Full-Wave Rectifier Test-Bed Circuit Board

In my previous post we have seen the final part of the schematic of the full-wave rectifier test-bed. Today we will see the Printed Circuit Board (PCB) design before it is sent to OSH Park for manufacturing.

The board is a relatively difficult one to wire due to the choice of many through-hole components with large pads. What you see below is a manual design that has a slightly wider trace width and isolation space (0.01 inches). We could have less than 90 vias with a less aesthetic component layout but at the low frequencies at which the test-bed will be working trace-length minimization is not that relevant.

rectifier_test_bed_pcb

Interestingly, the auto-router of EAGLE performs relatively well. It does not know about nets with higher voltages and mechanical constraints but it computes an amazing result (not shown in this post as it is outside the scope of the project). In my opinion manual wiring still delivers better wiring but I shall investigate if professional designers use automatic or semi-automatic PCB routing and how exactly automatic routing works (this is a nice application of AI algorithms).

Full-Wave Rectifier Test-Bed Schematic, Part III

In this post we will discuss the the third and final part of the full-wave rectifier test-bed schematic.

The final major subsystem in the schematic is the fault-injection and retraction. Most faults are short-circuits and open-circuits and we use Panasonic AQW212 solid state relays to simulate them. The Arduino Due can source enough current for them, so there is no need of external transistors (the maximum allowed DC current on all Arduino Due outputs is 130 mA, while a relay LED consumes at most 3 mA).

rectifier_test_bed_schematic_part3_relays

For injecting the R_1 parametric faults we employ an AD8400 digital potentiometer by Analog Devices. Notice that the potentiometer has substantial wiper resistance of 50 Ω in its zero position, so we use a relay in parallel (K5-2) to simulate short-circuits. The use of the AD8400 is very simple and it does not require any external components. It is going to be programmed directly by the Arduino Due and its digital interface is connected to the Arduino Due's digital I/O pins.

rectifier_test_bed_schematic_part3_digipot

Last, we have the 26-pin male IDC header that will provide the connection between the test-bed and the Arduino Due shield.

rectifier_test_bed_schematic_part3_conn

In our next post we will discuss the physical design of the Printed Circuit Board (PCB).

Full-Wave Rectifier Test-Bed Schematic, Part II

In my previous post, we started drawing the schematic of the diagnostic test-bed. Today, we will extend the schematic with signal acquisition, so the analogue voltages and currents at points N_1 and N_2 get digitized and transferred to the Arduino Due that we will use as a data collection hub and a controller.

Without further ado, the ADC connections:

rectifier_test_bed_schematic_part2_adc

The ADS131E04 is a 24-bit delta-sigma Analog to Digital Converter (ADC) that supports rates of up to 64kSPS. It can sample simultaneously from 4 channels. This ADC has been designed by Texas Instruments for industrial power monitoring.

We plan to sample at 6 KHz. Our main frequency is 60 Hz (the AC frequency), so we plan to over-sample 100 times.

Consider, first, the power supply requirements of the ADC. We need analog and digital power lines. The analog one can be single-ended or double-ended. As we do not want to design our own power supply we will use the one of the excess power of the Arduino Due. The linear regulator of the Arduino Due provides 5V and 3.3V. The ADC requires 3V to 5V for the analogue supply and 1.8V to 3.6V for the digital one. We will supply both with 3.3V from the Arduino Due, but to reduce the RF noise, we will do the splitting on the Arduino Due shield. Similarly, we will have two ground planes (AGND and DGND) that we will wire separately and ground near the power supply of the Arduino Due.

If you read carefully the datasheet of the ADS131E04, you will notice that the design decision to use the Arduino Due 3.3V power supply (single-ended) results in decreasing the useful resolution of the ADC. As the ADC is designed to be differential (for monitoring AC signals in a plant), not providing a negative power supply means that we have to waste half of the dynamic range. Providing 3.3V instead of 5V for the analogue supply also limits us on the upper range of the input signal. As a result, we will use only 22-23 bits of the 24-bit ADC.

All capacitors are ceramic (except the 22 μF that can be tantalum) and are used to filter-out unwanted noise. Talking about capacitors, one shall not forget the power supply ones, which should be placed close to the power supply pins of the ADC:

rectifier_test_bed_schematic_part2_pscaps

The final part of the data acquisition is the voltage and current conditioning. For the voltage part, it is easy. We have to scale the input AC range with a simple resistor divider:

rectifier_test_bed_schematic_part2_voltage_conditioning

For the current measurements, we need to convert the current to voltage. To do that, we use the miniature 200 mA CSLW6B200M Hall-effect current transducer designed by Honeywell. The current sensors have a built-in common-base amplifier (actually two transistors) and the output voltage is between 0V and 5V which is somewhat unfortunate as we cannot use part of the ADC's fully-differential range.

rectifier_test_bed_schematic_part2_current_conditioning

Notice the use of the resistor/capacitor pairs R3, C12; R7, C8; R10, C9; and R13, C15 as anti-aliasing filters. We have used the wonderful online RC low-pass filter design tool to calculate the values of a low-pass filter capable of attenuating frequencies greater than 6 KHz.

Full-Wave Rectifier Test-Bed Schematic, Part I

It is time to start implementing the full-wave rectifier test-bed. We will use the CadSoft EAGLE package for preparing the schematic and the board designs.

Our schematic will have three sheets. The first one is the rectifier itself, the second one is the analog-to-digital conversion of the current and voltage, and the third one contains the fault-injection relays, digital potentiometer, and the interface connector.

This is the first part of the schematic, the rectifier itself (right-click on the image and choose "view image" to zoom-in):

rectifier_test_bed_schematic_part1

The test-bed is going to be connected to 120V/60Hz AC power. The first thing is to bring the mains voltage down to safe level and this is going to be done by the TR1 transformer. The voltage of the output of the TR1 transformer is 6.3V RMS. The role of the F1 fuse is to blow in the case of a short-circuit. Its value is 500 mA. The other components are already described in the preliminary design. Notice all EAGLE schematic cross-references where we will connect the ADC, the fault-injection relays and the potentiometer.