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:
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
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
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
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
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
More experiments, more time, hopefully more fun.