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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?

  • in reply to shabaz

    Thanks Shabaz,

    Got it and thanks for the clear explanations. I suppose I have been getting away without some of these things as my circuits are not critical and are low speed. But then again I have a box of things I never made work as well. 

  • in reply to fmilburn

    Thanks. Those loops will be very handy... which leads on to another question. What probes work best...

  • in reply to BigG

    I use the 2.54 header pins for test points. You can use the straight or the right angle depending on the need. I set the hole for the pin in the PCB layout and the silkscreen but don't necessarily place the pin in production. On the first production run, you have all the pins but as you become familiar with the board, some points don't prove out as you hoped. Having the hole to place the pin that you don't use doesn't cost you anything.

  • in reply to BigG

    Any really.  I use micro-grabbers with my DMM and normal scope probes.  As I mention, if you position two close together you can even use your scope probe with the pigtail attached.

  • in reply to fmilburn

    The damage mechanism in chips is due to excessive local heat. Most chips have some ESD protection which defines how much excess voltage & current they can take before being damaged. A steady-state DVM is much different to an ESD event, but it provides some clue about the power dissipation ability of the circuit.

    You can actually measure the voltage and current supplied by a DVM in resistance mode using other DVMs to see if it seems excessive.

    I think it is probably the protection circuitry in chips that is most vulnerable. An input pin is generally going to have a high impedance, so only the protection circuitry will conduct current when the rails are low.

    An output pin could be high or low impedance. If it is high impedance it would only have a similar issue to an input. If it is low impedance it should be able to handle the small current the DVM can supply. (The resistance reading may be weird though)

    I can't recall ever damaging a chip by measuring continuity in-circuit, but I don't make a habit of it, and I always evaluate if I think it is safe.

    It may be a situation where a cheap meter that runs off 3V is safer than a higher voltage meter.

    You also need to consider what happens when making resistance measurements with the circuit powered up - this could damage your meter.

  • This could be a story for the Project14 Prototyping Techniques theme ...

  • in reply to BigG

    I usually design the test points to accommodate the probes I have, usually a DVM or an oscilloscope.

  • in reply to Jan Cumps

    It would… I would read it for sure. 

  • 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.