# [EVDL] LED driver that runs off of 12V



## EVDL List (Jul 27, 2007)

From: Rod Hower
> Thought somebody might want to roll their own,,,
> http://electronicdesign.com/Articles/ArticleID/16763/16763.html

It always amazes me how complicated they can make driving an LED.

This circuit uses dozens of parts and ICs to drive some 3.6v LEDs from a 12v battery. A plain old resistor for each three LEDs in series also does the job nicely. The resistor will drop 12v - (3 x 3.6v) = 1.2v which means the overall circuit is 90% efficient -- higher than this switching regulator circuit. It automatically draws no current if the battery goes dead because the LEDs stop conducting.

Yes, the light output varies with battery voltage; but how often is that a problem. Car tail lights also vary as the car's electrical system wanders around 12-14v. An ordinary 3v flashlight bulb could be used as the series resistor as a crude constant current source, for a 1-component solution.


--
"Excellence does not require perfection." -- Henry James
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart-at-earthlink.net

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## EVDL List (Jul 27, 2007)

Well it does use a lot of parts but you could use a 34063 to drop it 
down to about 5 parts. You could also use a 555 at a fixed duty cycle 
with a small resistor in series to limit peak current.
--
Martin K



> Lee Hart wrote:
> > From: Rod Hower
> >
> >> Thought somebody might want to roll their own,,,
> ...


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## EVDL List (Jul 27, 2007)

> It always amazes me how complicated they can make driving an LED.
>
> This circuit uses dozens of parts and ICs to drive some 3.6v LEDs from a 12v battery. A plain old resistor for each three LEDs in series also does the job nicely. The resistor will drop 12v - (3 x 3.6v) = 1.2v which means the overall circuit is 90% efficient -- higher than this switching regulator circuit. It automatically draws no current if the battery goes dead because the LEDs stop conducting.
>
> Yes, the light output varies with battery voltage; but how often is that a problem. Car tail lights also vary as the car's electrical system wanders around 12-14v. An ordinary 3v flashlight bulb could be used as the series resistor as a crude constant current source, for a 1-component solution.
>

Yeah, for non-EE hobbyists, the simple resistor is easier. For a big
automaker, though, the proper driver allows them to have bright/dim
settings that look the same regardless of voltage. It also allows them
to put the driver on a common circuit board and only run two wires to
the LEDs in the tail light. That way, the only components in the light
itself are about 8 LEDs in series.

I agree that a resistor is much simpler and is a great choice for most
homemade EVs, but personally I'd do the regulator circuit anyways.

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## EVDL List (Jul 27, 2007)

Because your solution does not have adequate regulation.

At 14.6V, the LEDs will take over 3x the current. At 11v current drops 
to 1/6th. A regulated voltage source helps, but most regs drop quite a 
bit of the voltage, and there is still a regulation issue due to the 
thermal coefficient of forward voltage in the device as well as the 
device's inconsistent Vf. In practice a 3.6V LED may actually be 
2.9V-3.8V (depends on spec sheet), so even with a regulated 12V the 
current could anywhere from 20ma to 110mA so we don't know if it will be 
bright enough to be seen or if it will burn itself out from 
overcurrent. If you only use 2 3.6v LEDs in series and increase the 
ballast resistance then it becomes much better regulated, but less 
efficient.

Cost is not as big as it seems for mass producers. Ever take apart the 
base of a $3 110V CFL tube? Holy crap there's a lot in there. They 
kept the cost down (well in part by not using ASICs like this LED 
drivers, but still).

ZXLD1360 is a good, simple LED regulator but it's only 36V IIRC. 
There's gonna be a ZXLD1362 out eventually with a much higher voltage 
rating.

HV9910 is a great reg, high voltage rating, but you need to buy an 
external switching MOSFET with it so it's a bit more complicated and 
expensive.

Danny



> Lee Hart wrote:
> 
> >From: Rod Hower
> >
> ...


