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Re: [GPSL] Seeking advice on nichrome cut-down from microcontroller

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  • Mike Manes
    I got the spes for Dave s FET, and it is fine for the pyro initiator that he uses on the LOHAN project, but with +3Vgs, it won t pull the 5A that we see on our
    Message 1 of 21 , Sep 27, 2013
      I got the spes for Dave's FET, and it is fine for the pyro initiator
      that he uses on the LOHAN project, but with +3Vgs, it won't pull the
      5A that we see on our burners. It MIGHT be better to use a Darlington
      power BJT, which will saturate with less than 3V Vbe, but has a Vce-sat
      like 1.5V @ 3A Ic with Ib of 3 mA. E.g., Fairchild KSD560 in a TO-220
      package.

      BTW, Bill posted a query about a magnetic cutdown device that was
      mentioned in one of the Vin Lally papers. I couldn't find any more
      details about it either, but speculate that it might operate much
      like a maglatch relay, which uses a permanent magnet to hold the
      relay closed (or open), with a coil surrounding the PM carrying
      current which creates a field that BUCKS that of the PM, thus
      allowing the magnet to let go. This would work for a light balloon
      payload, which can't supply a continuous electromagnet current
      in flight, but can supply a brief, high-current pulse.

      My battery-operated programmable furnace thermostat uses maglatches
      to close the 24 VAC control circuit.

      73 de Mike W5VSI

      On 9/27/13 2:43 PM, wb8elk@... wrote:
      >
      >
      > I have used the logic-level gate RFP12N10L FET (less than a buck from
      > Digikey) for nichrome cutdowns (and other things) quite successfully for
      > years now activated by a microcontroller output. It is not in a SMT
      > package however (TO220). The big brother to this one is also available
      > from SparkFun and can be used as well (RFP30N06LE). The important thing
      > to note about it is that when you select a FET that the Gate voltage to
      > turn the device fully on has been fabricated to be activated by logic
      > levels (i.e. 3 to 5 volts) and it has a very low Rds. Make sure that any
      > device you plan to use this way has a logic-level gate...otherwise you
      > will not fully turn the device on as Mike Manes mentioned in his
      > previous post.
      > The SMT one listed in the posts by Anthony and Dave from the UK should
      > be just fine for your purposes.
      > - Bill WB8ELK
      > -----Original Message-----
      > From: Michael Hojnowski <kd2eat@...>
      > To: GPSL list <GPSL@yahoogroups.com>
      > Sent: Thu, Sep 26, 2013 9:34 pm
      > Subject: [GPSL] Seeking advice on nichrome cut-down from microcontroller
      >
      > Hiya,
      >
      > I've got a microcontroller and radio set up to send aloft on a balloon.
      > We hope to let it go for an hour or so, and have it cut itself down, and
      > then foxhunt the transmitter.
      >
      > I tried using a pair of mosfets in parallel between the microcontroller
      > and the nichrome we want to use for cut-down, but the mosfets heat up,
      > and the nichrome doesn't. I'm more of a software guy than a hardware
      > one. What have people been using for switching their cutdown wires?
      >
      > Thanks for any suggestions!
      > Mike
      > KD2EAT
      >
      >
      >
    • HarryM
      Here’s the MosFET I use... http://www.onsemi.com/pub_link/Collateral/NTMD6N02R2-D.PDF A DTMF receiver turns on a 555 timer that triggers both gates. 73 Harry
      Message 2 of 21 , Sep 28, 2013
         
        Here’s the MosFET I use...
         
         
        A DTMF receiver turns on a 555 timer that triggers both gates.
         
        73 Harry
        KC5TRB
      • Mike Manes
        Yep. That s a good one, Harry. 73 de Mike W5VSI
        Message 3 of 21 , Sep 28, 2013
          Yep. That's a good one, Harry.
          73 de Mike W5VSI

          On 9/28/13 11:23 AM, HarryM wrote:
          >
          >
          > Here’s the MosFET I use...
          > http://www.onsemi.com/pub_link/Collateral/NTMD6N02R2-D.PDF
          > A DTMF receiver turns on a 555 timer that triggers both gates.
          > 73 Harry
          > KC5TRB
          >
          >
          >
        • HarryM
          It was recommended to me by Michael Gray, KD7LMO. God rest his soul! 73 Harry Yep. That s a good one, Harry. 73 de Mike W5VSI
          Message 4 of 21 , Sep 28, 2013
             
            It was recommended to me by Michael Gray, KD7LMO.
            God rest his soul!
            73 Harry
             
             

            Yep. That's a good one, Harry.
            73 de Mike W5VSI

          • Michael Hojnowski
            Gang, As the Cornell Amateur Radio Club is getting into balloons, we re (ok, ok, I am!) intrigued by long-flight (multi-day) pico balloons. Being in the US,
            Message 5 of 21 , Oct 3, 2013
              Gang,

              As the Cornell Amateur Radio Club is getting into balloons, we're (ok,
              ok, I am!) intrigued by long-flight (multi-day) pico balloons. Being in
              the US, APRS seems like the right technology to use for telemetry, but
              we're struggling a bit to work out a low wattage transmitter. The lower
              the wattage, the longer we figure we can track. It doesn't seem like
              reaching an APRS rf gateway will be difficult, even at low wattages,
              from a great height.

              We're not afraid to build something, if need be. Does anyone have
              suggestions? Crystals in the right range seem to be as rare and
              expensive as hens teeth. I feel like we're missing something.

              Mike / KD2EAT
            • Anthony Stirk
              Evening Mike, Try decase the Radiometrix HX1 it comes in about 2g and power it using a step up. You re still going to be pushing it for a pico. Alternately try
              Message 6 of 21 , Oct 3, 2013
                Evening Mike,

                Try decase the Radiometrix HX1 it comes in about 2g and power it using a step up. You're still going to be pushing it for a pico. Alternately try a custom radio design based on the SI446x chips  which are the same ones we are using on our picos over the pond. I believe these can be run down to the 2 meter band but I've not tried doing APRS on one. 

                Cheers,

                Anthony M0UPU


                On Thu, Oct 3, 2013 at 8:11 PM, Michael Hojnowski <kd2eat@...> wrote:
                 

                Gang,

                As the Cornell Amateur Radio Club is getting into balloons, we're (ok,
                ok, I am!) intrigued by long-flight (multi-day) pico balloons. Being in
                the US, APRS seems like the right technology to use for telemetry, but
                we're struggling a bit to work out a low wattage transmitter. The lower
                the wattage, the longer we figure we can track. It doesn't seem like
                reaching an APRS rf gateway will be difficult, even at low wattages,
                from a great height.

                We're not afraid to build something, if need be. Does anyone have
                suggestions? Crystals in the right range seem to be as rare and
                expensive as hens teeth. I feel like we're missing something.

                Mike / KD2EAT


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