Electronics > Projects, Designs, and Technical Stuff

DIY Modular Test Equipment Project

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void_error:
I was doing thermal calculations for the different components which are going to heat up, like the linear voltage regulators for the +/-15V rails on the Aux Analog Power Supply module and the sine/triangle output driver of the Waveform Generator, the LMH6321.

Since the LMH6321 comes in a surface mount package there's a limit of how much power it can dissipate with only the PCB as a heatsink and also trying to stay away from the SMT heatsinks.

The datasheet says 300mA maximum output current, that's 15V into a 50 ohm load, assuming a dead short on the output. The output of the buffer doesn't swing to the supply rails so it's not really going to have to output more than 250mA. Worst case is outputting half the supply voltage - 7.5V - and that's 1.125W.

If the using the PCB as a heatsink doesn't do the trick I've got another solution which is already implemented in hardware. I can set the output current limit depending on the set output voltage, all in the micro on the UI board.

void_error:
Connectors, connectors, connectors... and some more connectors.

This is when the 3D preview comes in really handy. I'm trying to make the boards as versatile as possible which means they must accept both straight and angled connectors. For the whole modular assembly of boards straight 0.1 inch headers will do the job just fine, no drama here. Angled PCB headers are not a problem either, unfortunately they're not keyed and you can plug stuff into them the wrong way around and we all know what happens afterwards.

Messing around with connector placement over the past two days trying to make the boards accept different types of 0.1 inch pitch connectors I found that the best placement for the connectors is at the top and bottom, with none going off to the sides.

This allows using both angled and straight pin headers as well as the keyed KK 2.54 connectors from Molex pictured below. Just making sure I don't plug the power connectors in backwards... :scared:

void_error:
Looks like the first layout pass for the UI board wasn't good enough and it also made the Waveform Generator board layout not to easy so it was back to almost square one. I wasn't expecting everything to be perfect first try anyway. Not even second try but I'm on the right track.

Being a modular design implies the use of a lot of mounting holes and a lot of boards that have to stack up.
Since the UI is actually the front panel it'll most likely be the only board which has connections on only one side (back side, the front side is the front panel). Most of the other boards will have connectors to both sides.

I managed to stuff a lot of components in a small space for cost reasons but I realized that for more flexibility I'd need more mounting holes, 8 just wouldn't cut it.
This would allow for extra combinations between modules stacking up in more ways, like this:

[UI] : [Waveform Generator] : [Aux Digital Supply][Aux Analog supply]

or

[UI] : [Aux Digital Supply][USB - UART]

or

[UI] : [Digital Isolators] : [Aux Digital Supply]

For the bigger modules like the DC Electronic Load or the Lab Power Supply the data connections (SPI/I2C) will be made using ribbon cables. I'm trying to keep the off-board wiring to a minimum.

void_error:
Took a small but needed break from this project and it seems to have paid off.

The UI board seems to be sorted out, only a few more minor tweaks required... maybe(*)... wait, did I say that before?

Anyway, I moved on to another board that has to fit on the back of the UI board and that's the USB-UART module.
It's nothing fancy, just a PIC16F1454 with a few connectors and some optocouples so I thought why don't I make it more versatile, able to support some add-on modules such as RS-232 or RS-485 outputs.

Back to the Waveform Generator board again, I'll have another go at it this weekend probably. The tricky part is getting everything correct, both mechanically and electrically. In more detail, that boils down to BNC connector and other component placement relative to the LMH6321 output buffer while keeping the analog and digital signal paths clear of each other and at the same time have enough room to fit two power supply boards on top of that which will have to share a few components in some configurations (don't want to go into more details, it's a bit hard to explain).

(*) If you look below you might notice U5 (EEPROM) can be moved and J5 (Fan Connector) taking its place. Also, I color-coded a few traces as follows:

* YELLOW:   +12V
* ORANGE: +3.3V
* PINK:       UART
* GREEN:    I2C
* BLUE:       SPI
Here's the UI board as viewed from the back, the display and controls along with the buzzer are on the other side:



And in 3D, with an example of the possible connector configurations:

pmc:
I just registered, and read this topic. So here goes the n00bish first post...

A couple of things:

1. You were looking for an alternative to DipTrace and free 3D modeling.
For schematics & layout, check out EasyEDA. From the little I've played with it, it seems to work. And it's free.
For 3D, check out OnShape -- a relative newcomer offering no-apologies parametric CAD for free. They claim to import step. Free accounts limit *private* documents. I like it, for what that's worth.


2. Maybe I'm just not understanding what you're doing. It sounds like a UI front panel, some stackable modules and a framework for making arbitrary other stackable modules. Then I imagine that means a UI panel can manage a mixed stack behind it. Is that the idea, or did I get lost?
If I'm not completely lost, then here's a thought: (assume easy answers to fab questions and) arrange modules to launch their real-world connectors/terminals from one side (both?) and angled ~45deg forward. The result would be stacks with a UI panel on the "front" face, and behind that along the "side" of a stack, all the external connectors facing front-ish.
(You're placing connectors at top and bottom not sides - but maybe the constraints that lead that way apply more to stacking connections than to external connections?)
Or more simply just go straight out the side - at cost of requiring more directly perpendicular access to the side of a stack.
Something kinda like this, but with the knobs on the front and connections down the side: http://w140.com/tekwiki/wiki/File:Tek_211_front.JPG

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