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8429Re: [tracker2] Re: 5v buck converter?

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  • Barry L. Lankford
    Sep 2, 2009
      Erich J. Ritzmann wrote:
      > For 5vdc 1 amp, I am using a 7805 with a couple of caps -- 0.33 uF
      > between the the input and ground, a 0.1 uF between the output and
      > ground -- tied together on a bit of perf board which I place inside
      > the APO3 power relay box. Or, if not using an APO3 I put the circuit
      > into a 35mm film container. I don't remember what the total cost was,
      > but it couldn't have been a lot as I ended up a drawer full of 78XXs.

      You may get away with this linear power supply scheme if all you're
      powering is a tracker and a low power hockey puck GPS, but if you try to
      get much more current than that, you'll probably run into heat problems.
      You may even melt the case you put it in or more likely the regulator IC
      will go into thermal shutdown mode.

      You don't mention a lot of details of the components used, specifically the
      case style of the 3-terminal regulator or whether any heatsink is used, but
      from what you describe of housing the circuit, it doesn't sound like
      there's much opportunity to get rid of heat. It sounds as though there's
      little or no air circulation inside the case, and possibly no heatsink at
      all. If you're using a version of the 78xx that has a thermal mounting
      tab, just the tab itself isn't much of a heatsink.

      And you'll have a lot of heat, especially if you try to run such a circuit
      at anywhere near the 1A current you mention. You're dropping about 8.8 Vdc
      across the regulator (13.8 - 5.0 = 8.8 Vdc). At 1A, that's 8.8 Watts, and
      that's a lot of heat to get rid of. It's not just a matter of efficiency
      to save the vehicle battery, it's the heat produced.

      To try to put this amount of heat into perspective, imagine (or actually
      try it) holding onto a 2W night light that's been on for a several minutes,
      then imagine holding on to 4 or 5 of them.

      Just because the 78xx device data sheet says the max output current can be
      up to 1A, that merely means that IF ALL OTHER DESIGN REQUIREMENTS ARE MET
      (total power dissipation, etc.) IN THE TOTAL DESIGN, then you can have up
      to 1A on the output pin. Meeting all the other design requirements
      frequently means you won't be able to have up to 1A output.

      Even at the OP's desire to run 400 mA, that's 3.5+ Watts, still a lot of
      heat (think of it as two 2W night lights). If you use a regulator that has
      a thermal mounting tab and provide a heatsink, you'll still need to provide
      very good ventilation to the heatsink to let the heat out.

      There's also the issue of reliability to consider. Running an IC at very
      high temperatures will greatly decrease the reliability of the device, and
      I'd be concerned that a regulator failure might put the full 13.8 Vdc on
      the GPS/Tracker. I don't recall the most common failure mode of the 78xx
      line, but if it *CAN* fail shorted (input-to-output), you might be putting
      a costly GPS at risk of damage.

      Even if you design or obtain a high-efficiency SMPS that runs cool, if you
      have a high value mapping GPS powered by it, I'd still consider adding a
      overvoltage crowbar on the output, fuse the input and include surge
      protection devices and polarity protection on the input to protect the
      power supply and everything it's powering from the sometimes "undesirable
      pecularities" of vehicle power (I used to work in an electronic engineering
      department for a car manufacturer).

      As for putting a switchmode power converter in a DB9, I think that'd be
      difficult to say the least. You might be able to use one of the higher
      switching frequency SMPS devices (>1MHz) to reduce the amount of inductance
      required and SMT components to squeeze it inside a DB9 backshell, but it'd
      be very close if it is doable at all, and you'd surely not have enough room
      for any protective devices. There are some special types of DB9 backshells
      that provide more space than the common ones you usually see, but you'd
      still probably be better off putting it all in a separate small case wired
      inline with the cable to the DB9

      I've bought a few close-out Belkin cellphone mobile power adapters and the
      ones I've got will put out about 500 mA at 5.1 Vdc. I've been able to take
      the PC board out of the lighter plug housing and (very carefully) saw away
      a lot of unnecessary PC board material to end up with a board not much
      bigger than a common postage stamp. It'd easily fit in a 35mm film can,
      and it wouldn't produce enough power loss to heat the thing up. I've put
      mini phone jacks, mini-switches and coaxial power receptacles into DB9
      backshells, but I still wouldn't be able to stuff one of my cut-down and
      stripped cellphone power converters into an ordinary DB9 backshell.

      One often useful modification I've made to the Belkin cellphone adapters is
      to leave it in the lighter plug but replace the 2 conductor cord with a 3
      conductor (or better still a shielded pair) cable. Run the regulated 5Vdc
      to one of the conductors and ground to another (or the shield) and connect
      the remaining conductor to the circuit side of the fuse in the lighter
      plug. That gives both unregulated 13.8 Vdc and regulated 5 Vdc at the end
      of the cable. I find that useful for some projects, and might reduce the
      number of lighter plugs to just one.

      I haven't looked at the power requirements of the Nuvi line, but most of
      the larger *handheld* GPSes Garmin makes, such as the GPSMAP60/76CSx, will
      run very well from straight 13.8 Vdc vehicle power. In fact, many of them
      accept a wide range of input voltage, something like 8 to 36 Vdc (or
      thereabouts). Note that those with the extended input voltage range will
      NOT run from just 5 Vdc at the DC power receptacle.

      Barry N4MSJ

      > On 1-Sep-09, at 3:30 PM, gsdatplace wrote:
      >> --- In tracker2@yahoogroups.com, Chris Kantarjiev <cak@...> wrote:
      >>> I'm interested in packing a 5V buck converter into a DE-9 shell, to
      >>> take 12V in and provide power to
      >>> the attached GPS - typically 60-80mA, but it would be nice to
      >>> handle the Nuvi's load of 400mA.
      >>> Can anyone suggest a simple design or readily available unit?
      >>> Thanks!
      >>> 73 de chris K6DBG
      >> Simplest and cheapest I've found (and I bought everyone I could)
      >> Duracell makes a beautifully tiny cigarette lighter to USB charger
      >> adapter. 12V in, 5Vdc at up to 1A out. I've taken a couple apart
      >> now, one is mounted inside my OT2 (like I said very small switching
      >> supply) off the power control output which powers up and down my
      >> nuvi based on profile switching or remote commands. Picked them up
      >> for $6.99, I'm sure it must have been a pricing mistake (in my favour)
      >> Belkin makes a tiny one as well.
      >> http://www.reghardware.co.uk/2009/04/24/duracell_belkin_usb/
      >> Garrett
      >> VE6RKY
      > --
      > Erich J. Ritzmann/VA3XTO
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