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## EVDL List (Jul 27, 2007)

How about something like a Hartley style oscillator. I built a dc-dc
converter with 1 centertapped transformer, two transistors and an opto,
1 diode and a cap.

This is from memory, Lee I'm sure can fill in the gaps.
The opto's output was in series with a resistor and the battery (and
a switch) to the center tap, each end went to the transitor back to the
battery.
The base of each transitor was connected tot he other transformer
end (maybe a resistor there)
This is a self oscillating circuit, the natural imbalance gets it
going then the saturation of each half of the tapped side of the
inductor causes the opposite transitors to switch.

The output is then recified.

I made this circuit in a 3/4 inch square that took a 9V battery and gave
me +/- 5 for communcation testing.


Now that I think About it that was 2 diodes.

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## EVDL List (Jul 27, 2007)

> Jeff Shanab wrote:
> > How about something like a Hartley style oscillator...
> 
> Sure; that will work. In fact, there are of course dozens of circuits
> ...


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## EVDL List (Jul 27, 2007)

> I tell the kids in my BEST class, "The first solution is the worst
> solution." It's the one you arrived at without thinking; by doing
> whatever seems easy or expedient or fashionable at the moment. But take
> some time to *think* about the problem. How *many* ways can you find to
> solve it? Often, your second or third or fourth ideas turn out to be
> better choices!
>
> If you think about a problem long enough, you will frequently arrive at
> a "best" solution; one that is so obviously right that once you've found
> it, you'll slap yourself on the forehead and say, "Of course! That's it!"

I have a similar theory, but mine consists of thinking things through until=
I have come up with what I term an elegant solution. Not only does it get=
the job done, but it's clean and simple. Sometimes for expediency sake I =
do go with the first solution, but I am never more satisfied and proud than=
when I put in enough thought time up front to come up with the elegant sol=
ution. Sometimes it takes days or weeks of chewing on a problem until it f=
inally comes.

damon


_________________________________________________________________
Help yourself to FREE treats served up daily at the Messenger Caf=E9. Stop =
by today.
http://www.cafemessenger.com/info/info_sweetstuff2.html?ocid=3DTXT_TAGLM_Oc=
tWLtagline

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## EVDL List (Jul 27, 2007)

> 
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On
> Behalf Of damon henry
> Sent: Friday, October 05, 2007 1:50 PM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] LED driver that runs off of 12V
> 
> 
>> I tell the kids in my BEST class, "The first solution is the worst
>> solution." It's the one you arrived at without thinking; by doing
>> whatever seems easy or expedient or fashionable at the moment. But take
>> some time to *think* about the problem. How *many* ways can you find to
>> solve it? Often, your second or third or fourth ideas turn out to be
>> better choices!
>>
>> If you think about a problem long enough, you will frequently arrive at
>> a "best" solution; one that is so obviously right that once you've found
>> it, you'll slap yourself on the forehead and say, "Of course! That's
> it!"
> 
> I have a similar theory, but mine consists of thinking things through
> until I have come up with what I term an elegant solution. Not only does
> it get the job done, but it's clean and simple. Sometimes for expediency
> sake I do go with the first solution, but I am never more satisfied and
> proud than when I put in enough thought time up front to come up with the
> elegant solution. Sometimes it takes days or weeks of chewing on a
> problem until it finally comes.
> 
> damon
> 

Well, it's been ten years and with just a little more planning I'll be ready to start my ev conversion.... ;-)

Somehow, that seemed funny when I first thought of it. Now it just makes me sick.


--
Stay Charged!
Hump
I-5, Blossvale NY

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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

> Tim Humphrey wrote:
> >> Well, it's been ten years and with just a little more planning I'll
> >> be ready to start my EV conversion... ;-) Somehow, that seemed funny
> >> when I first thought of it. Now it just makes me sick.
> ...


