Installing Cabinet LEDs in my Workvan

[FvnnyL3tt3r1ng]

For the inside ceiling of my workvan:

So the LED light fixtures I bought from harbor freight use 3 AAA batteries in series or ~4.5V. There are two separate circuits of these on the fixture so all 6 lights be powered.

I'm not sure on the power draw (yet) but I need to step down my battery/alternator input (12.5-14v) to the 9v needed for the lights rigged up in series.

Alternatively, I could step up to 18v and wire all 4 circuits in series as well.

After a little research it seems that they sell "Buck converters" specifically for this purpose. I'd rather just solder a few components together from my home supply if I can do it a simpler way.

I know the linear conversion method is said to be inefficient and generates heat but I don't see that being a big problem considering I am only using these lights typically less than 10 minutes at a time.

Could anyone reference me a circuit or give me some pointers on this? Could I just solder enough diodes to induce a big enough voltage drop? I'm thinking 6 of them??? Would I also need to size a resistor?

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

I'm not sure about your arithmetic. You say you have six lights, and each one needs 4.5V. You have two circuits, with presumably three lights on each circuit. 3 × 4.5 is 13.5.

Dropping your automotive supply down to 4.5V using diodes is not a good idea because any disturbances on the supply (and there are many of them) will become an even larger proportion of the 4.5V supply. A linear regulator will waste power and dissipate heat; how much depends on how much current your lights draw.

For your application I recommend using a buck switching regulator with an adjustable output, set to 4.5V. Then connect the lamps in parallel. This is closest to how they are designed to operate.

You can pick up these regulators very cheaply on eBay. Go to eBay and search for LM2596. It's the switching regulator IC that's used on many of these boards, so searching for it will pick up all the ready-made boards that use it. The simple regulator boards (without digital display, etc) are less than $2. Make sure you get an adjustable one.

There is a potential danger with powering these circuits from an automotive supply. They are normally only rated for +40V DC input voltage, but in an automotive environment, voltage spikes and surges can easily be negative, or can exceed that voltage, and the most likely failure mode of a buck regulator is to go short from input to output, which would apply 12V across your lamps and probably blow them up. I recommend using a series diode (http://www.digikey.com/product-detail/en/1N5407-T/1N5407DICT-ND/190951) in the positive supply to the regulator, followed by a voltage clamping device (http://www.digikey.com/product-detail/en/V24ZA50P/F3079-ND/1009392) across the input to the regulator.

 

[FvnnyL3tt3r1ng]

Ok, I'll check it out tomorrow,thanks!

P.S. The 6 I was counting were the diodes in series...just speculation.

 

[FvnnyL3tt3r1ng]

Does it matter that the minV for the Varister is 20.7 or was your intention just to present me with an example? Also the Diode is listed @ 1V forward, would this be the voltage drop during normal operation?

 

[KrisBlueNZ]

I thought that varistor would be suitable. The minimum voltage is just the lowest voltage where it will start to clamp. It doesn't mean that it needs at least that much voltage! As long as its maximum clamping voltage is less than the maximum allowable input voltage of the regulator, it will do its job.

Yes, the diode will drop about 1V during operation. That will waste a bit of power. What is the total load of all the lamps? If they're designed to be powered from three AAA cells, they probably won't draw more than 1W each. That's 4W total, multiplied by regulator inefficiency is about 5W, which at 11V is 460 mA, which for a 1V drop is 0.46W. So it will get a bit warm, and will waste about 10% of the total power used.

 

[FvnnyL3tt3r1ng]

Excellent! Thank you for the quick response.

I don't think I have any AAA batteries to test the power draw right now. I just looked at the fixture and I was incorrect in my original post description. There are two reflectors containing 3 individual LEDs in each one, total of 6 LEDs per circuit (two circuits per fixture). Power requirements are the same however. There was no writing on the fixture denoting power draw and I can't find them online either.

