LED's and diodes question

lordraiden

Newly Enlightened
Joined
Jan 24, 2013
Messages
89
Hi all. I'm trying to build a home brew LED lighting system for a converted bus (ie, school bus to motorhome conversion. FYI, they're a very cool way to roll. :D) and one of the questions I have is this. The power system aboard is all 12v, both for the starter battery, and the onboard solar charged 12v "house" power. (they're identical to allow me to switch back and forth between the two if needed) Anyhow, the lights I want to hack into the power system are 6v 1.5w LED "pucks". I know that if I apply 12v, all I'll end up with is blown lights, and quite possibly a house fire. However, I also know that you can wire in line a few diodes and get a proper voltage reduction. The problem is, I'm not familiar enough with diode ratings to know for certain how many olms of diode I need to reduce it from 12 to exactly 6v. I've looked at the diode calculators on the web, but what they tell me makes about as much sense as brick dust in a birthday cake. I do think from a mechanical mindset, and have lots of hands on experience with electricity, but when it comes to things like this, I'm lost. Can you guys help me figure this out? I'd really appreciate it. Thanks.
 

Steve K

Flashlight Enthusiast
Joined
Jun 10, 2002
Messages
2,786
Location
Peoria, IL
Generally speaking, I think you'd be much better off by using a simple linear voltage regulator upstream of your pucks instead of some diodes in series.

If you do want to use diodes, something like the cheap 1N4001 (or higher voltage versions like the 1N4002, 1N4004, etc) would be suitable. The problem with using diodes is that they only lower the voltage while there is current being drawn. The pucks will pull current in surges, so when there's no current being drawn, the voltage at the puck's input will be 12v.

Also.. you might find out that there are some nasty transient voltages on a vehicle's 12v bus. I'd recommend a diode in series with your electronics just to block negative transients, and a 5 watt 18v zener diode across the power (but after the series diode) to clamp positive voltage transients to a semi-safe level. It would make it safe for a LM317 regulator, at least. The cathode of the zener would go to the positive 12v, and the anode would go to the negative side.
Actually, there's a decent application note from Vishay about Transient Voltage Suppression diodes (similar to zeners) for automotive uses..
http://www.vishay.com/docs/88490/tvs.pdf
 

lordraiden

Newly Enlightened
Joined
Jan 24, 2013
Messages
89
Hmm, it sounds like the diodes may be more than I want to fuss with. So, moving to the topic of voltage regulators, what kind or model of regulator would you suggest? I found some dc to dc converters on Ebay that allow for stepping down to 6v. Would those work? They call them a "Boost Buck Step Up Step Down Voltage Module".
 

JohnR66

Flashlight Enthusiast
Joined
Aug 1, 2007
Messages
1,052
Location
SW Ohio
I assume the LED pucks with a 6 volt supply may have some sort of driver inside. You can use the DC converters if you don't exceed their current rating. Good quality converters won't be cheap. Beware of the imported cheap junk (ebay). For a regulated solution, I use the LM338 5 amp IC or the LM1084 5 amp low dropout IC. The voltage across the in and out termals times the current through it is turned into waste heat watts, so they are not efficient and you need a heatsink with fan cooling.
 
Last edited:

Steve K

Flashlight Enthusiast
Joined
Jun 10, 2002
Messages
2,786
Location
Peoria, IL
A good solution would require detailed knowledge of the lights that you were using. If you are using individual LEDs, you could try just wiring them as 3 LEDs in series with a suitable resistor in series with the LEDs. Each string like this could be wired across the battery power. That would be reasonably efficient and do a tolerable job of controlling the current through the LEDs. I'd still have a diode in series with the battery power to protect the LEDs from reverse voltage spikes.

A buck mode current regulator rated for use in automotive applications might be good too. I don't know if anyone makes them, though. Most stuff is designed for clean electrical power, such as battery power.

A LM317 can be configured as a current regulator, and could be used in place of the resistor mentioned above. It would provide a more controlled current, and can withstand positive input voltages up to 36v or so. A bit more complicated, requiring some ability to assembly electronics.
 

lordraiden

Newly Enlightened
Joined
Jan 24, 2013
Messages
89
SteveK, it's funny you should mention an automotive voltage regulator. I actually found some offered by Autozone that take 12v car power and convert it to 6v for certain applications. I wasn't sure what to think about those, but since you've mentioned them, I guess they're worth checking out at least. If nothing else they might prevent a solution to my issue. :) Plus, since they're built for cars, they likely would take into account for the variability you'd experience from vehicle power. JohnR66, surprisingly enough, there's no driver. Just 3 big old LED's wired to the battery connector. I guess the big old "6v power only!" sticker was enough to convince people not to hit them with too much voltage. lol.
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
surprisingly enough, there's no driver. Just 3 big old LED's wired to the battery connector.

First of all, if the whole puck is rated at 1.5W, those aren't 'big' LEDs ;-).

Second, if there's nothing there but the LEDs, you really don't want to connect them to a voltage regulator. Some sort of current regulation would be better. You want about 250 mA. The LM317 in current regulator mode and a 30-ish volt transient suppressor would work well.

A very simple and pretty effective circuit to drive and protect them would consist of two 12 ohm resistors and a 10V unipolar transient suppressor diode (or zener diode, which is almost the same thing). Using 1 watt resistors and a 300-600W suppressor (transient rating, not steady-state!) would protect from the normal day-to-day stuff pretty well, though the resistors would run pretty hot. 2W or 3W resistors would run cooler.

To protect from crazy stuff like reverse voltage and 24V jump (lots of tow trucks connect their 24V system to your 12V system to jump you!) you'd need beefier parts. One of the resistors would dissipate around 12W under reverse voltage, and under 24V jump it would dissipate 27W, while the transient suppressor got 15W. A 12W resistor isn't too bad, but the others would be pretty massive parts.
 
Top