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12v relay that doesn't consume current...

Discussion in 'Electronic Basics' started by Ric, Dec 1, 2003.

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  1. Ric

    Ric Guest


    I have a sailing boat equipped with 12v solar panels. I would like to use my
    solar panels to turn off my anchor light when the sun rises (the anchor
    light is an all-round 12v white light at the top of the mast which must be
    displayed at night - and which uses a lot of precious electricity hence my
    wish to turn it off as soon as the sun comes up).

    Obviously it would be simple enough to connect a 12v relay in series with
    the anchor light switch so that as soon as the solar panels power up, the
    relay trips out the anchor light.

    BUT this is not a very elegant solution, because it would mean that all
    through daylight hours the relay will be powered and therefore drawing
    precious mA from my panels (I know it is only about 50mA but I'd really not
    waste it if there is a better solution).

    Can anybody suggest a neater solution? I had thought of getting a second
    relay, and rigging it so that it only closes the line between solar panel
    and light-control relay when the anchor light switch is on. Or is there a
    more advanced 12v latching relay that would be an easier solution?
  2. Ian Stirling

    Ian Stirling Guest

    What sort of power?
    50ma is a very heavy relay.
    Look around some electronics catalogs.
    What sort of electronics skills do you have?
    Carefully matching the relay with the load, and picking the lowest current
    one may be fruitfull.
  3. Ric

    Ric Guest

    Yes, it is 10W
    Oh I can make things from a schematic
    I'd rather find a zero-currnet loss solution if I can.
  4. The voltage on the panels goes up during the day, right? If it gets to near
    12V, you can use a P channel mosfet. Connect the gate to the solar panel
    voltage, the source to your 12V supply, and the drain to the lamp. The other
    side of the lamp goes to the common ground. When the voltage on the gate
    gets near 12V, it'll shut off the lamp, and won't consume any power. It'll
    dim as the voltage is coming up, which may be what you want.

    There are very low resistance mosfets you can get. Get the lowest resistance
    one you can, which will cut down heat, and prevent the mosfet from chewing
    up battery power while the lamp is on.

    Bob Monsen
  5. Maybe you can have the relay pulse off an SCR circuit. This way it would
    only have to operate for a strictly limited time. The scr can be turned on
    using a resistor-zener-network so that it fires when the voltage difference
    between the solar panel and the battery exceeds a preset limit, a relay
    could turn the SCR (and itself) off when an appropriate solar panel voltage
    minimum is reached. This would consume a _very_ small current, the only
    thing to care would be to set the voltage on / off limits so that they do
    not overlap. The schematics below illustrates the general idea, you'll need
    to do some testing and tweaking with the technical details such as zener
    voltages. The relay is wired as to disconnect the light circuit.

    View with fixed font:

    --------------o/ o----------o---o---o---.
    +12V Switch | | | |
    BAT 37K.-. --- --- |
    | | --- /|\ |
    | | | --- |
    '-' | | |
    1K ___ | | | |
    .---|___|---o---o---o- |
    | \---
    __ | Triac /\/
    ---------o--|>|--' ---
    +12V |both zeners ca .-----. |
    SOLAR | __ 7-8V each | __|_ |_
    '--|<|--. | | |_ _ /
    | | |____| /
    | |Relay| |
    .-. 1K | | |
    | | o-|>|-o-|<|-o
    | | | | |
    '-' o-||--' |
    | | |
    | / o
    o-o-o---o---| Light
    | | | > NPN o
    37K.-. --- --- | |
    | | --- /|\ | |
    | | | --- | |
    0V '-' | | | |
    GROUND | | | | |

    P.S. I know, it is not zero-current, but very near to zero,
    so I hope the idea will be of some help.

  6. John Fields

    John Fields Guest

    I'd use an LM393 as a window comparator driving an R-S latch driving a
    low RdsON MOSFET. 3mA for the LM393 and pullups, another 1mA or so for
    the reference divider, and basically nothing for the logic and the
    MOSFET, so you're talking about 3mA total (72mAH per day) plus, the
    MOSFET snaps on and off, so it's not dissipating any power during the
    transitions. I'll draw you a schematic if you like, but I'll need to
    know how you're switching the lamp (Battery+ to SPSTNO switch to wire up
    the mast to lamp to wire down the mast to battery- ?) and the logic you
    want to apply to the switch with the new circuitry behind it. I'd
    suggest something like a three position switch labeled ON-AUTO-OFF. In
    the ON position the lamp would always be on, in the OFF position it
    would always be off, (pretty much like it is now) and in the AUTO
    position it would turn on at dusk and turn off at dawn. Maybe also (for
    another mA or so) a circuit with an LED which would come on if no
    current got through the lamp when it was supposed to be on?
  7. Dave Holford

    Dave Holford Guest

    Pick up a latching relay - pulse to change state and no consumption
    until you change it again.

