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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

  • I always build in test points now that can take either a test terminal soldered in - I have around 100 of these that cost a couple of quid from CPC and are reusable - or can take a probe tip and pigtail.  When I was designing my Instrument Control Board I spent a lot of time breadboarding sections first which also allowed me to create some test Arduino code; I then went on to prototyping the power section with a PCB to do a comparison test between various approaches.  That's not as expensive as it sounds: the parts can be desoldered afterwards and reused and the PCB manufacturer is dirt cheap from China.

    That doesn't help for shorts across pins though  I never used to give a second thought about continuity or resistance testing pins on SMD ICs to make sure no shorts across pins existed and a good connection to the pad had been made as a visual inspection is often awkward.  More recently I've wondered about this approach: a multimeter does this by providing a voltage and in some cases that might be more than the IC is designed for.  It's not often clear from the DMM manual what the voltage is.  On the other hand, I don't know if this is ever likely to be a problem, e.g. continuity testing with a 9V DMM signal across an IC designed for 5V.

    For the ICB when I first received the actual boards, I did like Shabaz and measured continuity across points to ensure that there were no shorts where there shouldn't be.

  • in reply to Andrew J

    That is an interesting concern about measuring continuity where ICs are in place and I've never given it consideration.  Does anyone know the answer to that?  I've not experienced a problem with it to my knowledge.

    The detailed design process and documentation you create for your projects would be helpful in developing a test plan and selecting test points. I'm more apt to do that with software/firmware than hardware.  Maybe that is part of the reason I have problems with even simple circuits at times?

  • in reply to fmilburn

    I've never been able to get an answer to that concern either!  So far, I've not blown anything.

  • in reply to fmilburn

      What do you think about the point Andrew raises on measuring continuity with a DMM and ICs in place?

Reply Children
  • 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.