13.8-12.0 vdc to 12 vdc and 5vdc

Discussion in 'The Projects Forum' started by livnlo, Aug 26, 2008.

  1. livnlo

    Thread Starter New Member

    Aug 26, 2008
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    new to the board but have been reading for the last few hours and i am in need of advise. what i need to build is a dc to dc regulated power supply. this setup is going into an off road race car to record the in car camera. the input power will vary from 13.8 down to 12 volts. the camera needs12vdc at 500ma. the recorder needs 5vdc at a low of 1000ma and a high of 2000ma. it was recommended not to use 2000ma as this might over charge the battery in the recorder and to use 1500ma. i want to regulate the camera supply so i wont have any issues if the voltage goes up over 12vdc. i am not worried about how efficient it is other then having to run fans or a large heat sink. i would like to put it in a project box that is 5" x 2.5" x 1.5" if possible. my main concern is protecting the equipment. cost isn't so much of an issue unless your talking about hundreds. do i just go get a 7805 - 7812 - some 10uf caps and 1" heat sinks??? it just seems to simple. and for a total newbie question how do i control or limit the milla amps to each device, threw the regulator rating??
    any suggestions or advise is greatly appreciated and thank you in advance for your time
     
  2. SgtWookie

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    That's do-able ;)
    Down to 12? Is there no alternator in the car?

    Usually, anytime an alternator is running, it's putting out at least 13.4v even if no accessories are on. Immediately after starting, voltage can go up to perhaps 14.4v, depending upon how depleted the battery is.

    A fully charged battery will measure 12.6v-12.7v.

    That's odd - maybe they were talking about if you were using a "wall wart" type power supply. Wall warts are not typically regulated supplies.
    For your application, neither would work.
    A 7812 linear regulator has a "dropout voltage" of 2. That means in order to ensure a properly regulated 12v output, a minimum of 14v must be input. Otherwise, if the input dropped below 14v, the output voltage would be 2v less than the input voltage.
    You need a low dropout voltage regulator, or better, a DC-DC converter.
    I found a 12v .5A DC-DC converter, P/N OBR12SC12, that's small for $30 on Mouser.com:
    http://www.mouser.com/Search/ProductDetail.aspx?qs=4E3eX0Dht1i/pgNIeSJHDQ==
    Datasheet:
    http://www.mouser.com/catalog/specsheets/618OBRSC.pdf
    It's 78% efficient, which isn't bad - considering that it'll accept anything from 9v to 18v in, and output 12v at 1/2 amp. You won't get that kind of efficiency from a linear regulator - and in a race car, you want as little power wasted as possible. You won't need a heat sink for it, and it only weighs 12 grams.
    It'll need to be mounted on a circuit board, but so would most anything.

    The 5v supply is another matter. The 7805 is only rated for 1A output. If you really need 1.5A, you'll need something like an LM317 which is rated for that, or better yet a dc-dc "buck" converter. The LM317 will require a good-sized heatsink, because at worst case, it will be dissipating a good bit of power; if your alternator is putting out 14.4v and the regulator output is 5v @ 1.5a, the regulator will have to drop 14.4v-5v= 9.4v times 1.5A = 14.1 Watts of power. It would be nice if that 14.1 Watts could be used to power the rear wheels instead of heat up the driver's compartment.

    However, it's quite late and frankly I'm a bit too tired to design a dc-dc buck converter supply right now.

    Well, normally what you do is build a supply that can output at least the current required at the working voltage rating of the device. In your case for the camera, it requires 12v, 500mA, or 0.5A. That one's in the bag with an off-the-shelf solution. Someone else might have a suitable 5v 1.5a DC-DC converter, but like I said, I'm tired.
     
    Last edited: Aug 26, 2008
  3. SgtWookie

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  4. blocco a spirale

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    A simple shunt regulator is worth considering for the 12V supply. The 500mA current is a little high so it won't be very efficient but if efficiency is not a priority...
     
    Last edited: Aug 26, 2008
  5. SgtWookie

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    A shunt regulator would initially be less expensive, but I feel that it would cost in the long run due to the power consumed. It could also lead to an unstable/low voltage condition if the alternator failed, which may very negatively impact the operation of the camera.

    I'm not very familiar with racing. I can only imagine that the camera could be used by the pit crew to aid in diagnostics while the driver is zipping around the track. If that's the case, then one would want to ensure that the camera will keep working down to exhaustion of the battery. Since a 12.6v battery is considered completely discharged, the dc-dc converter I suggested will keep the camera going even after the battery is flat.

    Another consideration may be camera operation while starting the engine. This might result in an input voltage level low enough to knock the camera offline, even if using the dc-dc converter I suggested. But with a shunt regulator, losing the camera for a few moments would be virtually certain.

    While our OP said that they really weren't too concerned with efficiency, and it might be argued that what power might be wasted using linear regulators is negligible compared to the many kW's generated by the vehicles' engine, I'll suggest that every milliwatt saved from waste is equivalent to adding that power to the engine; every ounce of weight saved equates to improved acceleration, braking and cornering.