 

[FvnnyL3tt3r1ng]

100mA per circuit for a total of 400mA between the two fixtures.

 

[KTW]

There are many 12volt LED's available, just plug them in or wire them up to 12 volts.

 

[FvnnyL3tt3r1ng]

True, but I had already purchased the fixtures ($14) and had no other use for them. I also am getting back into electronics so it's a fun project to get my feet wet.

 

[sndscientist]

If it were me I'd use 7805 with 1n4001 in series with the output. it would give you about 4.3ish volts. at 400Ma it shouldn't dissipate very much heat. also if these are used as work lights they more than likely won't be on constantly also they probably won't be on while the vehicle is running. if you are worried about automotive spikes put a nice 25v 2200mf cap across the input. it will smooth out the spikes and lags nicely.

 

[FvnnyL3tt3r1ng]

So you'd 86 the buck converter and just wire up the +Vcc with 7805+1n4001? Would it be easy to come by a Cap that size at radioshack?

 

[FvnnyL3tt3r1ng]

Well I dropped by RadioShack on the way home and the largest they have are 4400uF caps rated at 35VDC ($6) I believe so I just wanted to clarify you're recommending a 2.2 Farad capacitor correct?

 

[Gryd3]

If it were me I'd use 7805 with 1n4001 in series with the output. it would give you about 4.3ish volts. at 400Ma it shouldn't dissipate very much heat. also if these are used as work lights they more than likely won't be on constantly also they probably won't be on while the vehicle is running. if you are worried about automotive spikes put a nice 25v 2200mf cap across the input. it will smooth out the spikes and lags nicely.

Remember that 'power' is a function of voltage and current...
So a 5V output will need to drop 9V. At 400mA this is almost 4 Watts with the motor running! You could get away with only wasting 3W with the motor off, but still... this thing is going to get HOT!
Additionally... the 25V Capacitor on the input side of things will only go so far, as the spikes and dips in an automotive supply can be quite large. If you were to isolate the power line from the battery prior to using a cap that would be ideal... otherwise your 2200mF cap will discharge back into the vehicle for anything else that may be demanding power. Remember that the positive wire in most automobiles are common, so without using a diode to prevent your fancy capacitor from feeding your headlights when your radiator fan kicks on you are not really gaining anything except maybe a bulky high pass filter to ground.

I certainly agree with Kris' recommendation of using a buck converter. They are cheap, will run way cooler, and have a higher output current capability. (Which will directly result in a better operational life)
If you are concerned with the buck converter failing you can use a diode on the input as suggested to prevent negative spikes. A capacitor after the diode will help smooth out longer spikes and ripples. (If the buck converter does not have one.)
As for the output, put a fast-acting inline fuse in place. This may not save all of your lights, but if the buck converter fails, or you accidentally short a wire this fuse will save some of your bacon.
(As far as the voltage clamp is concerned that Kris mentioned... it's simply for preventing or mitigating spikes... This is used in surge protectors. If the voltage gets too high the 'clamp' will begin to conduct. Large enough surges will destroy the clamp, but the clamp will prevent the voltage from exceeding a certain threshold. The rating is usually chosen to prevent catastrophic failures to due much higher than expected voltages.)

 

[sndscientist]

no i'm sorry not a 2.2 farad, a 2200uf again sorry i've been building for a long time i just haven't had to explain in a while. yes a 7805 regulates between 4.8 and 5.1 volts that i have seen. now remember that as with any power components it will get warm. if you are really only planning to run it for a few mins at a time a small heat sink or attaching it to the vehicle body should suffice. again if it were me, and i admit i am a tad unorthodox with some of my installs i would sink it directly to the vehicle body. the tab of the 7805 is common to the ground so it won't hurt anything. the only reason i'd "86" the buck converter is honestly because i look for the lowest cost way to do things. if you were using $100 worth of led pods, yes i would also agree to use switching supplies for max efficiency. but being that these are made to run on 3 AA batteries, and you stated a max on time of around 10 mins per run i'd hit the regulator.

