Electronics > Projects, Designs, and Technical Stuff
DIY Modular Test Equipment Project
void_error:
--- Quote from: max_torque on October 01, 2016, 09:43:02 pm ---whilst proper eurocard rack enclosures are expensive, i bet you could do a simple DIY solution using std back plane connectors and some laser cut acrylic or simple ally frame etc??
Design you "processing" pcb as the back plane, with the IO and numerous serial bus connections, and have the various cards slot into that!
--- End quote ---
That would be possible but I would lose compatibility with solderless breadboards and protoboards which both use 0.1 inch spacing. The reason for the modules having 0.1 inch headers is if someone wants to hook up a module (like the waveform generator) to their own "processing board" like an arduino or raspberry pi they wouldn't need a custom made adapter board. They also have to fit in an off-the-shelf enclosure.
The other issue with plugging the modules at 90 degrees is that all the different IO would have to be on the processing board so they would be accessible on the front panel and they would take up a lot of space, not to mention the mechanical part, securing all modules so they don't flop around. That's the reason the current mechanical layout was chosen, as it only needs one surface to attach to, in the case of the waveform generator which has a 3D model shown shown here, is the front panel.
I also have to mention that all the bits of test equipment here are not precision driven but more by price, ease of construction and versatility so some compromises have been made.
void_error:
Some significant progress has been made on the DC Electronic Load module.
The microcontroller for the "smart" version is going to be a PIC16F18345, again, a newer part, chosen for the pin reallocation feature and the multitude of peripherals, most notable being the availability of two hardware SPI ports, one used for communicating with the UI board, the other one handles the peripherals.
Having all the versions on one board is going to be a bit tricky and it'll involve a few jumper links.
void_error:
The DC load will be split into two, one board with the analog stuff and temperature sensors, the other board will include the ADCs and DAC with the digital isolator and shift registers, haven't decided exactly what goes where but I'll make it in such a way so it will be shared with the lab supply.
I'm also thinking about a purely analog version, but that implies analog temperature sensors and comparators for the over temperature pretection as well as some solution for fan control.
void_error:
Made some block diagrams of what I have in mind so far. Hopefully I find a way around the tricky DC load requirements...
While I was still at it I decided to make one for each piece of test equipment.
If anyone's wondering what Q7 & Q15 are they're the last stage outputs of the shift registers used for Chip Select multiplexing.
DAC_I, ADC_I etc. are the chip select signals.
For the Waveform Generator I chose +7.5V to feed the linear regulator powering the square/PWM output driver but that might change. I could just slap a 5V linear regulator and ditch the +5V output from the Aux Digital Power Supply. It'll be cheaper too.
void_error:
The DC load final schematic is beginning to take shape.
Some changes were made to accomodate analog control so the digital temperature sensors were replaced by a pair of LM56.
The nice thing about them is that they have two built-in comparators with an input and an output available externally for each of them and the obvious way to go was to slap an SR flip-flop (with an inverter on one of its inputs) and turn each temperature sensor into a schmitt trigger with the input being the temperature and presettable high/low temperature thresholds.
It even has an internal voltage reference but it's not going to be used as there's already a more accurate LM4132-4.1 on board.
The UI board will read the temperature via two MCP3001 ADCs, the cheapest I could find with a SPI interface and external voltage reference input. 10bit is probably overkill anyway.
What's left to add is a PWM circuit for fan control for a fully analog version.
With all of those things in place I had an idea...
On paper a single DC Load module should be able to dissipate 100W but it should be tested after the thing is built. How? The answer lies in some lines of code that have yet to be written. I'm thinking about a test mode where you connect the load to a 150W+ voltage source and start "cranking" the current up while keeping a close eye on the temperature. Once things start warming heating up to a predefined temperature reduce the the current until thermal equilibrium is reached (in other words temperature reaches a stable value) and read the input power.
Another thing worth mentioning is the availability of two current ranges implemented like Dave described in his EEVblog #931. Look it up on youtube if you haven't already.
The SPI Isolation board will also include fan control/monitoring as well as the MCU for the "smart load" configuration along with isolated power. Nothing fancy, just a tiny MCP1416 driven pulse transformer with a few diodes and caps feeding two LDOs. The tricky part is having the whole lot of variants on a single board. :scared: I've previously done something similar and it's so easy to screw it up. Triple-checking won't cut it.
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