    The main issue was to protect the equipment by supplying it with the correct power, and keep the component size reasonably small. While they could probably work out a cheaper solution if they researched long enough, about $65 for the basic components that will do everything required and more, while being even smaller than specified and generating little heat - they could do a lot worse.
     
  6. livnlo

    Thread Starter New Member

    Aug 26, 2008
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    thank you all for the recommendations and advise. the camera in the car is just for viewing after the race is over. i am not worried about power consumption as the car does have an alternator and two large odyssey batteries. the camera itself needs constant power to operate, the recorder does have a lithium ion battery so it just needs power to keep the battery topped off so to speak. most races last anywhere from 4 to 6 hours unless its the baja 1000. after reading some more on the specs of the camera it shows a power consumption of only 200ma, im not sure if that affetcs what you would recomend. if you want to look at what im using here are the model numbers.
    camera - speco cvc-6805ex
    recorder - archos 605
     
  7. SgtWookie

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    The camera in the car isn't used until the race is over? Why even carry it then? :confused:

    Isn't it being used to at least tape the race?

    As far as the 200mA vs 500mA specs; it doesn't hurt to have overcapacity. That will let the dc-dc converter run even cooler than it would with a full load.

    If you're not going to be running the camera while driving though, there's not much point in having such an efficient design. Just shut the engine off and turn on the camera. You'll be within 5% of the specified 12v supply from when the battery is fully charged until about 50% discharged.
     
  8. livnlo

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    Aug 26, 2008
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    the camera records the whole race while driving for video and 2way audio communications with the driver. this setup it not intended for use to diagnose the car if there is a problem. check out this link and see what we are doing with this setup.

    http://www.youtube.com/watch?v=0GWOnjFnikM&feature=related
     
  9. blocco a spirale

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    A switching regulator is definitely the best solution, I just wasn't sure how involved livnlo is prepared to get with the electronics.

    Another possibility may be a capacitive charge-pump (to overcome the regulator fV) followed by a 317 linear regulator. I have no idea whether this would supply enough current but it wouldn't need to boost the supply voltage by much so it might just work.

    Just another thought that would avoid doing any electronics at all - a small inverter (almost free these days) running a couple of plug-top power supplies.
     
    Last edited: Aug 26, 2008
  10. SgtWookie

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    Pretty impressive ;)

    Well, I'll stand on my initial recommendations. With that kind of a race, you really do need to keep weight down and efficiency up.

    You're going to need to build it very rugged though. Looks like the car gets a pretty good beating. Having the regulators so efficient helps, because you won't have the added weight of heat sinks, fans, etc - that would otherwise help to beat it to death. Once you get it working (fairly simple matter of connecting things up) you could encapsulate it with some JB Weld or potting compound to help protect it from vibration.
     
  11. SgtWookie

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    Agreed - that's why I looked for an off-the-shelf solution that all he'd have to do was plug the thing into a circuit board and make a couple of connections. It's almost the same level of simplicity as 7812/7805, but the former pair really wouldn't work nearly as well in this application.

    That's a possibility, but I'm not aware of capacitive charge pumps that output 500ma. 200mA or 250mA might be do-able, but then there's more selection of external parts, laying out a circuit board, getting it etched - suddenly, you've spent quite a bit more than the complete OTS board would've cost in the first place.

    That's true, too. I have a small 400W unit that seems to be pretty efficient. However, it has a fan in it that pulls air through it for cooling. Looks like they're spending lots of time on trails that are extremely dusty, and might turn to mud at a moments notice. In that case, a sealed unit that doesn't need a fan for cooling would be a good bit more rugged and reliable, were it assembled/potted correctly. You would have to zip-tie the wall warts to the converters to make sure they couldn't fall out. They would certainly take up more space; and if they broke loose could become little missiles flying around the cabin.

    The flip side is that converters and wall warts are available practically everywhere.
     
  12. livnlo

    Thread Starter New Member

    Aug 26, 2008
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    space and weight is a concern but how much weight could this power supply actually be when its done?? i really didn't want to use an inverter and have two ac/dc transformers, bulky and that just seems like i didn't want to spend the time to make it as good as i could be. and as far as my skill level, i am sure i am know where near your levels but with the proper direction i am sure i can build it. now for the next issue, i leave for the race this Thursday and cant find those parts locally. i can run the recorder off the built in battery for about four hours but the camera needs to be hooked to the car. can you come up with something i can build with readily available parts??
     
  13. blocco a spirale

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    The total power consumption is only around 10W, so a fanless 50-100W inverter would be ok.

    For the charge-pump idea, I was thinking of something pretty simple such as a 555 driving a couple of transistors driving the usual voltage-doubler type diode+capacitor arrangement and all built on a bit of stripboard. It might just work but this is one for experimentation and isn't guaranteed.
     
