Electronics > Projects, Designs, and Technical Stuff
DIY Modular Test Equipment Project
void_error:
--- Quote from: void_error on August 24, 2016, 05:00:03 am ---The digital supply board had a few changes. Initially I wanted to use LTC3824s but there wasn't any room for all the external components needed so I ditched the chip and went on to look for something else. Found the TPS54360 which comes with a really handy spreadsheet which does all the math for you. Now I can fit both regulators on one board quite easily.
--- End quote ---
Nope... even more changes. The way I want to sync the two regulators, one with an inverted sync signal, was a bit tricky with the TPS54360 as the timing resistor pin also serves as a sync pin and needed more parts than I had board space so it was back to suitable part hunting.
LT3680 seems to do the job - 12V @ 2A for one of the regulators and 5V @ 3A for the other one. And everything fits. (Note: @ means at up to here)
A dedicated sync pin makes life much easier.
Of course, the component values can be recalculated for different voltages, who knows where else I might need one of these boards. The sync logic runs off a separate 3.3V LDO (LM3480) and you might be wondering why I chose to run it off the 12V rail. Here's why: 12V also powers the fans on the DC Load and on the Lab Power Supply and those two don't need a 5V rail.
I'll be playing with these a bit more in LTspice to find the proper values for the compensation network components. Would have been easier if LT just made a spreadsheet like TI did.
Back to the dreadful-to-route waveform gen board... nothing changed there except for the LDO for the 50MHz oscillator which was a MCP1700, now it's a LM3480. Once I finish moving component groups around to get the best possible placement and finish the layout I'll have a 3D model of the whole assembly. Finally.
The other power supply board required by the waveform gen is not really worth talking about too much as it's just a pair of 7815/7915 linear regulators, a pair of 78L05/79L05, a bunch of filter caps and some diodes. Oh, and two small heatsinks.
Should I add some PTC fuses (where?) just to be on the safe side or is it a bad idea?
void_error:
Eye candy time!
Although I can't call anything finished yet I'm close enough. Lots of minor tweaks and some double-checking is still required for every single PCB but I finally got as far as being able to slap everything together in 3D with all the connectors in place. There's still a long way to go.
Color coded for visibility reasons:
The white board is the front panel user interface. The waveform generator board is the blue one with the BNC connectors sticking out.
The 5-pin connector on the green board (USB-UART module) goes to a USB panel mounted connector. One side of the optoisolators is powered by USB +5V while the other gets power from the 3.3V rail on the user interface board.
The purple board provides the analog supply rails: +/-15V and +/-5V. The two TO-220 regulators will have some small heatsinks between them and the board. Filter caps placed below the heatsinks. There's still room for a frequency counter module ;)
There's more than enough space for those bulky inductors.
The two black connectors on the top accept terminal block headers(?). The wires will be coming from a 18V center-tapped transformer. I managed to keep wiring to a minimum ;D
Connectors on top for easy probing and firmware upgrade. Might include a serial bootloader at some point...
void_error:
Haven't worked on any part of this project at all for a few days and that's when ideas start coming and it's time to start tweaking.
I was looking for parts on Farnell/element14 website and found out they have a lot of quadrature rotary encoders to offer which made me want to change things a bit, forget the Bourns PEC11-L which is available at distributors which have high shipping cost to my country. Alps seem to have some nice encoders at a decent price also with included push option so I'm going with one of those. $5 shipping is the same as TME charges. If I can stick to these two distributors I'm happy and so far they seem to have all the parts I need.
Speaking about tweaking the designs I'm starting to think the UI board doesn't need a SEPIC converter if I use a charge pump to generate the -5V rail on the lab power supply board instead of an inverting DC-DC converter which would have used the LT3580. The charge pump wouldn't be a big deal as I could just use some logic gates driving small dual MOSFETs and clock them with the SYNC signal. That would eliminate the requirement for coupled inductors which aren't as easy to get as regular ones.
The LT3580 would be then replaced by the LT3680 which is also used on the aux digital supply board, reducing the BOM count.
With the waveform gen configuration being almost complete it's time to focus on the next two groups of modules I have planned, the DC Load and lab power supply and sort out the mechanical aspects.
Since I want the DC load itself to be modular there are two possible setups:
* Have the modules in parallel for more power
* Have the modules grouped and isolated from each other, like a multi-channel power supplyThe first setup can contain a local microcontroller (on the DC load PCB) or not. The micro would be used for constant resistance or constant power modes. Paralleled modules will share a single DAC .
The second setup will include some form of isolated DC-DC converter and logic isolators.
For the lab power supply things will be a bit simpler, each power supply module is going to have an isolator+ADC&DAC board on top of it.
Another piece of test equipment that I intend to design and build, another module in this case, is a mains Wattmeter.
void_error:
Over the last few days I've been trying to figure out how to make the DC load modular and spent a lot of time looking for suitable parts.
Had to stick with SPI because there are not many I2C ADCs available.
The tricky part is making the modules daisy-chainable for the two main configurations - with or without an on-board micro.
Why? Because there is only one Chip Select line available.
With an on-board micro it can be done in firmware, each microcontroller will have an address. The peripherals will be on a second SPI bus.
When there is no on-board micro I'll be using cascaded 74HC595s to multiplex the Chip Select signals for the DAC, ADCs & Temperature sensors.
In the end it all boils down to a lot of jumper links used depending on the board configuration.
I still have no idea if passing the analog signals from one DC load board to another is a good idea for "slave" boards containing only the analog stuff and temp sensors with the "master" board having the optional micro and DAC/ADCs.
void_error:
Before thinking of finalizing the DC load design I thought I'd check if the load modules scale up nicely and are stable in parallel with the help of LTspice.
The actual control loop will be using an OPA2727 but for the simulation I used the closest LT part I could find within 5 minutes which is the LT1213.
Everything seems to scale up nicely with no parasitic inductances/capacitances so a 400W DC load comprised of 4 DC load modules appears to be possible. Many thanks to Jay_Diddy_B for thoroughly explaining how to get the loop stable.
On the digital side I had a few issues with designing the module for operation with or without an on-board microcontroller but most of them are now dealt with.
One of the issues was multiplexing the fan rpm signal on the non-micro configuration but I had some free pins on the 74HC595 shift register used for multiplexing the ADC/DAC/Temp sensor chip select pins and used one to enable/disable the output of a 74LVC1G126 3-state buffer. A schematic would probably explain this better but it's not finished yet.
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