automotive lighting issues / concerns

[Axkiker]

So im considering purchasing one of the bridgelux led arrays that is rated for 23.5 min 27.5 max V dc

I have the ability to use (2) 12 v auto batteries in series to get the needed 24 v.

What issues do you all see with me running this straight off the 24v.

I have read all about transient voltages and reverse currents etc. Do you feel the threat is great for this setup. If so what do you propose to do to eliminate the threat.

By running it straight off 24v I can eliminate the need for a driver which greatly reduces the price of my product.

thanks

 

[Alaric Darconville]

Is this an automotive lighting product? Or some kind of fixed lighting? Most cars use 12V electrical systems (military and other special vehicles may be 24V). That you ask about "transients" and whatnot makes it sound like it could be an automotive product, or possibly it's for some sort of solar system.

If it's fixed lighting (like for a house or shed or something), ask a moderator to move it to the fixed lighting section.

 

[John_Galt]

Why not use a boost driver? TaskLED is where I plan on sourcing my Hyperboost, for an ongoing project.

Seems like it would work well. What is the current needed for this array?

 

[Axkiker]

Its actually for a boat project. The boat has 2 12 v batteries so I can just wire in series if needed.

Still the same alternator etc as a car.

 

[Axkiker]

Why not use a boost driver? TaskLED is where I plan on sourcing my Hyperboost, for an ongoing project.

Seems like it would work well. What is the current needed for this array?

I need 2.1 A of output for a single chip


I got on the task led site and dont see anything that would work... unless im missing something

 

[Diesel_Bomber]

Running 24v devices off of two 12v batteries can be done, but doing so while maintaining a 12v power source and long battery life is more tricky.

12v nominal is the most common voltage for cars, trucks, and boats the world over, and hence there is a bewildering variety of appliances and doodads in existence to use 12v power. I can't imagine it'd be that difficult to find a solution to your lighting needs in a 12v package, which would not require re-wiring batteries every time you want to use your lights or complicated relay switching systems.

 

[Scott Packard]

It's 2.105V/cell for Lead-Acid, then about 15% above that for charging, so about 29V charging 29.4v max).
You're going to need to do some regulation in order to keep the system max voltage from frying your load.

 

[Diesel_Bomber]

It's 2.105V/cell for Lead-Acid, then about 15% above that for charging, so about 29V charging 29.4v max).
You're going to need to do some regulation in order to keep the system max voltage from frying your load.

Doesn't matter, he only has a 12v alternator on board. He won't be doing any charging while the batteries are in series, only while in parallel. The 24v lighting would have to run off the batteries while they're in series and not charging.

 

[Axkiker]

Okay im lost now...

I thought that a typical car alternator charged at roughly 13.5 - 14V

So I thought that while the engine was running and alternator charging that each battery would be at roughly 14V Max

So that put in series would be 28 V Max

do i havbe something wrong ???

 

[Steve K]

in my work designing electronics for heavy equipment, we assume that a 12v battery can vary anywhere from 9v to 18v, and a 24v system can vary from 18 to 36v. The voltage transients can be very large! Use a series diode to block negative voltages, and be prepared to withstand input voltages of up to 100v or more.

regards,
Steve K.

 

[Scott Packard]

> I thought that a typical car alternator charged at roughly 13.5 - 14V

Typical, but not the whole story. If you Google you can find the daily charging voltage can be 14.2-14.5V.
As ambient temperature goes down the internal battery resistance goes up. The charging circuit will raise the voltage slightly to compensate.

Then, there's the abnormal circuit. What if one diode goes bad? What about high resistance grounds?
You typically don't see these voltage fluctuations in your dashboard lights because they have a regulator on them (I've seen the bad diode on my dashboard lights though, and I've had to diagnose and fix a high resistance ground.)
Also, briefly full-fielding an alternator is an acceptable testing procedure a technician can perform, and the other circuits in the vehicle survive that.

 

[Axkiker]

Okay guys give me some options.

I assume one option is a driver..... my concern though is I hear horror stories about the drivers not beign able to take the spikes etc in and automotive env.

What kind of circuity could be used to run straight off the 24V electrical system

thanks

 

[Steve K]

You can drop the voltage to a level that is safe for your driver by using a regulator. A regulator IC, like the LM317, isn't usually rated for the 100v or so that might be present. My preference is for a simple zener diode followed by a current amplifier. This is nicely illustrated in the OnSemi zener diode data book:

http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

See figure 8 on page 38. Unfortunately, they show a negative voltage regulator. To get a positive voltage regulator, use a NPN transistor instead of PNP, and flip the zener diode around. The following pages show some more sophisticated regulators. The NPN transistor should be rated for 100 or 200 volts. You'll probably need to heatsink it too, depending on how much current you'll pull through it, and the voltage drop across it.

A big transorb or zener diode at the input would help clamp some of the really big spikes too. It might even be enough to let you use a LM317??

by the way, page 89 shows some of the typical transients in automotive environments for 12v systems. For 24v systems, roughly double the voltages.

There are some nice buck regulators designed for automotive use too. Designed to handle input voltages of up to 80vdc or so.

regards,
Steve K.

 

[Axkiker]

what I dont get is that I currently have leds running straight off my 12v boat battery and they are holding up fine.

the set is a total of 8 leds in a housing. 2 sets of 4 running in series then the 2 sets in parallel to the battery

If I remember correctly the forward voltage is 3.4 each

so that should mean the unit could accept 13.6V

I have never had an issue....

the setup was cheap and I honestly wanted to see how long they would live. I just assumed that they would have blown by now with all the horror stories you hear about transient and reverse voltages....

