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Re: [beam] Re: Gate Boost Solar Engine Questions

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  • wilf rigter
    Hello all, I just posted GBSE3, a tested MOSFET solar engine with a voltage doubling gate driver.
    Message 1 of 9 , Sep 10, 2006
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      Hello all,
      I just posted GBSE3, a tested MOSFET solar engine with a voltage doubling gate
      It uses the same number of parts as the original GBSE with a single 1381voltage trigger but works quite differently.
      The  solar cell charges the main storage cap (C1). The voltage of C1 is filtered with D1 and C2 at the input of U1 (a 1381L).  The output of U1 is directly connected through R2 to the gate of Q2 a logic level MOSFET.
      When not triggered U1 output is low and holds the MOSFET gate at 0V. 
      When U1 triggers (at 3.7V), the output rises to 3.7V and this signal is connected through R2 to the MOSFET gate. 
      The rising voltage of the 1381 output is also capacitive coupled through C3 to the emitter of Q1 which switches on and generates a 6V pulse at the gate of Q2.
       This 6V pulse is high enough to turn the IRLZ14  MOSFET fully on and efficiently drive the 8 ohm motor winding I used in the prototype. 
      With C1=10,000 uF, the duration of the unloaded motor on time is about 250 ms, set by the value of C2. With C1=0.5F, the duration of the motor on time is about 3 seconds unloaded and 1 second when heavily loaded. Because, C1 is so large, the voltage drops relatively slowly and the on time is more determined by the C1 and the motor impedance than C2. 
      The novel element of this circuit is Q1, used as a non-inverting transistor switch, which is driven by a logic level signal and together with  C3, D2, R1 and R2 generates nearly double the input voltage at the gate of the MOSFET. This is a cheap solution for boosting MOSFET gate drive that may be suitable for other applications.     
      Keep in mind that the boosted gate drive is capacitive coupled but with the components shown has a very long time constant suitable for "pops" of motor current of several seconds duration.
      Note that R2 pulse the gate to 0V when the 1381 is off and when triggered also connects the 1381 input voltage to the MOSFET gate.   
      This is an important feature to ensure the MOSFET is fully off during charging while the dc drive of the gate with the 1381 input voltage addresses a potential problem that plagues SEs of this type.  The problem is that if the 1381 is triggered and the C1 voltage continues to rise (ie very high charge current and low motor loading), the motor on time would be much longer than the boosted gate pulse duration which might hang the circuit until the C1 voltage is reset.
      In practice, the direct drive of the MOSFET gate with R2 ensures that the gate drive is sufficient to operate the motor while the C1 voltage is high and will discharge C1 when the voltage drops enough to cause the motor to stall.  It does require that the MOSFET should at least partially turn on at the reset voltage level of the 1381 that is used for the SE. The partially on MOSFET with a stalled motor should have a drain current that is greater than the solar charging current to discharge C1 below the reset voltage of the 1381.  I tried to hang the GBSE3 circuit in the schematic using a IRLZ14, under these unusual charging conditions but it always recovered normal operation.         
      I have not tried this circuit with a wide variety of motors or solar cells, so feedback on performance with different components would be highly appreciated. Neither did I have time to test the circuit with PWM which would seem a natural application for this circuit.
      Let me know how it works for you.
      ----- Original Message -----
      Sent: Sunday, September 10, 2006 3:04 AM
      Subject: Re: [beam] Re: Gate Boost Solar Engine Questions

      it works ok as shown but there is a problem getting the time constants right. Without fine tuning, it can hang because the main cap discharge is too small for the timing cap chosen and the 1381 trigger (left side) does not reset.  Now that I understand the subtlies, I  will make it bulletproof (in the morning) .
      ----- Original Message -----
      Sent: Saturday, September 09, 2006 6:20 PM
      Subject: Re: [beam] Re: Gate Boost Solar Engine Questions

      I found some 1381s and should be able to check it out later tonight. I will let you know the results.
      ----- Original Message -----
      Sent: Saturday, September 09, 2006 1:12 PM
      Subject: [beam] Re: Gate Boost Solar Engine Questions

      Hi Wilf,

      Just got a chance to implement your design for the new GBSE. I am sorry to
      report that I could not get this design to function at all. I tried it with
      both a 3 Volt and a 6 Volt solar cell, but could not get the voltage to rise
      above 2.4 Volts at the MOSFET. It seems that both 1381's are activating,
      but the motor does not run at all. I must admit I was in the middle of
      something else at the time and did not experiment too much beyond changing
      solar cells to see if I could raise the Voltage in the circuit. See what
      you can come up with!

      Thanks Again

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