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How do you start up an untested circuit board?

fmilburn

I've been known to throw a circuit together, apply power to it, and fry something.  Or the circuit just doesn't work and the problem could be anywhere in that mess of wiring.  But I used to work on complex and potentially hazardous projects where we couldn't afford those types of mistakes. There were mechanical, electrical, civil, chemical processing, and instrumentation disciplines, and towards the middle of the design specialists in commissioning and start-up were brought in.

I recently sent out a PCB design for manufacture without prototyping it first.  The schematic is shown below but it is just an example.  My question is more general to starting up any untested circuits.

Schematic

Time for me to get my act together.  I put together a plan with the following approach:

  • Break the circuit into logical sections that can be tested sequentially
  • Describe the test plan for each section with the expected outcome
  • Solder it up a section at a time and test the section before moving on

The actual plan used is shown below.

Commissioning and Startup Plan

One lesson from this was that the test plan should have been made before the PCB was sent out which would have allowed for dedicated test points.  Not shown in the plan (but they should have been) are the bench power over-voltage and over-current settings.

Fortunately, this board went together and started up without a problem but it got me thinking.  What did I leave out?  What tips do you have for planning and starting up an untested circuit?  Do you know of any good resources on the web?

Parents

  • For production designs of products which will be small and use 0402 parts I often start with a development or prototype board based on 0603 minimum sizes.

    I use lots of test points for power and key analogue signals. I use little Molex picoblade 10 way connectors for digital stuff (pins 1 and 10 are ground and 8 signals in between). I can connect these with simple adapter leads to a logic analyser.

    SPI and I2C buses often cause problems so I like to take them out on a connector and generally try and make sure that any spare pins on an FPGA or processor are connected to a debug connector.

    On things that will only be built in smallish numbers (<100) then I would leave all the debug stuff on the final design.

    This is a prototype board, the real device will fit in the space defined by PL1 and 2. None of the Molex connectors or test pins will be retained.

    image

    This is a GPIB or IEE-488 to USB interface board. I won't ever make that many so the test pins and debug connectors will stay forever.

    image

    MK

  • in reply to michaelkellett

    Thanks Michael,

    I have come to the realization that I should be providing more test points. 

  • in reply to fmilburn

    This statement is universally true. 

    And something else I just remembered (which has to happen during layout), if there is space on the board, more text is also always appreciated by oneself when bringing the board up. With the designator also a net name to every testpoint. Signals to connector pin. Also very important a visible pin 1 marker at all connectors, ICs, more-than-3-pin-components. Function of jumpers in text. Also useful for high pin count ICs, every 5th pin a little dot and every 10th pin a little line (flashback when QFP-144 was a lot of pins).  Also more GND pins with label, to clip GND probes to the board close to the measuring pins. 

    Unrelated, if the board has mounting holes, small nylon stand-offs are useful to raise the board above the work surface to prevent anything on the bottom side make contact with little conductive pieces lying on my messy workbench. 

Reply
  • in reply to fmilburn

    This statement is universally true. 

    And something else I just remembered (which has to happen during layout), if there is space on the board, more text is also always appreciated by oneself when bringing the board up. With the designator also a net name to every testpoint. Signals to connector pin. Also very important a visible pin 1 marker at all connectors, ICs, more-than-3-pin-components. Function of jumpers in text. Also useful for high pin count ICs, every 5th pin a little dot and every 10th pin a little line (flashback when QFP-144 was a lot of pins).  Also more GND pins with label, to clip GND probes to the board close to the measuring pins. 

    Unrelated, if the board has mounting holes, small nylon stand-offs are useful to raise the board above the work surface to prevent anything on the bottom side make contact with little conductive pieces lying on my messy workbench. 

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