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## EVDL List (Jul 27, 2007)

----- Original Message ----- 
From: "Lee Hart" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Friday, October 05, 2007 5:39 PM
Subject: Re: [EVDL] LED driver that runs off of 12V




> > Tim Humphrey wrote:
> >>> Well, it's been ten years and with just a little more planning I'll
> >>> be ready to start my EV conversion... ;-) Somehow, that seemed funny
> >>> when I first thought of it. Now it just makes me sick.
> ...


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> > storm connors quipped:
> > > The perfect is the enemy of the good.
> >
> ...


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## EVDL List (Jul 27, 2007)

> > The perfect is the enemy of the good. 
>
> It was very crude; a pickup truck with an aircraft surplus generator, a
> dozen 6v golf cart batteries, a homemade contactor controller, and a
> homemade transformer-rectifier charger. But it worked! It got me
> started, and taught me a lot about what to do (and what *not* to do).

Hmm... "but it moves." <g>

The first EV I came into contact with was a few years earlier... 1970, as I
recall. The first Earth Day in Eugene.
http://www.encyclopedia.com/doc/1E1-EarthDay.html

It's ironic that my theme was noise pollution... and no one seemed to get
it. (And today we are debating noise traffic may not be loud enough.) That
was when I saw the first EV... a Toyota converted by EWEB (our utility) and
displayed at our high school. They had a similar system to yours, Lee, but
their main problem seemed to be in making sure they were in the proper gear
before starting out. It (apparently) was fairly easy to get into too high a
gear and twist things off in the driveline. (It had happened at least a
couple times previous.)

That was when I first learned of the "horrid" torque available to a proper
sized motor... something that is used to advantage in the 1/4 mile EV racers
today.

It would have been easier to be a radical, if my mom hadn't been 2 steps
ahead of me.

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## EVDL List (Jul 27, 2007)

Hi Danny,

I do not know if you actually measured a white LED recently,
they have a very large internal resistance. The regulation
is not as bad as you would like us to believe.

Still, because I am a tinkerer, I built my own regulator.
Since LEDs are current-controlled devices, I built a
current source.
It is pretty simple to create a current source that drops
less than one Volt and is pretty stable in its controlled
characteristic - a resistor to measure the voltage drop 
caused by the current and you have a pretty solid value
that only changes a few percent when ambient temp changes.
(To be precise: 1% for every 3 deg Celcius change)
If you want to make it perfect (temp compensated) it costs
you more than two diodes and two resistors plus a transistor.
Such a current regulator will keep the brightness of the LEDs
constant and you can easily use 3 white LEDs in series and
have no impact from fluctuating voltages, so your LED will
not burn out prematurely or dim unnecessary for all reasonable
voltage levels (say, 11 to 15+V)

See the crude drawing below (watch with fixed font)
The component between the e-b-c is a NPN transistor
with at least Hfe = 100, preferably higher.
The good old BC-547 will do perfectly.
R is the current sense resistor and will drop
approx 0.7V, dependent on the type of diode D used.
If you use two schottky diodes, the total drop across the
diodes is less than 1V, minus 0.7V for the b-e drop, so 
the resistor will then get less than 0.3V
The R-bias is just to get some current through the diodes
and open the base of the transistor. Around 33k Ohm should
be enough to get everything to work, assuming the LEDs
do not need more than 50 mA (I actually use two parallel
strings of 3 LEDs in series to regulate 6 LEDs in my
bicycle headlight).
For the indicated 0.7V and 20 mA LED current, the resistor
R needs to be: 0.7 / 0.020 = 35 Ohm. (33 Ohm nearest value)

NOTE that the efficiency and the voltage drop for this
regulator is determined by the drop across the R and the
e-c drop of the transistor.
Since many transistors are happy to go down to 0.2V, the
total drop of this regulator can easily be less than 1V
and with two schottky diodes, it would even be around 0.5V.

