim the led's? with a 1/2 watt resistor or to use a voltage regulator with a lower output voltage? for instance a 10v output.

Either one will work (and there are several other ways as well). The resistor is simple and will get the job done. Unless you've already done so, you might need to try a few different resistance values to get the brightness of your tail lights to the desired level.

If you use a lower output voltage regulator, you might want to consider an adjustable regulator instead of a fixed 10V output. That'd allow you to easily change the output voltage in case 10V ends up making the LEDs too bright or too dim. The LM2941T is an adjustable version of the LM2940T-12 fixed 12V regulator you're considering. The output voltage is adjustable using the ratio of a couple of resistors. And for testing purposes you could use a potentiometer.

Originally Posted by Matt21lutz

but thank you for all the help so far as well! i greatly appreciate it!

You're welcome!

Originally Posted by kikkegek

I have already build a couple of these tail lights for myself.

see my sig for the end result.

here is a link to the build topic of it on HID PLANET
http://www.hidplanet.com/forums/show...or-my-Suburban

Nice job! I didn't get a chance to read through all the details, but it looks like you used the LM2940T-12 regulators that Matt is considering, correct?

[Matt21lutz]

Originally Posted by ray_mcavoy

You're right ... they'll definitely be dimmer than normal with 56Ω resistors on a 12V supply after you add in the diode drop. And without some additional data on the LEDs you're using, it's hard to say just how dim they'll be.

The LED's that im using are from superbrightleds.com, http://www.superbrightleds.com/morei...-mcd/356/1303/ they are EXTREMELY bright when wired correctly, ive got one small board sitting on my wall hooked up and working, i use them as a night light when i'm working late at night lol well with a resistor and witch they're a night light and then flip the switch for full brightness.

You can put some of the 56Ω resistors you already have in parallel to obtain lower resistances. To calculate the total resistance, add the reciprocals and take the reciprocal of the sum. Rtotal = 1 / ((1/R1) + (1/R2) + ... + (1/Rn)). So 3 of those 56Ω resistors in parallel = 18.7Ω ... and that's pretty close to the 15Ω I calculated earlier. Using 3 parallel resistors for each LED string obviously isn't something you'd want to do for your final design (it'd be a waste of resistors & board space). But I thought I'd mention it because it's something you could do for preliminary testing on a string or two of LEDs without having to buy more resistors.

as far as wiring in parellel... yeah that'd be too much for me, i could do it but I'll just end up buying the right resistors online for 10 cents each or something like that lol

Either one will work (and there are several other ways as well). The resistor is simple and will get the job done. Unless you've already done so, you might need to try a few different resistance values to get the brightness of your tail lights to the desired level.

If you use a lower output voltage regulator, you might want to consider an adjustable regulator instead of a fixed 10V output. That'd allow you to easily change the output voltage in case 10V ends up making the LEDs too bright or too dim. The LM2941T is an adjustable version of the LM2940T-12 fixed 12V regulator you're considering. The output voltage is adjustable using the ratio of a couple of resistors. And for testing purposes you could use a potentiometer.

I think I might try the adjustable one, I could always set it to 12v and use a resistor if that is what i go with.