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Help: Electronic Relay for 12V automotive application

Discussion in 'Electronic Components' started by Dab, Jan 23, 2004.

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  1. Dab

    Dab Guest


    I'm looking for something (transistor) suitable for use as a sold state
    relay for a 12v automotive application to power driving lamps. Maximum
    wattage 110w (2 x 55) total (only if I can't find the right bulbs), or
    nominally 50w (2 x 25) total. Maximum current output 4.5 - 10 A

    I know that there are some SS electronic relays, but I was thinking of
    building something myself to try and keep the relay as small as possible
    (motorcylce applicaion).

    Anyone have any suggestions on components? I can live with a small voltage
    drop as it will help extend the life of the bulbs.

    Thanks for any advice.
  2. Tim Wescott

    Tim Wescott Guest

    The natural choice is a MOSFET, but the application isn't trivial. Your
    load of 10A should be no problem, at least.

    Presumably you want to switch the +12V and use the frame for return. This
    means that you need to either use a P-FET which is easy to drive, but don't
    conduct as well, or you need to use an N-FET with an IC called a "high-side
    driver". At this point you can just pick out parts and go.


    Automotive electrical systems suffer from something called "load dump".
    This happens either when the car is switched off or when a corroded battery
    connection is slightly intermittent. When this happens the voltage on your
    "12-volt" rail can jump up to 60V, or down to -60V. I assume that a bike is
    going to have all the same issues. You have to design around this, or your
    system will have a short life.

    Have fun!
  3. Tim Dicus

    Tim Dicus Guest

    Hi Dab,

    I don't know whether you are controlling the power supply or the ground to
    these bulbs, but I will presume you are controlling the power supply. The
    ground control is easy.

    Have you looked at International Rectifier products?

    Search for IRF9Z34. It is a P-Channel HEXFET capable of the current and
    voltage you require. And being a P-channel device means you will not need
    the high side voltage like a N-channel device.

    It is rated at -50 volts, continuous drain current of -13 amps at
    100degreesC. Power dissipation of 88 watts. Drain-Source ON resistance is
    ..14 ohms.

    That would be a 1.4 volt drop at 10 amps, producing about 14 watts on the
    TO-220 package.

    You can use multiple devices in parallel, so you can reduce the total device
    resistance (and heat) and increase the total current capability. Using two
    in parallel would reduce the voltage drop to .7 volts at 5 amps per device
    for a power dissipation of 3.5 watts each.

    Most International Rectifier parts, including this device, can be obtained
    online through Digi-Key

    Hope that helps,

  4. Tim Dicus wrote...
    Whew, I was going to say, sheesh! 0.14 ohms isn't very good.
    14 watts is a lot, even wiith a heat sink the FET will get pretty
    hot. What's more, as its junction temperature rises, Rds(on) goes
    up, e.g., by a factor of 1.5x at 115C, turning 14 watts of loss
    into 21 watts. Sheesh!

    There are much better p-channel TO-220 FETs available from IR, such
    as the IRF4905, which I keep in our stock at the Institute. This is
    a 55V 74A FET with Rds(on) = 0.02 ohms max at 25C. Very nice. Ooops!
    So nice that DigiKey is fresh out of stock! OK, let's try another.

    How about ST's great STP80PF55, rated at 55V 80A and only 0.016 ohms,
    which mean only 1.6W of loss at 10A. Yes much better. DigiKey has
    1980 of these in stock, p/n497-2729-5-ND, for only $1.92 each qty 10.
    Get the datasheet,

    Actually, when used in an automobile, etc., there's another concern,
    and that's short-circuit protection. Without any foldback current
    limiting or thermal limiting, etc., the FET can get into some serious
    danger. Clearly your goal is to have the fuse blow before the MOSFET
    dies. This iffy proposition is one reason the automotive industry
    has turned to more sophisticated intelligent high-side switches.

    One rugged part that's nearly an ultimate is Infineon's BTS555. This
    is a 62V 165A, 0.0025-ohm intelligent FET in a 5-lead TO-220 package.
    Its impressive 2.5-milliohms is achieved with a N-channel vertical
    power FET and an internal charge pump. It includes both current and
    thermal limiting, and reports back any faults. This amazing part is
    sold by Future and Avnet, but they want you to buy 250 of them. :>(

    Hmm, Future also offers the BTS550P in tubes of 25 for $7.72 each.
    It's nearly as impressive, rated at 115A with R(on) = 3.6 milliohms.
    This means it'll dissipate only 0.37 watts at 10A (including operating
    power) so it doesn't even need a heat sink! Cool indestructible stuff.

    - Win

  5. Winfield Hill wrote...
    DigiKey offers many of the smaller high-side switches in the Infineon
    BTS series. With built-in thermal limiting, these are excellent for
    serious automotive and industrial use. In the table below, the Imax
    rating is at an 85C case temp. DigiKey stocks six TO-220 parts,

    part no. Imax ohms price (single) comments
    -------- ---- ---- ------- --------
    BTS409LI 2.3A 0.20 $2.82 5-leads, all-purpose protected switch
    BTS410F2 1.8A 0.22 $3.70
    BTS432E2 11A 0.038 $6.79
    BTS611L1 2.3A 0.20 $3.16 dual switch, 7-leads, can be paralleled
    BTS621L1 4.4A 0.10 $3.24 dual switch, 7-leads, can be paralleled
    BTS650P 70A 0.006 $6.14 7-leads, excellent for 10A lights, etc.
    BTS660P 44A 0.009 $6.46
    BTS550 115A 0.0036 not offer by DigiKey, 5-leads, impressive
    BTS555 158A 0.0029 amazing best-in-breed, not offer by DigiKey

    The BTS650P would certainly be a good choice for Tim, with an I^2 R
    power dissipation of only 0.6W at 10A. In stock at DigiKey for only
    $5.61 qty 25, it's a good item for everyone's engineering parts bin.

    Infineon has many other attractive parts in their BTS smart-switch
    series, but they come in various types of surface mount packages.

    - Win

  6. Good one Win!
  7. Ross Mac

    Ross Mac Guest

    I would use a hockey puck ....
    There are tons of them on Google....
  8. Guest

    You are better off using an automotive relay.
    20 to 40 amp relays are available for less than
    3 dollars. They are simple, cheap and effective.
    You could even use 2 10 amp miniature relays,
    one for each bulb. (While one would work, at
    110 watts that's close to the 10 amp rating. I
    prefer a wider margin.)

    Do you have a reason to use solid state vs
    an electromechanical relay?
  9. tech-guy

    tech-guy Guest

    Do you have a reason to use solid state vs
    If you just want to turn on/off once in a while, mechanical is the
    lower cost method. If you want higher on/off cycles (flashing lights)
    or a relay that is unaffected by vibration and shock, then a solid
    state relay like a
    or other dc solid state relays would work fine.
  10. JeffM

    JeffM Guest

    [response to a post from Jan 24 2004]
    Do you really think this dude is still hanging around
    looking for a solution to his problem?
    Before posting to Google Groups.
    learn a little about Usenet.
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