R ____ e ___ c 3 white LEDs
+---|____|-----|___|------|<|--|<|--|<|---+
| 0.7V b| |
| D D | ____ R-bias |
+----|<|--|<|----+----|____|--------------+
| |
|__o negative 12V supply positive o

Cor van de Water
Systems Architect
Proxim Wireless Corporation http://www.proxim.com
Email: [email protected] Private: http://www.cvandewater.com
Skype: cor_van_de_water IM: [email protected]
Tel: +1 408 542 5225 VoIP: +31 20 3987567 FWD# 25925
Fax: +1 408 731 3675 eFAX: +31-87-784-1130
Second Life: www.secondlife.com/?u=3b42cb3f4ae249319edb487991c30acb

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Danny Miller
Sent: Thursday, October 04, 2007 12:42 PM
To: Lee Hart; Electric Vehicle Discussion List
Subject: Re: [EVDL] LED driver that runs off of 12V

Because your solution does not have adequate regulation.

At 14.6V, the LEDs will take over 3x the current. At 11v current drops to 1/6th. A regulated voltage source helps, but most regs drop quite a bit of the voltage, and there is still a regulation issue due to the thermal coefficient of forward voltage in the device as well as the 
device's inconsistent Vf. In practice a 3.6V LED may actually be 
2.9V-3.8V (depends on spec sheet), so even with a regulated 12V the current could anywhere from 20ma to 110mA so we don't know if it will be bright enough to be seen or if it will burn itself out from overcurrent. If you only use 2 3.6v LEDs in series and increase the ballast resistance then it becomes much better regulated, but less efficient.

Cost is not as big as it seems for mass producers. Ever take apart the base of a $3 110V CFL tube? Holy crap there's a lot in there. They kept the cost down (well in part by not using ASICs like this LED drivers, but still).

ZXLD1360 is a good, simple LED regulator but it's only 36V IIRC. 
There's gonna be a ZXLD1362 out eventually with a much higher voltage rating.

HV9910 is a great reg, high voltage rating, but you need to buy an external switching MOSFET with it so it's a bit more complicated and expensive.

Danny



> Lee Hart wrote:
> 
> >From: Rod Hower
> >
> ...


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## EVDL List (Jul 27, 2007)

Lee Said:

> I tell the kids in my BEST class, "The first solution is the worst
> solution." It's the one you arrived at without thinking; by doing
> whatever seems easy or expedient or fashionable at the moment. But
> take some time to *think* about the problem. How *many* ways can you
> find to solve it? Often, your second or third or fourth ideas turn out
> to be better choices!
>
> If you think about a problem long enough, you will frequently arrive
> at a "best" solution; one that is so obviously right that once you've
> found it, you'll slap yourself on the forehead and say, "Of course!
> That's it!" 

OMG, truer words were never spoken. I was just throwing out alternate
ideas as part of the brainstorming process. I agree that circuit in
magazine is there to say "hey look what new chip we have" you need one,
but after reading the article I see it has some benefits, just not for a
tail light. It is more towards a back lit battery operated device.
Battery monitor, constant current control and dimming.

On a funnier note, I am kind know for the hand on the forehead. Years
after people have left the machine shop where I used to work I heard
about stories where people asked how I was doing and if I still had a
hand-print on my forehead.

I am up do design 10 on my active charge balancer and monitor, even the
name changes. I have yet to pick up the soldering iron. I wish I could
do it in 4, but I need more experience for that!

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## EVDL List (Jul 27, 2007)

White LEDs don't have a "very large" internal resistance. Its behavior 
is very nonlinear and can't be measured by a simple multimeter, a 
multimeter will give nonsense readings. They do have a dynamic 
resistance figure but even that has a meaning only in a fairly limited 
context.

That regulation circuit you use is fairly effective, it's been used 
before. You need to calculate the heat in the transistor. It may or 
may not be high. Actually it's probably a pretty good choice for LED 
taillights.

Danny



> Cor van de Water wrote:
> 
> >Hi Danny,
> >
> ...


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