now the 1n4001 will give you a voltage drop of around .6 volts. just right in series with the regulator output. it will get you right around the 4.2v - 4.7v range. while 4.5 is ideal if you measure some AA batteries you will see them around 1.6volts. 1.5 in nominal and what they shoot for but it's hard to control the acid and differences in the metal. also they do run fine on 1.2 volt Nimh batteries. i'm telling you this because the acceptable range suddenly becomes 3.6v - 4.8v. so if you build it and it's 4.3 volts or 4.6 it's not an issue. granted the lower voltage you get the longer the leds should last. however these are harbor freight parts. they fail whenever they want for whatever reason they want

Remember that 'power' is a function of voltage and current...
So a 5V output will need to drop 9V. At 400mA this is almost 4 Watts with the motor running! You could get away with only wasting 3W with the motor off, but still... this thing is going to get HOT!
Additionally... the 25V Capacitor on the input side of things will only go so far, as the spikes and dips in an automotive supply can be quite large. If you were to isolate the power line from the battery prior to using a cap that would be ideal... otherwise your 2200mF cap will discharge back into the vehicle for anything else that may be demanding power. Remember that the positive wire in most automobiles are common, so without using a diode to prevent your fancy capacitor from feeding your headlights when your radiator fan kicks on you are not really gaining anything except maybe a bulky high pass filter to ground.

this may be true but in my lifetime of installing things in cars if the line voltage falls enough that my "fancy capacitor" discharges back into my radiator fan or headlights then i have a problem much more serious than if my lights work in the back. likewise if it spikes high enough to destroy a 7805 i'd be more concerned that the regulators running the pcm would get hurt rather than the leds in the back. having that said. if you choose to run a buck regulator by all means go for it. i won't say that anything is better or worse. but from my point of view efficiency in an automotive system dealing with less than an amp isn't an issue. as for heat dissipation. the tab on the 7805 is ground. it's really nothing to screw it on to a part of the van.

unless i read wrong the OP isn't asking for help building a super efficient mission critical life support device that resists all forms of EMF and solar flares, he or she is asking for help hooking up a few harbor freight led pods (which ironically enough i have here too) to run off of his van.

(apologies for the grammatical errors, it's been a long day)

 

[FvnnyL3tt3r1ng]

Great discussion! I did order a buck converter for about $3 (Shenzen, China) so I will end up throwing that in. I don't believe I mentioned I will have an output of 9v from the converter so I will be soldering battery terminals of the LEDs in series. This shouldn't be an issue should it?

It seems there is a chip in the fixutre that allows for two different light outputs after a pushbutton is pressed (High,Low,Off). I'm not worried about using the Low output and I already tested that the circuit will turn on HIGH just by applying power after I soldered the pushbutton contacts together with a wire. I am curious which chip you think this is? I will be taking a closer look at it after I break open the 2nd fixture to solder that pushbutton's terminals hot all the time.

Thanks for the help

 

[sndscientist]

dumb curiosity, which HF light is it? the IC might cause you a problem, if it's a momentary switch and the ic controls everything. there is no telling what might happen if one turns on before the other or if one is dim and the other is off.

 

[FvnnyL3tt3r1ng]

It's the model that has 4 rotating pucks attached to a rectangular plastic housing. It's advertised for under cabinets.

 

[sndscientist]

i just took a look at them, they are neat little things. only potential problem you might have with a series circuit is that they do draw a bit of current while off. meaning unless they are doing the exact same thing at the exact same time you very well might burn them out.

 

[FvnnyL3tt3r1ng]

Maybe I should re-think the series layout. I guess I'll just order another converter rated for the 4.6V

 

[Gryd3]

Maybe I should re-think the series layout. I guess I'll just order another converter rated for the 4.6V

Or an adjustable output would be perfect
Series could produce some odd results depending on how the lights are wired internally.