Biomedical Engineering Reference
In-Depth Information
HI-POT
Touch-Proof Banana
Connector, Male
HV OUT
Touch-Proof
Banana
Connector,
Female
DEVICE
UNDER
TEST
HV RETURN
Figure 3.30 The high voltage produced by T2 of Figure 3.29 is applied to the device under test through a touch-proof banana connector.
The current that leaks through the DUT's barriers is returned to the hiPot's sense resistor via a second touch-proof connector.
fl
flows from the HV OUT terminal, J1, and through
J3, the HV RETURN, and then through R9, the current-sensing resistor to ground. The
direction of the current
flowing through the device under test
fl
flow is reversed on the opposite phase of the ac waveform applied
to the device under test. The voltage that appears across R9 is proportional to the current
fl
fl
flowing through the device under
test appears as 7 V RMS across R9. This voltage is bu
flow through the device under test. A current of 1 mA
fl
ff
ered by IC1A and full-wave
recti
ed by IC1B and IC1C. The peak of the current-sense voltage across R9 is compared
to the dc voltage level from R13 by IC1D. If the peak current-sense voltage is greater than
the voltage from R13, the output from IC1D will be
fi
12 V dc, a logic level high. This
logic high causes IC2B to be reset, setting the AC_ENABLE output low, switching off
K1
and the high voltage to the device under test. IC3A provides a power-on reset circuit, so
the control
ff
fl
ip-
fl
op, IC2B, always powers on in a reset or off
ff
condition. Switch SW3 pro-
vides a way to stop the test by resetting the control
fl
ip-
fl
op manually to the off
ff
state.
op is a logic low, meaning that the
comparator output is a logic low, and SW3 is open. SW4 is pressed and a logic level high
is input to the clock input of the control
To start a test,
fi
first the reset input to the control
fl
ip-
fl
op. The Q output will switch to a logic high
state. This high output on AC_ENABLE will switch K1 on, applying high voltage to the
device under test. As the control knob on the variac is increased, the high voltage to the
device under test is increased. If the current
fl
ip-
fl
flowing through R9 (i.e., the current leaking
through the device under test) increases above the level set by R13, the output of IC1D
switches to a high state, resetting the control
fl
fl
ip-
fl
op and switching K1 off,
ff
, disconnecting
the high voltage from the device under test. Because the reset input to the
fl
ip-
fl
ip has pri-
ority over the clock input to the
op, the Q output cannot be turned on anytime the cur-
rent-sense voltage is greater than the trip level set by R13. This is done for safety purposes,
so that if there is an overcurrent condition, the high voltage must
fl
ip-
fl
rst be reduced to the
point where the overcurrent condition is removed before the test can be restarted by press-
ing SW4. IC3B and IC3D are connected in parallel to provide enough current to drive the
high-voltage-indicator LED, D9.
Utmost care should be exercised when using a hiPot tester. High voltage below the cur-
rent-trip threshold can give a nasty or lethal shock. In addition, dielectric breakdown carries
the associated risk of ignition. Be prepared to deal with emergencies. In addition, consider
fi
 
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