New Project Feedback

Discussion in 'The Projects Forum' started by charabb, Oct 10, 2010.

  1. charabb

    Thread Starter New Member

    Oct 9, 2010
    6
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    Hello. This is my first time to post, but after reading around a bit I am looking forward to the feedback.

    I have a vehicle wiring project that I'd like feed back on.

    The vehicle is an SUV in which I have an equipment box with certain electrical needs:

    1.) First it needs a heater to keep certain supplies from freezing at night (I live in the South, so nights are usually the only time that heating is needed). The truck is a diesel with a block heater. To supply heat the box has a AC heater installed with a thermo-cube thermostat to regulate and maintain a minimum temp of 35 degrees. To supply power a deditated AC line will be mounted under the hood and exit via the front grill (much like the current block heater line does) and end with a power strip. The block heater will plug into this power strip with some form of switching (yet to be determined) that will allow it to be turned as needed. Another extention cord will also plug into the power supply and run back to the box to supply AC power to the heater there. The truck will be manually plugged into AC supply on cold nights.

    2.) Second there needs to be a DC power supply to the box. This will easily come from the batteries of the truck back to the box and terminate at a "power supply" strip to allow multiple applications. I'll probably run a "always hot" line and a "ignition switched hot" line.

    A - First application will be lights. For this, I will just find some 12vdc lights and wire them directly to the power strip with appropriate switches.

    B - Second application is dc supply to power a charger for a piece of cordless equipment - however, I would also like to be able to use the AC power to supply charging when the truck is plugged in on cold nights. This is the most challenging application. My thought were:

    For AC: the charge already has a AC power supply with appropriate current modification (110AC to 5v DC 1000mA). I can simply plug the charger into the same AC supply coming to the heater (but not be controlled by the thermo-cube).

    For DC: I need a good way to regulate down the 14-12volt supply from the batteries. Any easy suggestions??

    There there should never be a time that the AC is plugged in while the truck is running (and therefore also supplying DC), but I figure that the dc supply probably needs to be "disconnected" while the AC supply is charging and so that when the truck is not running the charger will not be drawing current off the batteries. To do this I was going to put a relay in place between the battery and whatever converter application I came up with to drop the dc voltage. The relay power will be switched with the ignition, that way, when ever the truck is not running the relay will disconnect the dc power supply from the battery to the charger.

    I have attached an image/diagram of what I am proposing.

    Thanks for thoughts from anyone.

    [​IMG]
     
  2. wayneh

    Expert

    Sep 9, 2010
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    This is all a bit confusing, but I'll ask a stupid question regarding 2B: Why not just use a cheap inverter (to take 12v DC to 110v AC) to power up your AC adapters? It's maybe not the most efficient way to go but it's what everybody does.
     
  3. bitrex

    Member

    Dec 13, 2009
    79
    4
    Sounds like you're well on your way to a solution. Here are my thoughts:

    Straightforward enough.

    Buck converters that take a wide range of input voltages and put out 5 volts in the current ranges you require are commodity hardware. For example:

    http://cgi.ebay.com/DC-DC-Converter...ltDomain_0&hash=item2eb05f2f7e#ht_3196wt_1059

    Don't know about the quality, but hey the price is right! :cool: Lots of similar items available on Ebay and sites like Mouser and Digi-Key as well.

    To have a foolproof system, you should account for all the possible states and make sure the output isn't undefined when something "abnormal" happens. Sure, there should never be a time that the AC is plugged in when the truck is running, but I sure wouldn't trust myself to always remember!
    It seems like there are 4 states that things could be in:

    Truck not running, AC supply not plugged in. Load disconnected? I'm assuming that because you don't want to run down the battery.

    Truck not running, AC supply plugged in. Load connected to AC-DC power supply.

    Truck running, AC supply not plugged in. Load connected to DC-DC converter.

    Truck running, AC supply plugged in. Oops, someone made a mistake! What should the default be? Should it default to connecting the load to the DC-DC converter, turn on a warning light, or do something else?

    It would be straightforward enough to build a system to handle these states with digital logic, or better yet, a microcontroller driving solid state relays. However, it can probably be done with just relay logic with a little thought. As a first attempt, how about this: put a break before make DPDT in line with the load, with one set of poles going to the AC-DC converter, and one set of poles going to the DC-DC converter. The relay's drive signal is connected to the AC-DC converter's output, arranged so that when the AC line is plugged in the relay connects the AC-DC converter to the load. Then, between the DC-DC converter and the first DPDT put another DPDT relay, with its drive taken from the "engine running" ignition line, arranged so when the engine is running the relay connects. Put another DPDT relay in line with the AC-DC converter, and connect its drive to an _inverted_ version of the ignition line, so that when the ignition is on the relay disconnects.

    If I've thought this out correctly (I'm sure someone will point it out if I haven't :D) it should account for all the states, with the "error" state handled by having the load disconnected (AC-DC line disconnected, DC-DC line disconnected because the AC powered relay is pulled in). It would not be too difficult to sense the "error" state and turn on a warning light, simply connect a "NOR" gate using the drive lines from the AC-DC conveter's relay and the DC-DC line of the "main" relay. A "0" on both lines would indicate that the system was in an error condition. Pulldown resistors would be added to make sure the NOR gate input was always well-defined.
     
