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voltage regulator, 12 volts in, 12 volts out

Discussion in 'Electronic Design' started by CampinGazz, Jan 25, 2004.

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  1. CampinGazz

    CampinGazz Guest

    I've got a lcd TV that uses a switched mode PSU to power it in the home,
    takes 230 volts AC (im in the UK) and outputs 12.33 volts at 4 amps max,

    i bought this TV to go in my motorhome i'm building, and thought i'd just
    make up a straight plug to go from a 12 volt power point i put in to power
    the TV,
    but someone did this with the same model TV that i have in his motorhome,
    and got smoke out of it...

    What happened with his TV, he'd always used the mains PSu when he was
    plugged into shore power, and when he didnt have shore power connected, he'd
    plug the TV straight into the 12 volt leisure battery (lead acid, 12.8 volts
    fully charged, 10.5 volts discharged)
    when he had the TV on battery power alone it worked fine, it worked the
    entire time the battery had usefull power in it, right down to 10.5 volts,
    (no CRT 12 volt tv will do that, they like to die at 11.5 volts)

    Anyway, one day he forgot the mains PSU for the TV, and was hooked upto
    shore power, so he connected the TV to the battery as he would when not on
    shore power, his battery charger was on-line, a switched mode unit set to
    13.8 volts, and upto 12 amps output,

    after about 5 minutes of the TV being on, smoke poured out and it died.

    we can only assume the TV didn't like the 13,8 volts it was being fed,
    thinking the TV was designed to be used with the supplied mains PSU, so it's
    expecting 12.33 volts and no more.

    Now obviousely i dont want to blow my TV, but i need to run it off the 12
    volt battery as i'll be doing a lot of boondocking (camping with no hookups,
    so battery power conservation is a high priority.. so the obvious of using
    an inverter to take 12 volts to 230, then take it back down the 12 volts is
    very wastefull)

    but i also have a high output charger on board, it can put upto 14.8 volts
    into the battery bank for upto 2 hours while it equalizes the battereis,
    then it tapers the voltage down to 13.8 volts once the batteries are fully
    charged, but going back into 14.8 volt boost mode if anything pulls more
    than 20 amps from the batteries.

    and i'd like to be able to have the TV on when i'm driving to keep rear seat
    occupants ammused, and the van's alternator will be feeding 14.8 volts into
    the leisure battery bank (dedicated alternator for the leisure batteries,
    fed through a smart regulator that mimicks the mains chargers 4 stage charge

    So waht i need to do is build a 4 amp voltage regulator that gives no more
    than 12.33 volts out to safeguard the TV,

    That's fine when the batteries are being charged via the mains or alternator
    chargers, but when i'm off grid, the battery voltage will be anything from
    10.5 to 12.8 volts (prolly 13.5 volts for a little while after they have
    been charged..surface charge)
    and all the voltage regulators i know of need the input voltage to be 1 or 2
    volts over the output voltage.

    is there a regulator that can handle 4 amps, that can ensure no more than
    12.33 volts ever gets to the TV, but when the input voltage to the regulator
    drops below the 12.33 volt limit, it drops out of the circuit and allows
    what ever voltage is available to go to the TV.. as long as it's under 12.33
    volts that is,
    and if the cahrger comes back on line or i start the engine with the TV
    powered, it will instiantly cut back in, and ensure 12.33 volts only ever
    gets to the TV.

    How can i do this?
    i'm wondering if it wasnt just over voltage that killed this other blokes
    TV, but a less than perfect supply from the charger, when the alternator is
    on line it will be a very dirty power source as it's far from smooth DC it
    produces, so i'll need a smoothing circuit as well.

  2. Well, you certainly described the problem in detail! Yes, it is possible
    to make a device that will do what you want, but it's a custom design
    job and I'd rather leave it to people with experience of precisely this
    type of circuit to advise you.

    My approach, for what it's worth, would be an LDO regulator (maybe it
    has to be discrete to get a really low drop-out voltage), possibly with
    the addition of a relay, driven by a comparator whose reference is 12.33
    V or a bit less, to cut out the regulator entirely when the voltage is
    low enough.

    But it's possible that the TV failed for a reason other than excessive
    input voltage. Such coincidences do happen.
    No, you don't need any smoothing, all the while the battery is there. It
    acts like a HUGE capacitor and quite effectively removes the ripple from
    the rectified supply. If the battery became disconnected, the charger
    output voltage might well go very high, and that would kill other stuff
    as well as the TV.
  3. N. Thornton

    N. Thornton Guest

    A vehicles electrical system gets spikes of upto 60v on it, theres no
    way you want to connect your tv to that.

    Regards, NT
  4. Ian Stirling

    Ian Stirling Guest have quite a bit of car stuff.
    Go and search for "cigarrette", and you may find a suitable power
  5. John Larkin

    John Larkin Guest

    Dump the TV for a good book? They run directly on solar power.

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