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Coil-Driver Results

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  • davidj95650
    In message #1260 I indicated that I had not made accurate measurements of the power input/output comparing a bridge driver to the pulsed driver. I have since
    Message 1 of 1 , Sep 8, 2004
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      In message #1260 I indicated that I had not made accurate
      measurements of the power input/output comparing a bridge
      driver to the pulsed driver. I have since made some
      measurements that seem to be reasonably accurate. I say
      "reasonably" because this pulsed driver generates much
      interference with every item of electronics in the vicinity:
      the TV, the radio, the regulated power supplies providing
      the control and supply voltages. I have since found that
      intense isolation of the power supply leads allows the
      power supplies to function without going into a crazy
      mode of operation where the output develops much line-
      frequency ripple and the output voltage decreases
      dramatically. Anyway, the results are as follows:

      H-Bridge:
      RL = 1,720 ohm, Vo = 258, Po = 39W, 82% efficiency
      RL = 2,860 ohm, Vo = 324, Po = 37W, 88% efficiency

      Pulse-at-start (instead of at end)
      RL = 1,720 ohm, Vo = 321, Po = 60W, 76% efficiency
      RL = 2,860 ohm, Vo = 351, Po = 43W, 83% efficiency

      RL is the load resistor connected across the output
      capacitors.
      Vo is the voltage on the output capacitors.
      Po is the calculated power in the load resistor.
      Efficiency is the ratio of output to input power.

      I attribute the difference in efficiency to the fact that
      the MOSFETs on the H-bridge are lower voltage, lower on-
      resistance devices, and the clamp diodes are Shottky. The
      MOSFETs on the pulse-driver are high-voltage, higher on-
      resistance, and the diodes have higher voltage drop than
      Shottky.

      The H-bridge is four MOSFETs configured so that each
      drive-coil is connected to power, then allowed to discharge
      at the same voltage as the supply voltage when the MOSFETs
      are turned off.

      BTW, I changed the drive-control logic to pulse the
      drive-coil at the start of the cycle instead of the end
      because it appears that this is more appropriate. The
      supply current using pulse-at-start is more nearly a
      steady value, except for the start of the pulse where
      capacitor CP supplies the initial voltage to charge the
      drive coil.

      I have taken pictures of the oscilloscope for the H-bridge
      and the pulse-driver. Go to "Files" then go to the folder
      "MESSAGE ATTACHMENTS", go to the folder "MEG2 Experiments",
      and open "HbrdCoil.jpg" for the current in the drive-coil
      driven by the H-bridge. File "PulsCoil.jpg" shows the coil
      current when driven by a pulse at the start. Files
      "HbrdIs.jpg" and "PulsIs.jpg" show the supply current for
      the H-bridge and pulse drivers.

      The current measurements are made by a Tektronix probe
      configured for DC operation. The zero-line for the current
      measurements is the second horizontal line on the oscilloscope.
      Note that the edges are much sharper, and there is an overshoot,
      for the current in the pulsed-coil configuration. This means
      that the core magnetic field is changing much faster than with
      the H-bridge: the flux is switching rapidly from one state to
      the other.

      Note that the supply-current measurement for the pulsed-driver
      is very similar to the current shown in the photos of the MEG-2.

      I am investigating allowing the coil-drives to overlap. This
      would cause an increase in the drive-coil currents since they are
      opposing each other, and consequently increase the fields associated
      with "leakage" even though the core magnetic field will essentially
      remain unchanged.

      More experiments, more time, hopefully more fun.
      David J.
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