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I have a problem when trying to boot my Raspberry (c03111) with a current sensor connected.

The Joy-it COM-KY053ADC is used to translate the analog signal of the current sensor (LEM Hass 50-s) to I2C (which works fine.)

It is powered via an external power supply using the 5V-pin, so it should get enough power.

When I disconnect the sensor from the analog input of the ADC the Raspberry boots.

This is my circuit:

Schematics

Do you know what could cause the problem?

Datasheets:

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2 Answers 2

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Having a voltage on ANY pin of a chip while it is not powered is generally outside the normal operating conditions, so normal operation is simply not guaranteed and funny things can hapopen.

I suspect that the ADC passes the voltage on A1 via the 3.3V or one of the SDA/SCL lines back to the Pi, which causes it to be outside its normal operating conditions and hence malfunction.

Check: your LEM sensor has a VERY low output impedance: 5 Ohm, hence it can deliver quite a current, which could cause the problem.

Fortunatley, your ADC has a very high input resistance (M-Ohms). I would start with a series resistor of 10k in the A1 line. That limits the current into the A1 input, but doesn't significantly affect the measurement.

If that doesn't help, add a bleeder diode (try 1N4148 for a staret) from the ADC A1 input to the 3.3V, and a 1k bleeder resistor between the 3.3V and ground.

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  • \$\begingroup\$ It works with the 10k resistor! Thank you so much! Was thinking about the inrush current, but thought it couldn't draw much current because of the high impedance of the ADC input. That was a mistake. \$\endgroup\$
    – kanduko
    Feb 7, 2021 at 22:27
  • \$\begingroup\$ The ADC input ITSELF was probably not causing this, but the input protection circuit, with an intentional or side-effect diode to the 3.3V rail. \$\endgroup\$ Feb 8, 2021 at 15:29
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Here is something i would test:

1)disconnect the Joy-it from the raspberry (let everything else as it is that you show) and try to boot the Pi. I am thinking that a connection from the Joy-it to the Pi is stopping the pi from booting, as you might have connected it to some pins that are responsible for booting/interfere with booting of the device

2)Measure the voltage on the pi the moment you try to boot it with the current sensor on. What does it show? does the voltage drop below 5V? The current that is being draw from the current sensor from your power supply might be too much for your power supply and the voltage drops.

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  • \$\begingroup\$ 1) When I use the Joy-it board without anything connected to the analog input pins the rpi boots. When I connect the sensor after the rpi has booted it works properly. 2) The strange thing is, that the rpi boots when the supply lines of the current sensor are connected and the output of the sensor is disconnected. Only when both, the supply line & the output line, are connected the rpi won't boot. I'm using a laboratory power supply to power it and I see no voltage drop while starting the rpi. And if the voltage drops shouldn't the rpi start after the voltage restores? \$\endgroup\$
    – kanduko
    Feb 7, 2021 at 19:38
  • \$\begingroup\$ Hm not sure about the voltage restores. On what pins do you connect the ADC to the PI? check out what @wouter said. Try to make RPI power up the Sensor and the ADC after it has booted (using a mosfet or transistor) \$\endgroup\$ Feb 7, 2021 at 21:09
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    \$\begingroup\$ Your idea was my fallback plan, but luckily Wouter van Ooijen helped with the problem! But also thank you for your time and input! :) \$\endgroup\$
    – kanduko
    Feb 7, 2021 at 22:29

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