Today I’ve been working on a radiation qualification test for three different types of bridge diodes.
The test set-up is as follows:
1) Pair of custom PCB’s located within the radiation test area
2) MCA24 cables (24 cores) linking the test area to a safe zone for the measurement electronics, 230V power is also available within the test zone.
3) Two identical custom PCB’s in the safe area, used for pin-out from the MCA24 cables with their Sub-D 25 (Think parallel port!) outputs
4) Two Arduino Mega 2560’s with 15 of their 16 analogue inputs used to measure the output voltages of the bridge diodes at regular intervals throughout the test, outputting blocks of data (100 samples per channel, taken consecutively) over serial at 9600 baud.
5) A Raspberry Pi B+ running command line raspbian, with modifications to auto-login and run a simple shell script at startup.
The Input voltage to the bridge diodes is approx 19V (from a 230/12 volt transformer with no load), and the RMS DC output is 4.8V. The 100Hz ‘lumpy’ DC is read into the Arduino ADC’s via a potential divider network with one 8.2K Ohm resistor on top and a 3.3K Ohm resistor on the bottom – the more observant will note that this will produce clipping at the peaks, however I hope with the lengths of cables (voltage drop and capacitance) between the test and safe zones, that this will not be an issue. In any case the aim of the exercise is just to verify that radiation doesn’t cause any unexpected catastrophic failures in the diodes, so a continuous saturation reading isn’t a particular issue as I’m looking for dramatic changes in behaviour.
Samples are sent over the serial port at 59 second intervals (there’s a microsecond offset too so that it will wrap over the 100Hz waveforms, rather than locking in at a particular offset.
In setting up the hardware (Arduino and Pi) I wrote this sketch for the Arduino’s and this script for the Pi. Along the way I found this helpful resource on how to login automatically on the Pi and run a script by default on boot. The other thing to add is that the script needs to be set as executable with sudo chmod +x logging.sh before it will run.
Photos of the setup to follow when I get it mounted.