Author Topic: !2 Volt to 6 Volt (Read 1424 times)

bilmac · « **on:** November 29, 2009, 05:16:44 AM »

I have several lights, LED and flourescent, that are designed to run on dry cells. I would like to hard wire them into my 12 volt solar system. Is there just one resistor that I can use for any application? Will putting a resistor in the circuit increase the power consumption?

All the LED and flourescent lights that I can find that are designed for RV's, or boats are twice or three times the cost of tent lights or the dry cell run closet lights for the home.

ShadowMover · « **Reply #1 on:** November 29, 2009, 04:21:16 PM »

The answer is yes, BUT. The but takes the form of a different sized resistor for each light, depending on the current it draws. The other but is: half of the power you use will be wasted heating up the resistor to 'drop' the voltage. I'll give you the simple but ugly solution first. Measure or determine from specifications, the amount of current your light would draw using the correct 6 volt battery. This current will be the Amps in the formula. Since you want to drop the voltage 7 volts ( most car batteries and 12 volt systems operate from 13 to 13.9 volts) the total voltage is the drop across the resistor and the amount across the light. 6+7=13

To calculate the resistance needed divide the voltage drop needed by the current. Translated this means divide the 7 volts you want to waste by the current in amps. Let's say your light draws 450 milli-amps as measured by your Harbor Freight digital multimeter. This is 450/1000 amps or .45 amp. Dividing 7 by .45 (7/.45)= 15.55 ohms. Lets call it 16 Ohms. If you wanted to be on the safe side and your voltage was actually closer to 14, you would drop it 8 volts. Doing the math 8/.45= 17.7, say 18 Ohms. This is the basic part. Now you have to figure out how much heat that resistor is going to have to dump. Resistors can only dump their rated wattage or they will burn up, and lose their magic. Using the formula for watts Volts x Amps in the last example an 8 volt drop carrying .45 amp would be 8x.45= 3.6 watts. It's OK to use a higher wattage resistor than needed, in fact it is recommended. A 5 watt resistor would do the job. The resistor is hooked up in series with the Light. It doesn't matter which pole it's on, but it's standard to put it in the + plus side
Plus----R----L-----Ground

This site has the information and the formulas:
http://www.the12volt.com/ohm/ohmslaw.asp

Plan (B)etter would be to get a voltage regulator chip and hook it up to do the automatic regulating. This method wastes less power. To get 6 volts you would use the LM7806 chip. The 06 at the end indicates the voltage it puts out. One problem is the amount of current these units will deliver before they 'lose their magic'. They are limited to about 1 Amp (1000 mill amp).

Plan Best is to get 12 volt rated lights. I guarantee you that using resistors or regulators , one day you will be asking yourself, "What's that smell"? , as the magic leaves your old 6 volt light.

I have recently seen the LM7806 regulators on on-line auction.

Matt · « **Reply #2 on:** November 29, 2009, 09:36:52 PM »

Great post ShadowMover...

The lm7806 idea is not a bad one and is the one I would use if I did not want to buy new lights. You can get 10 of them for about 4 bucks shipped on ebay. Each of these are rated at 1.5amps so you could use them in parallel and get 15amps out of all them together. with adequate heat sinks you could ramp it up to get 20amps or more.

I have a hand held monitor that I built to aim CCTV cameras as I mount them. I took an old portable DVD player LCD and built a composite input for the camera. I also took 2 of the DVD player batteries and use them to power the whole thing. The Batteries are 12v but the panel only needs 7.5v and the camera needs 9v so I have a single lm7808 with a resistor to get about 7.6v for the panel and I have 4 lm7809 for the 3amps needed to power the IR bomb and camera. I get great life out of the batteries and have not had a problem with the regulators. Though I agree if you put to much load on them they will let the magic smoke out and stop working but if you build it with your needs in mind you can avoid this problem.

ShadowMover · « **Reply #3 on:** November 30, 2009, 08:38:47 PM »

Thanks for the 'atta boy' on the post.

I'm not sure hooking 10 voltage regulators together in parallel would work to give you a safe and reliable set up. What might happen, is that one of the ten would try to provide a little higher voltage than the others, and would likely kill itself trying to do the job. After it went, the next highest would try, until they all popped. It would be like trying to tow your car with ten pieces of wire , each slightly a different length. The longest would pop first, then the next. I'm not sure that would happen, but I'd do some reading before I tried it. When they hook transistors and diodes in parallel they usually add a ballast resistor in series with each one, to try to even out he current. A few ohms would do it. Finding these low ohm high wattage resistors is a trick sometimes, and they cost more than you might think. This is probably more work and $ than most people want to put in to a money saving idea.