  4. bitrex

    Member

    Dec 13, 2009
    79
    4
    If he's going to be using the AC adapter to charge equipment overnight, it's probably best to have it plugged into the house line to avoid running down the battery. I don't know exactly what the energy capacity of the equipment he's charging is, but automotive batteries don't like to be deep-cycled.
     
    Last edited: Oct 11, 2010
  5. wayneh

    Expert

    Sep 9, 2010
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    Right, I meant for when an outside AC feed is unavailable. For instance when I'm using my laptop in the car and need to recharge. PLug in the inverter, plug in the computer's AC adapter, and voila. I think part of the OP's question is how you might switch from one state to the other without manually unplugging from one power strip and plugging into another.
     
  6. bitrex

    Member

    Dec 13, 2009
    79
    4
    Ah, I see. I guess if you were to use a DC-AC inverter to power the charger when the truck was running it might simplify things. Then you'd have to devise a switching arrangement where the mains input to the charger is switched between the DC-AC inverter and the house mains depending upon whether the truck was running or not.

    One advantage of my proposal above is that you're always working with low voltages.
     
  7. bitrex

    Member

    Dec 13, 2009
    79
    4
    Eeeh, something I forgot to think of - I'm not certain from the OP's post exactly how the charging circuitry is set up. Is the charger for the equipment _built in_ to the AC adapter, or is the charger a separate piece of equipment that just takes a DC supply? If it's the former, my idea won't work. :(
     
  8. charabb

    Thread Starter New Member

    Oct 9, 2010
    6
    0
    Hi Guys - thanks for your thoughts!

    To answer a few questions:

    YES - I want to make the only "manual" part of this the plugging in of the AC main to the truck on cold nights.

    YES and NO - The equipment charger is designed to be plugged into AC power supply. The AC cord (with transformer box at the plug) plugs into the back of the charger base and the equipment cradles in the base for charging. I will have to open it up the cradle and and a DC input so that I can maintain use of the factory AC line input, but I did not think this too big of a problem.

    I like the idea of keeping the current small, but also to use an inverter I would have to manually unplug the (only) AC line I have to the cradle from the inverter to the main AC on cold nights.

    From what I understand you are suggesting a second relay that when the AC main is plugged in will switch off the DC line - correct? I presume that this will require a different relay than the 12v standard auto relay that I was planning on using for the switch on.

    I will check out the buck converter for the 14/12v to 5v dc drop... another suggestion I found elsewhere was to use a cell phone charger that plugs into the cig. lighter. I tested mine with my voltmeter and it showed 5v out... think one of these might work?

    thanks again
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Yes, I have several of those and they're very handy for all the USB-powered devices. I recommend the ones that have the USB port available directly from at the adapter, rather than with an attached cable. This is more versatile because newer devices are using smaller and smaller USB connectors. Having the port allows you to use your own cable with the right connector, instead of being locked into one output.

    These adapters can be found very cheaply, and I've even built my own with the 7805 inside an Altoids tin. That approach is less elegant since it burns off excess voltage as heat, but that's not a problem with cellphones and iPods and such.

    About the only caveat I have about recommending these is that SOME devices, most notably iPods, need to see a certain signal on the data pins in order to initiate charging. In the devices I make, I include pull-up and pull-down resistors to satisfy this problem. The store-bought versions I have DON'T WORK with my iPod.
     
  10. charabb

    Thread Starter New Member

    Oct 9, 2010
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    Well, this is not for a cell phone or mp3 player... it is for a piece of equipment that has a rechargeable battery that is removable.. the cradle takes 5dcv 1000mA (according to the notes on the box/plug on the AC line)
     
  11. wayneh

    Expert

    Sep 9, 2010
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    I think the typical cheapo adapter is 500mA or less. You may have to look longer to find one with a 1A rating. Or just take the chance that the cradle rarely draws a full amp, and see if a cheap adapter can do the job. Got an ammeter on your multimeter? You could check it.
     
  12. charabb

    Thread Starter New Member

    Oct 9, 2010
    6
    0
    yes, my multi has an ammeter... but i've never used it before... do I simply plug in the adapter and then stick the leads to the end?
     
  13. wayneh

    Expert

    Sep 9, 2010
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    NO!! The ammeter must be in SERIES with the load. The ammeter will complete the circuit and internally uses a low ohms resistor, across which the voltage is measured to compute the current. Connecting that resistor across the poles would essentially short your device and overload your meter.

    If your meter has multiple ranges, start with the biggest one (eg. 10A) and then move down to the mA range if that's appropriate.
     
  14. charabb

    Thread Starter New Member

    Oct 9, 2010
    6
    0
    okay, that makes sense...
     
  15. charabb

    Thread Starter New Member

    Oct 9, 2010
    6
    0
    Looks like the cradle draws a max of 250mA and a min of 150mA. I has two locations for charging - "fast" and "slow". When the equipment was in the slow side it drew 250mA... in the fast side it drew between 150 and 165mA... so maybe the cell phone charger would work... I don't think mine have a mA rating on them... any way to tell for sure?
     
  16. wayneh

    Expert

    Sep 9, 2010
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    I checked my cheapo 12v-to-USB 5v adapter and it was rated 1A, not the 0.5A I thought before. So maybe you'll have no problem.
     
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