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DIY Modular Test Equipment Project

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void_error:
Everything is starting to come together nicely. The DC load schematic is almost finished as well as the SPI isolation board.
I've also started working on the linear part of the lab power supply aka the post-regulator to make sure the SPI isolation board fits on top of it and the whole thing seems to end up around 100x80mm.

The pre-regulator is going to be on a separate board which will also output the +/-5V rails required by  the lab power supply as well as a bias voltage around 5V above the pre-regulator voltage.

There's still a long way to go.

void_error:
Sorry for not posting anything lately... that doesn't mean I abandoned this though.

Some changes are in place, mainly changing the bus used to link all the modules together to something similar, a simplified version of what prasimix proposed here, which he's developing with a bit of assistance from me.

For the simplified version of the bus only 15 lines are necessary, as described below for the UI board:


* CLK - clock line for feeding a clock signal to other microcontroller in the bus
* GPO - General Purpose Output - user configurable output, can be used to control fans, etc.
* GPI - General Purpose Input - user configurable input, can be used for measuring fan rpm, etc
* SDA - I2C data line
* SCL - I2C clock line
* 1-WIRE (Note 1) - for connecting devices using Maxim's one-wire bus
* TX - UART TX - configured as open drain to allow multiple master devices
* RX - UART RX
* ST_CP - 595 shift register storage pulse, latches the shifted data to the ouputs
* SH_CP (Note 2) - 595 shift register clock, driven from the SPI clock line
* QS (Note 3) - SPI data output to 595 shift register
* MOSI - SPI data out
* MISO - SPI data in
* SCLK - SPI clock
* nCS - SPI chip select output

Notes:
1. the one wire bus uses a UART module
2. SPI output available via Peripheral Pin Select (PPS)
3. on peripheral boards (waveform gen, etc) it will connect to the 595's data input via the bus input connector, while the 595's last bit shift register output (QS) will connect to the bus output connector and to the data input on the next board for daisy chaning, a schematic will explain this better

Changes have been made to the UI board to accommodate the new bus connection which uses 5V signalling. The microcontroller used will be a PIC18F65K40 or any of its bigger brothers in case more RAM is needed. Ditched the switchmode regulator in favor of cheaper LDOs. Since the LCD runs on 3.3V while the MCU rins on 5V some level shifting was required. This was achieved by making the I/O pins open drain and setting the input levels to TTL, with pull-ups to 3.3V.

The Waveform Generator has changed a bit, with the square wave output fed from a comparator connected to the first amplification stage after the DDS filter, capacitively coupled. What that means there's no PWM capability on that output anymore, that will be available on the Frequency Counter module, the next one I'll be working on. I'm also using a dual output reference, a REF3140 since it's easier to bias things.

As far as connectors are concerned no changes will be made, sticking to ribbon cables for data and keyed headers for power.

If anyone's still following this I have to say I want to see this completed as soon as possible but with so many modules I have to make sure everything fits together before throwing money at it. I don't want to have a second revision right after I build the first one so I'm taking my time to make sure it works first go.

void_error:

--- Quote ---Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic.
--- End quote ---

Looks like I haven's posted anything in here for quite some time...
Well then...

After running into the 500-pin limit of the free non-profit version of DipTrace more than enough times I decided to give KiCad another go after watching some random presentation on YouTube so here goes me redrawing everything. There won't be any lack of drawbacks however, because you can't use sheets in KiCad yet(?) and although there's a workaround using hierarchical sheets you can't copy |O anything from the root sheet so unless you start messing with the files you've got your work cut out so I'm not going to do everything twice and since this is not a review of KiCad I shall end the rant here.

What I'm going to end up with due to the lack of sheets is huge schematics :scared: but at least they'll be separated into functional blocks.

Creating all the missing components is going to take some time as well.

More updates coming soonTM.

void_error:
Things look like they're coming together nicely. Since I have no pin limit anymore I'm going to put the Waveform Generator and the Frequency Counter on the same PCB while keeping the mounting on the back of the UI board.

The Lab Power Supply schematic is almost finished, the only thing that's preventing completion is that I'd like to have as many components / functional blocks shared between all modules, making PCB layout, firmware and ordering of parts less tedious. I'm also keeping the option of having it work without digital control while keeping switchmode regulator sync, fan control with OTP (Over Temperature Protection) and switching off the pre-regulator for low output current settings.

It has been previously mentioned that there were going to be two AUX power supply modules, they're now going to be merged into one and used to power the Waveform Generator + Frequency Counter or the DC Load modules. Where there are linear regulators there will be the option to choose between TLV1117/LM317 and 78xx for the positive supply rails and LM337 and LM79xx for the negative supply rails. There might also be a fan control circuit on that, haven't decided yet.

Since I've mentioned the DC Load I might as well get into some details, first of which will be the 'weird' supply voltages of +7.5V and -2.5V for the op amps (OPA2727, also used in the Lab Power Supply) since their maximum supply voltage is 13.2V along with +5V/-5V

On the ADC side I ended up using only one with a 74HC4051 multiplexer for all measurements on both the Lab Power Supply and the DC Load. There's a list of pin-compatible ADCs and DACs a few posts back.

As you've probably noticed, updates on this will be fairly random.

void_error:
It's Aux Power Supply time.

With some of the modules using more power rails than you can poke a stick at (you need more sticks) some form of power sequencing will be required, not so much because the power rails need to all come up at exactly the same moment in time but for that fault scenario when one of the rails outputs a voltage out of spec which could cause current to take the shortest path through the input protection diodes of some ICs and weird stuff could happen.

A simple solution to this is to use a window comparator circuit (LM393) for each rail and some P-channel MOSFETs paired with a delay circuit (CMOS 555 or some monostable using logic gates) and a cheap voltage reference (TL431 or LMV431).


--- Quote from: void_error on July 08, 2017, 04:03:40 am ---It has been previously mentioned that there were going to be two AUX power supply modules, they're now going to be merged into one...

--- End quote ---
That turned out to be not such a good idea. I want to be able to use the boards for other things and not having them only 20% populated.

One more thing worth mentioning is the use of a 74HC138 3-to-8 line decoder to drive the CS pins of the SPI peripherals on the boards that use them, which means I can use only one 74HC595 where I would have otherwise needed two and adding an extra level of idiot-proofness.

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