Matt · « **Reply #4 on:** December 02, 2009, 08:13:40 AM »

Quote from: ShadowMover on November 30, 2009, 08:38:47 PM

Thanks for the 'atta boy' on the post.
I'm not sure hooking 10 voltage regulators together in parallel would work to give you a safe and reliable set up. What might happen, is that one of the ten would try to provide a little higher voltage than the others, and would likely kill itself trying to do the job. After it went, the next highest would try, until they all popped. It would be like trying to tow your car with ten pieces of wire , each slightly a different length. The longest would pop first, then the next. I'm not sure that would happen, but I'd do some reading before I tried it. When they hook transistors and diodes in parallel they usually add a ballast resistor in series with each one, to try to even out he current. A few ohms would do it. Finding these low ohm high wattage resistors is a trick sometimes, and they cost more than you might think. This is probably more work and $ than most people want to put in to a money saving idea.

I have used multiple regulators many times and have never had a problem with them. Add a 104 ceramic cap on each one (at the Base) and I think it was a 100uf 35v cap I used on the output leg to smooth it all out. Though I do agree that without a few caps and resistors to balance it all out you might have over heating problems. I always use much larger heat sinks than needed on them though and they can operate at up to 125c. I found a TI article explaining the best method to parallel the regulators a few years ago which is where I got the idea from. But you can always use a LM123 which is a 5v 5amp regulator (I think) which will take much higher loads of course.

Even though many say don't do it it is possible and will work now for how long... who knows but I have had several going for years now with no problem... But don't get me wrong I have let the magic smoke out of more than one before...

Matt

torpedoman · « **Reply #5 on:** December 02, 2009, 09:40:06 AM »

easy way .. measure the resistance of the entire light string and put one resistor in the circuit before the first lamp you will drop half the voltage at the resistor and the other half will power the lights. but you are defeating your purpose the lights will draw more amps because they aren't hot enough to limit the current the heat of the filament limits amp draw that is why you have a surge amp draw when the lights are turned on.

ShadowMover · « **Reply #6 on:** December 02, 2009, 03:00:12 PM »

OK Matt, I understand you have tried it parallel regulators and made it work, using a TI article to guide you. That's handy knowledge to have. I also understand using the extra heat sinks. Thanks for sharing.
Torpedoman, I'm confused about your reference to using a meter to measure the resistance, as the resistance will change as the bulb heats up.. You know this because you then go on to mention they are not hot enough to limit current. I had thought about mentioning light bulbs as current limiters too, but the voltage surge when first turned on kept me from it. Unless your 'load' is something that can stand a little overload, like a bulb or a motor, don't use a light bulb to limit the current. They do an excellent job of limiting current to lead acid storage battery. More details if anybody wants them.

torpedoman · « **Reply #7 on:** December 02, 2009, 03:55:36 PM »

yes, getting old, use an amp meter and find out how many amps the circuit draws you know the applied voltage so you can find resistance using ohms law, get a precision resistor( gold last band) and put in the circuit before the first bulb and that will give you the drop you want.

bilmac · « **Reply #8 on:** December 02, 2009, 06:13:01 PM »

Thanks guys, I was afraid the answer was going to be complicated, and I was afraid that the resistor was also going to be drawing current, ah well there's only a few bucks difference for a 12 V light and it will be way better made.

bilmac · « **Reply #9 on:** December 16, 2009, 06:21:18 PM »

It just occured to me, why not just hook two 6V lights in series?

ShadowMover · « **Reply #10 on:** December 16, 2009, 08:29:19 PM »

Yes, that would work, if the units draw the same current. If they don't draw the same current then the one that draws the least will get too much voltage and go poof. In general it is a good idea.

bilmac · « **Reply #11 on:** December 23, 2009, 10:25:24 AM »

Well I bought another Wal-Mart tent light and mounted one on each end of a 3' 1x6. Ran the wires in a groove on the backside. Ditched the mickey mouse switches and put a good one in the middle. Wired them up series, and connected into one of the fixtures on the ceiling of the camper. Fastened the whole thing to the ceiling. Looks good, and puts out a worthwhile amount of light

I have the camper sitting on some property that I want to keep off the grid.Silly modern campers take a LOT of juice to keep them running. Been trying to keep the battery charged with a 15 watt solar panel. Wasn't enough, bought another, still wasn't enough, even when I adjusted them every time I went by. Now I have a bank of 4 15 watters and I don't think they will keep up either at least in the winter.

efremtags · « **Reply #12 on:** December 23, 2009, 04:12:47 PM »

rough rule of thumb with solar is you need appx 10x the wattage in solar than the load it operates. You also need a lot of battery to store the availble power (appx 3-5 days worth). Chances are you have too little battery as well..

