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254RE: [beam] D1 Solar Regulator was Re: Need help with the SIMD1

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  • Wilf Rigter
    Apr 26, 2000
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      Hi Travis,

      This circuit was build and tested yesterday, so hasn't got a long track
      record but it is based on earlier designs used by Jim Mullins in some of his
      wonderful creations. In my opinion, this is the most reliable of my solar
      circuits but feedback from builders will be most welcome.

      CHARGING

      The solar cell charges a 1F capacitor through a 1N34A Germanium diode to a
      maximum voltage of 5.5V. While the charging current flows through the
      diode the voltage at the cathode (stripe) is about 100mV negative with
      respect to the 0V line. This negative voltage is applied through a 100K
      resistor to the base of a 2N3904 NPN transistor Q1 and holds that transistor
      off. This cuts off the base current for the 2N3906 transistor (PNP) Q2 and
      the output of the regulator will be zero volts.

      SWITCHING

      Rapid switching is very important in this type of circuit because a circuit
      that is half ON draws power, draining the cap, but performs no useful work.
      On the SIMD1/ Solar Regulator, the output snaps on and off.

      At the end of the charging cycle, when the light on the solar cell
      decreases, the negative terminal of the solar cell starts to become more
      positive than the 0V line The base of the NPN Q1 must be at about +500mV
      (positive) with respect to the emitter which is connected to the 0V line,
      before it turns on and turns on the rest of the regulator. That usually
      happens in the evening but can be simulated by cupping your hand in front of
      the solar cell.

      When Q1 turns on, the PNP transistor 2N3906 - Q2 receives base current and
      it starts to turn on. The regulator output voltage at the collector of Q2
      increases to about + 2V when the red LED starts to turn on and to supply
      current to the base of NPN transistor 2N3904 - Q3. When Q3 turns on it
      "robs" base current from Q2. This in turn controls the base current for Q2
      and the regulator output will stabilize at +2V. The 10K resistor from the
      regulator output to the negative terminal of the solar cell provides
      positive feedback to the regulator turn on by loading the solar cell down so
      that it's output voltage drops even more and the regulator "snaps" on. Note
      that the red LED is used for reference voltage only and does not actually
      light up.

      DISCHARGING

      The output voltage at the collector of Q2 remains at 2V while the voltage on
      the main capacitor can range from a full charge at 5.5V to 2.1V at the end
      of the discharge cycle. If no load is attached to the regulator the cap
      voltage will drop very slowly because of leakage and a small amount of
      current required for the active regulator (<50uA). When a HC chip like a 240
      or 14 is powered from the 2V regulated output, the current for that chip is
      also very low.

      If the HC chip has a LED connected to the output which the same type of LED
      that is used for reference, then the current will be limited by a small
      voltage drop on the HC driver output. Since the regulated voltage is
      constant the brightness of the LED is also constant.

      When the voltage on the 1F cap drops below 2V, the regulator reference LED
      turns off and the base currents of Q1 and Q2 increase discharging the
      remaining charge on the cap and turning any attached flasher circuit rapidly
      off. At some point the voltage of the solar cell even in dim light is higher
      than the remaining charge on the cap and if there is sufficient light
      (usually in the morning) the charging cycle repeats all over again. If the
      sun is bright and the solar cell was shielded by your hand, then exposing
      the solar cell to the bright sunlight generates enough power to turn the
      regulator off and force the circuit back into the charging cycle.

      Hope this helps explain the operation.

      regards

      wilf
      > <<D1_SOLARREG.gif>> -----Original-----
      > From: Travis D. [SMTP:beam_bot@...]
      > Sent: Wednesday, April 26, 2000 3:01 PM
      > To: beam@egroups.com
      > Subject: Re: [beam] D1 Solar Regulator was Re: Need help with the
      > SIMD1
      >
      > I CAN'T WAIT TO TRY THIS CIRCUIT OUT...I REALLY HOPE
      > IT WORKS GREAT!!! One question though, How exactly
      > does it work?
      >
      > Thanks Wilf,
      > Travis
      >
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