  14. SgtWookie

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    With the parts I specified, very small and very lightweight.
    I'm not even suggesting that.
    Oh, boy. I have no clue where you are. Mouser is in Texas somewhere. I don't know where Digikey is. You could always have them overnight the parts, but that's expensive and you have very little time.

    That's a pretty tall order for such short notice, especially since we have no way of knowing what's available to you. How about your city and state? It's a good idea to put that in your profile.
     
  15. SgtWookie

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    I hadn't seen them that small, but sounds good to me.

    Yeah, with the very limited amount of time our OP has, there's not really any time for experimentation - and we don't have a clue as to what he might be able to lay hands on.

    Radio Shack really isn't a lot of help - their selection is so limited nowadays. :(
     
  16. SgtWookie

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    Well, what I'm thinking at the moment is using a couple of RS's TIP42G PNP transistors as pass transistors to charge up caps on the collectors.

    Use a couple comparators in an LM339 to switch some 2N3904's or MPS2222's on and off to pull down the bases of the TIP42G's.
    Use a couple of Zener diodes to set up the reference voltages for the comparators. They carry 100uH 2A chokes that might be used as inductors. Wouldn't be as efficient as toroids, but at this point options are running out in a big fat hurry.
     
  17. livnlo

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    Aug 26, 2008
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    sorry. i am located in phoenix, arizona. i called the few places here that i deal with and they both said go to digi key and mouser. the car leaves for the race 10:00am thursday so i have very little time left. i do have to try something so what i think i will try for the camera is to see if i can get it to run on 10 volts and then just use a 7812. tonight i will also test the voltage at the connector at hi revs to see where its at voltage wise.
     
  18. SgtWookie

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    OK.
    As already discussed, your camera with the 7812 is probably going to be running at 13.4v-2v, or 11.4v. At least the regulator won't be getting too hot. You should have a small heat sink on it.

    The 7805 is going to be maxed out. You will need a big heat sink for it. It's rated for 15Watts, and for this application it will need to dissipate 14.1 Watts. The tab is ground, so you don't have to worry about shorting it out to ground. You must use heat sink compound (Radio Shack has it), and a big heat sink. If you can find a copper Pentium heat sink with a fan at a computer supply store, and can drill and tap a hole (or use a sheet metal screw) in the bottom of it, and power the fan from the car's electrical system you might be OK.

    You could actually mount both regulators to the one heat sink. Put the 7805 in the center, and the 7812 near one corner.

    Copper heat sinks are much better than aluminum. They conduct the heat away much faster.

    You should use some capacitors on the inputs and outputs. For the input, use a 100uF. For the output, use a 10uF and a 0.1uF ceramic, tantalum or poly cap. The voltage ratings for the caps should be 25v or higher.

    This is just a stop-gap emergency supply kind of thing. I suggest you follow through and order the parts I recommended previously, so you will have plenty of time to build it and test it prior to the next race.
     
    Last edited: Aug 27, 2008
  19. blocco a spirale

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    I don't think a 7812 will work but a 317, which has a 1.2V drop-out, might. 12V in, 10.8V out. I think there is a good chance that the camera will work on 10.8V.

    Back of an envelope moment. The attached circuit should also work. The output will pretty much track the input but will not exceed around 11.8V.

    Edit: Put a 10k resistor on the base of Q2. The transistors are not critical - just about any small general purpose transistors for Q1 and Q2 and just about any small (or big) power transistor for Q3 with a hFE of 100 or more. No parts are critical, the max output voltage is equal to ZD1 + Q2 Vbe
     
    Last edited: Aug 27, 2008
  20. SgtWookie

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    According to National Semiconductor's April 2007 datasheet for the LM117/LM317, the dropout voltage for a 500mA load will be approximately 1.7v to 1.8v between 25°C-50°C. See the attached chart, excerpted from that datasheet.

    The LM78xx series datasheet from Fairchild Semiconductor does not contain such a chart; it merely states a 2.0V typical dropout voltage.

    The LM317 would likely be a better way to go for both regulators, but at this stage of the game and the rugged environment in which the regulators must function, I felt that the least complexity offered the best immediate solution.

    However, I've attached a pair of schematics, the 1st using 78xx regulators, the 2nd using LM317 regulators.

    The LM317 5v regulator uses a 510 Ohm regulator for the reference voltage path. This helps to make more current available for the 5v load. Both LM317 regulators must have at least a 10mA load on them to ensure proper voltage regulation (a 470 Ohm resistor would work fine) during output voltage adjustment via the pots. I suggest using multiple-turn sealed pots.

    [eta] BEWARE! With the LM317, the tab is Vout! This means increased complexity for mounting the LM317; you must use a TO220 mounting kit to electrically insulate the tab of the regulator from the heat sink. The kit consists of a thin thermally conductive sheet and a stepped washer to insulate the screw. Failure to use the insulation kit properly will likely cause rapid burn-out of one or both regulators.
     
    Last edited: Aug 27, 2008
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