Biomedical Engineering Reference
In-Depth Information
Warning!
This is an extremely dangerous device! It produces high voltages backed by
su
nitely a project that should
not be attempted by anyone who is not experienced with high-voltage/high-energy
devices. Just consider that TNT releases approximately 2 kJ/g. As such, a short across
the capacitor bank would de
cient energy to cause lethal electrical shocks. This is de
fi
nitely blow up the weakest link in the chain with the force
of a small explosive charge. Needless to say, any remains will turn into shrapnel that
can cause further injury or death. Consider that 2 kJ is the equivalent energy of drop-
ping a 100-kg anvil on your foot from a height of 2 m.
fi
R2. The capacitor bank consists of nine Cornell Doubilier type 400X212U450BF8 2100-
µ
F 450-V dc electrolytic capacitors in series-parallel arrangement. However, because of
its photo
ash rating, the PF212V500BF2B would be a better choice. Resistors R4-R6,
R9-R11, and R14-R16 equalize the voltage drop among all capacitors. These resistors
also bleed any remaining charge on the capacitors after the device is used. The capacitor
bank can be charged to a maximum of 1350 V, resulting in the storage of up to
CV
2
/2
fl
1.93 kJ. A blinking LED and a Mallory Sonalert II alarm module warn the user when the
capacitor bank is charged.
The desired capacitor charge voltage is selected by the user by pressing on the charge
switch. A keylock switch and an interlock switch also need to be closed to complete the
circuit to energize the charge relay K1, which in turn energizes the primary of high-volt-
age transformer T1. The complete high-voltage circuit of the stimulator is contained within
a
2
-in.-thick polycarbonate enclosure that acts as a blast shield in case of component fail-
ure. Interlock switch SW2 automatically disables charging whenever this polycarbonate
enclosure is opened to access the circuit.
The switching element is an International Recti
fi
er type ST330S16P0 high-power
thyristor (SCR). This speci
-state
voltage of 1600 V. It can handle a 9-kA peak current with a maximum
dI/dt
value of
1 kA/
fi
c device is rated for a maximum repetitive peak and o
ff
is SCR triggering is done through a MOC3010 triac-output optocoupler. Whenever
pushbutton switch SW5 is depressed, the optocoupler allows C4 to discharge between the
main gate and the center amplifying gate of SCR1. Diodes D5-D8 prevent the voltage on
the capacitor bank from reversing after the discharge.
Suitable coil inductances can be evaluated using the simpli
µ
fi
ed PSpice model shown in
Figure 7.25. The capacitors are each modeled as an ideal 2100-
Ω
resistor in series to simulate their high-frequency ESR. The heavy wire carrying the cur-
rent to the coil and the coil's resistance are modeled by a 100-m
µ
F capacitor with a 40-m
Ω
resistor (R_COIL).
Since International Recti
fi
er does not have a PSpice model available for the thyristor, we
modi
fi
ed the instance model for a C228A SCR (which is totally di
ff
erent from a
ST330S16P0) by setting the model parameters as follows:
• VDRM: max. nonrepetitive peak voltage
1700 V
• VRRM: max. o
ff
-state voltage
1600 V
• IH: max. holding current
600 mA
• VTM: max. on-state voltage
1.66 V
• ITM: max. one-peak nonrepetitive surge current
8380 A
•
dV dt
: max. critical rate voltage rise
400
10
6
V/s
• IGT: dc gate current required to trigger
200 mA
• VGT: dc gate voltage required to trigger
2.5 V
• TON: typical turn-on time
1
µ
s
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