ShadowMover · « **Reply #13 on:** December 23, 2009, 09:06:05 PM »

You can measure what goes in in amps and multiply by the hours and get amp hours. You can generally get the same amount, less the losses. So if you put in the output of four 15 watt panels which in perfect sunshine and angle would put out 60 watts (never will happen) at 13 volts that is about 4 amps (4 amps x 13 volts= 56 watts) If you run it in the sunshine for an hour, you put in 4 amp hours. Figure you are only going to get 75% efficiency from your battery, so you are down to taking 3 amp hours out. In a good 12 hour day you will be lucky to get 8 hours x 4 amps or 32 amp hours out of your set up. After discounting for sun angle, clouds, and battery losses you would be lucky to draw 24 amp hours. Have you ever put a meter on the lights you are using?

bilmac · « **Reply #14 on:** December 24, 2009, 11:47:19 AM »

I think I am getting the light situation under control, With my LED setup that will be about all I use, and only for an hour or two in the evening. What is disgusting about the trailer is that the furnace has to have a blower going or it will not operate. Then in the summer the refrigerator has to have electricity even to run on propane. These new outfits are designed to be hooked up to a tow vehicle all day and hooked up to 110 at night.

ShadowMover · « **Reply #15 on:** December 24, 2009, 08:13:01 PM »

Yes the blower heaters are a real pain, and very power hungry. My first old Nomad had a convection propane heater that was non electric. I'll bet they don't even make them anymore. It was easy to light with a match, and would heat the whole trailer up and then cycle on it's thermostat. Buy yourself one of those propane heaters with the glowing ceramic faces in it. They sell them for bathrooms and as space heaters. You can also just use a small Mr Heater Buddy and hook it up to a hose. I refill the one pound tanks and carry a dozen of them with me. Another ridiculous item is they put electric stoves in them??? Not mine, but some others. stupid

The controls on the fridge and water heater don't use much power, but yes you do need some power to run them. A word of advice, don't waste your money on nickel dime 15 watt panels, they are only good for keeping a battery up when it's not in use. Buy a 100 watt panel from eBay or some reputable place, get yourself a good charge controller too. Expect to pay about $4 a watt for the panel, and $25 to $50 for the controller. The controller is great when you are parked and not using all the power, otherwise it will just boil out the water out of the batteries.

efremtags · « **Reply #16 on:** December 25, 2009, 05:14:51 AM »

If you want to do it right:

1) dump the propane fridge and get a DC fridge designed to run from solar. The Steca 165 will operate off of about 200 watts of solar, and with more power can convert to freezer with digital thermostat.
2) dump the flooded battery and get a sealed battery designed for solar (not a trolling motor battery). In your application it will last almost a lifetime.
3) Get a real set of solar modules, but look online. I would not ebay. The prices have dropped enough to where new first hand stuff is cheap enough to buy. Prices have dropped almost 40% in the last 18 months. There's plenty of suppliers out there, look for the closest to you to save on shipping. Most of this stuff is freigt only do to size.

If I new your wattage, and location (nearest city) I would be happy to design the system for you. I do this for a living. I could design a system for just the lights or the hole trailer if needed (highly recommended to save gas on genset and reduce runtime). i find that in the long run it will be cheaper to design a system 1x with a goal in mind than to piece meal in bits and peaces.

Most of what was said above about battery charging is kinda accurate. Your panel delivers Amps not watts and that is what you need to determine battery recharge. #amp-hours out must be returned with additional 10% for over-voltage inefficiency. You also want to limit your discharge to 80% (about 11.8V on 12V battery) to prevent permanent damage to the battery.

I just cleaned out our office and got a bunch of returned and cosmetically damaged stuff that's been in storage for 3-4 years, and am building a parkin lot light in my yard on the far side where the house lights don't illuminate. That's how I am spnding the holiday weekends after he huntin season closes.