I’m tasked with building a bed of nails fixture for an existing Linux-based product with a wide variety of voltages onboard. DC input comes in at 18V, and we have several step-down/up converters and regulators on the board. I have test points and can add more points as needed, and would like to sample their outputs while executing shell commands on the product over SSH or serial.
I would like to protect the inputs from the board under test. I’ve had at least one case where the 18V input shorted to the 3V3 rail on a current sensing IC. This damaged the board being tested, and would have damaged or destroyed a simple GPIO input on the test fixture.
One idea I’ve had is before applying power, test all of the test points against each power rail and other test points using simple output -> input GPIO logic. This could identify simple shorts before applying power. I’d then need to detach the power rail pins because they will rise to 18V or more. This doesn’t account for other cases where a stray voltage might damage those pins. And adding something like an optocoupler would break the ability to feed inputs into the test points.
Does anyone have any recipes they’ve used with success to deal with such scenarios? What’s typical on a test fixture with mixed voltages?