  8. Ric

    Ric Guest

    Thanks John, and also for Dimitri for his suggestion. I'll look into both
    solutions, though I think sourcing the parts will be my biggest headache
    here in France as there is no Radioshack or Maplins here.
  9. Ian Stirling

    Ian Stirling Guest


    On the battery input put a .5 ohm resistor and a 1000uf 25V capcitor, to
    kill the spikes.
    Now, connect across the + and - two 100K resistors in series.
    This provides a voltage at the join of around half the battery voltage.

    You want to turn the light on when the voltage from the solar panels
    falls below this voltage.

    A BUZ11 MOSFET will handle an amp easily without a heatsink.

    You can use practically any op-amp as a comparator.
    One that springs to mind is the TLO61 (there are certainly better ones, but
    this will draw under half a milliamp) Connect the power pins up to + and -.

    To the inverting input through a 100K resistor connect the solar panel input.
    From the non-inverting input connect it to the junction of the two resistors.

    Now, connect the output of the opamp to the gate of the BUZ11.
    The drain (the middle terminal connected to the tab) goes to the bulb through
    a 2A fuse, and the other end of the bulb is connected to +.

    The source is connected to -.

    Total parts cost $5(US) or so.

    Should use well under a milliamp.

    Observe anti-static precautions when assembling.

    If you want to understand this, look at and
    consider buying/getting book from library.

    Basically, the BUZ11 acts like a relay with no current draw, and
    a 5V or so coil.
    Once the voltage between the source and the gate is over 5V or so, the
    BUZ11 turns on, and connects the source and drain with a resistance of
    around .05 ohms. (it's not quite as abrupt as a relay, it will turn on
    gradually from around 4V (where it might measure a few K) to fully
    turned on at 10V.)

    The op-amp compares the inputs and if the +(noninverting) one is higher than
    the -(inverting) one, the voltage of the output goes near the + supply,
    otherwise near the - supply.

    With the addition of external resistors this can do all sorts of stuff.
    Without feedback resistors it's pretty much a comparator.

    There may well be errors in this post, which I'm sure the denziens of this
    group will point out.
  10. John Fields

    John Fields Guest

  11. Dave Holford

    Dave Holford Guest

    A latching relay consumes no power unless the state changes.

    How does your question relate to that?

  12. Nick Hull

    Nick Hull Guest

    You can get zero current loss by using a simple wind up timer; turn the
    light on by winding the timer and it will tick off X hrs and turn the
    light off. This works as long as you are on the boat and turn it on
    each night.
  13. Why not just use a single P-channel MOSFET? The only issue is that there
    isn't any hysteresis, but that means the light flickers a bit when its going
    out and coming on. You probably don't care where it turns on and off, only
    that it does so.

    However, if you really don't want the thing to flicker, and you need to
    adjust the voltage where it comes on and off, use this circuit:

    You can use any comparator you can find (NOTE, not opamp, comparator, like
    an LM393 or MC3302 or LM311 for example), I think, as well as any p mosfet
    that can handle the current through the bulb. You should be able to buy it
    all at radio shack.

    The circuit as simulated turns on the anchor light at about 5V, and turns if
    off at about 10V. It uses almost no power when the light is off.

    You can predict when it'll turn on by setting the voltage using the variable
    resistor (pot, or trimmer.) Then, the on voltage is something like

    Call the variable solar panel voltage Vs. If the pot is adjusted to Vadj,
    then (referencing the link above)

    Von = Vadj * (R3 + R4)/R3 - Vcc * R4/R3
    Voff = Vadj * (R3 + R4)/R3

    where R1 = 470k, R2 = 1MEG, and Vcc = 12V.

    For the drawing above, we have Vadj = 7.2V, so

    Von = 7.2 * (1M + 470k)/1M - 12 * 470k/1M
    Von = 4.944V

    Voff = 7.2 * (1M + 470k)/1M
    Voff = 10.584V

    This means that in the morning, the lamp will turn off when the voltage gets
    to 10.5V, and in the evening, the lamp will turn on when the voltage gets to
    4.9V. Thus, it won't flicker.

    For a smaller range just make the ratio of R4 to R3 smaller. (obviously,
    when R4=R3, you have none at all)

    Bob Monsen
  14. John Fields

    John Fields Guest

    While the relay will consume power only when it changes state(s), the
    circuitry determining _when_ to switch will, AFAIK.

    I thought that since you glossed over that detail perhaps you had come
    up with a way to determine the presence of dawn or dusk effortlessly.

    I can think of a way to do it so that the detection circuitry only draws
    current during daylight, that way being to use the output of the PV
    array (in front of the battery) to keep the circuit working until dusk
    arrives and then when the relay switches over, disconnects the circuitry
    from the PV (in order to keep the circuit from chattering) until dawn,
    when the output of the PV goes high enough to bring the circuit back to
    life which then turns off the light and sits around waiting for dusk to
    shut itself off again.
  15. Ian Stirling

    Ian Stirling Guest

    No power... So electronics are out.
    Trivial answer is a mechanical clock.

    Let's start with a current relay driven by the current from the solar
    A latching relay should also be in there somewhere.

    Arrange the latching relay so that when in a state, it interrupts power
    to that states coil.
    Now, connect the current relays outputs to the change-state coils.

    Connect the lamp to the appropriate output of the latching relay.
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