 

[bshanahan14rulz]

I think your biggest problem with a direct-drive setup would be heat. As the LEDs heat up, their equivalent resistance goes down and allows more current through the LED. More current means more heat means more current, so-on and so-forth.

With a regulated supply, you won't have that problem because so what if the LED can allow a lot of current when it only has a safe amount of current to begin with.

Since your othe rlights are probably individual 5mm LEDs, their heat doesn't add up as easily. Since they don't use a lot of power, cheap resistors can be used to limit the current.

 

[Steve K]

the setup was cheap and I honestly wanted to see how long they would live. I just assumed that they would have blown by now with all the horror stories you hear about transient and reverse voltages....

the transients and such are based on automotive or similar applications. These apps include a bunch of electro-mechanical devices that can produce some nasty waveforms.

For instance, the horn. The horn is typically built with a coil that pulls in the diaphram. When the diaphram moves most of the way, it opens the contacts that are in series with the coil, suddenly interrupting the current and letting the diaphram move back towards the resting position. The sudden change in coil current generates a very large voltage spike (as does any sudden change in current in a conductor).

Another classic generator is the ventilation fan. When power is removed and the motor winds down, it generates a lot of brief voltage spikes. These have caused quite a few problems for parts that I've worked with.

"load dump" is another case. If you have a large load being driven by the battery and alternator, and suddenly remove the load, it takes a while for the alternator to respond and get the voltage regulated. In the meantime, the voltage can shoot up 50 to 100v. Very bad, and there's quite a bit of energy involved.

If you don't have these, or similar, devices on the boat, you've reduced a lot of the risks.

regards,
Steve K.

 

[Axkiker]

Well all of that makes perfect sense so that makes me wonder even more why these leds are still running

The boat has a horn
Typical automotive type charging system
2 fans to vent the bilge of gas fumes
1 bilge pump
1 trim tab pump
1 trim cylinder pump that i know pulls a great deal of current

The trim pump is ran multiple times when underway because you are always adjusting the trim of the drive...

Im kinda glad I put the leds on there because it has been an interesting experiment.

the transients and such are based on automotive or similar applications. These apps include a bunch of electro-mechanical devices that can produce some nasty waveforms.

For instance, the horn. The horn is typically built with a coil that pulls in the diaphram. When the diaphram moves most of the way, it opens the contacts that are in series with the coil, suddenly interrupting the current and letting the diaphram move back towards the resting position. The sudden change in coil current generates a very large voltage spike (as does any sudden change in current in a conductor).

Another classic generator is the ventilation fan. When power is removed and the motor winds down, it generates a lot of brief voltage spikes. These have caused quite a few problems for parts that I've worked with.

"load dump" is another case. If you have a large load being driven by the battery and alternator, and suddenly remove the load, it takes a while for the alternator to respond and get the voltage regulated. In the meantime, the voltage can shoot up 50 to 100v. Very bad, and there's quite a bit of energy involved.

If you don't have these, or similar, devices on the boat, you've reduced a lot of the risks.

regards,
Steve K.

 

[Axkiker]

Do you know of any good DC/DC step up power supplies designed to handle an automotive enviroment.

needs to be capable of pushing 2+ amps at 24v off of 12v

I believe this power supply is however im not sure if its really good for an automotive enviroment

http://cgi.ebay.com/Adjustable-DC-D...tem&pt=LH_DefaultDomain_0&hash=item335aec5c01

You can drop the voltage to a level that is safe for your driver by using a regulator. A regulator IC, like the LM317, isn't usually rated for the 100v or so that might be present. My preference is for a simple zener diode followed by a current amplifier. This is nicely illustrated in the OnSemi zener diode data book:

http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

See figure 8 on page 38. Unfortunately, they show a negative voltage regulator. To get a positive voltage regulator, use a NPN transistor instead of PNP, and flip the zener diode around. The following pages show some more sophisticated regulators. The NPN transistor should be rated for 100 or 200 volts. You'll probably need to heatsink it too, depending on how much current you'll pull through it, and the voltage drop across it.

A big transorb or zener diode at the input would help clamp some of the really big spikes too. It might even be enough to let you use a LM317??

by the way, page 89 shows some of the typical transients in automotive environments for 12v systems. For 24v systems, roughly double the voltages.

There are some nice buck regulators designed for automotive use too. Designed to handle input voltages of up to 80vdc or so.

regards,
Steve K.

 

[Steve K]

I'm not familiar with what is available as a packaged power converter. If you like to build electronics, TI has a nice boost converter that looks like it would do the job:

http://focus.ti.com/docs/prod/folders/print/tps61175.html

datasheet is here:
http://focus.ti.com/lit/ds/symlink/tps61175.pdf

I was hoping that they might sell an evaluation board, which is the quick way to get one of their parts up and running, but I don't see one mentioned.

Of course, some sort of current regulator would be needed, or just a series resistor.

Why not simplify things and use a LED array that can run from 12v?

regards,
Steve K.

 

[Axkiker]

I'm not familiar with what is available as a packaged power converter. If you like to build electronics, TI has a nice boost converter that looks like it would do the job:

http://focus.ti.com/docs/prod/folders/print/tps61175.html

datasheet is here:
http://focus.ti.com/lit/ds/symlink/tps61175.pdf

I was hoping that they might sell an evaluation board, which is the quick way to get one of their parts up and running, but I don't see one mentioned.

Of course, some sort of current regulator would be needed, or just a series resistor.

Why not simplify things and use a LED array that can run from 12v?

regards,
Steve K.

Well running straight off 12v would be preferable however the idea of one single array that produces mega lumens is nice... its makes the housing simple, the reflector simple, the lense simple, etc etc... Now if using multiple leds or arrays off of 12v would work and meet all above criteria then im all game... its just a matter of cost / ease of build / vs production