Texas Instruments AN-2154, Application Report

Page: 8 / 30

High Current In High Current Out
+6V
Gnd
J11
J10
J2 J1
Arc Detector
Design Description
www.ti.com
Figure 4. System Insertion
The board has a 1000V isolation capability. Therefore, the board could be connected at mid-string or on
the positive side of the string if the voltage does not exceed 1000V. However, this is not recommended
due to safety concerns regarding handling. The 6V power supply used to power the board is fed through
J10 and J11. The current carrying the string current connects through J1 and J2. See
Figure 5 . A 9V
battery can be used as a power supply for situations in which limited supplies are available. The 9V
battery will provide several hours of operation. The RD-195 can be installed into a BUD Industries PN-
1321-C Nema 4x enclosure for external evaluation. The enclosure will require appropriate modifications
for string level and RD-195 power interfacing.
Figure 5. Board Connection
14 Design Description
The analog signal path is shown in Figure 6 . The current is sensed through T3. U15A and U18A form a 4
pole Butterworth high pass filter, while U14B, U15B, and U18B form a 5 pole low pass filter. U18B and
U14A add gain to the system with low noise floor operational amplifiers. The output of U18B is the input to
the A/D conversion circuit. The filter has a cutoff frequency of 40kHz for the low limit and 100kHz for the
high limit. This bandwidth is necessary for the proper operation of the software programmed in the
microcontroller.
The op-amps are powered from a 5V rail provided by a linear regulator. The op-amps inputs are biased
using a reference voltage derived from another op-amp circuit shown in
Figure 7 .
The board also supports a self test circuit which produces a signal that mimics an arcing event. See
Figure 8 . The self test circuit produces a noise signal that can be controlled manually with switch SW1, or
with the CPU using the NOISE_EN input. For manual test (J9 open) press SW1 to generate a noise
signal, simulating an arcing event. For CPU test (J9 open) set NOISE_EN signal (AIO10) high to generate
a noise signal. In the RD-195 software this is done with “set Z 1” command. When an arc is detected, Z
resets, and turns off the self test circuit. Should no arc be detected, Z continues to stay at 1, and the self
test circuit remains enabled. Install J9 jumper to generate a continuous noise signal. +V should be
between 8 and 12 volts for proper operation of the noise circuit.
8 AN-2154 RD-195 DC Arc Detection Evaluation Board SNOA564F – June 2011 – Revised December 2012
Submit Documentation Feedback
Copyright © 2011–2012, Texas Instruments Incorporated