capacitance in long cables

Zener diodes are your friend.

Hello Duncan,

as i see on your pcb you use a 78L05 regulator. This can be in the heat of the sun overheat, because the limit is 100 milli amps. But test it and you can use otherwise the bigger one. Space is enough in your box. I think for principle clarification on the safety level for recognising high on your arduino, you can make a test with a potentiometer and measure the voltage where the arduino recognise high. My calculation says high level have to be greater then 2/3 of 3.3 volts of the arduino that means 2.2 volt . So normaly with your voltage on your data line it is ok ,but at longer cables there is not much safety level. If it is necessary you can play a little bit to shift the voltage with a pullup resistance to 3.3 Volts. It should be in the range of 5 to 10 Kohms i expect.

I wish you success.

Werner

Edited By Werner Schleidt on 15/09/2020 08:13:56

Have you checked the insulation of your three core cable? Each core to each other core, that's three tests. If you're working at <=12V then an ordinary multimeter should suffice but a 500V megger would be a better test of anything beginning to fail.

Another possibility is that you've got a high resistance contact to a pullup or pull down resistance possibly caused by a dry solder joint.

Looking forward to hear what you find.

Long cables? I remember testing 10 mile long multi pair quad audio cables for insulation at 2000V. When you failed to properly discharge the line under test and then improperly discharged it with your hand! Ooh that was painful but it still seemed to happen once a day.

No, nothing to do with your problem. Just a grey beard reminiscing.

Werner, it's a 5v Arduino, and we analogRead() the voltage

My colleague who does the solar panel has volunteered to fit a series resistor (of the order of 1 ohm) and big capacitor (across) to the supply so that it will soft start. Very easy to do, and can't see any downside

Edited By duncan webster on 15/09/2020 09:05:49

Latest up date, I've modified all 14 signals with snubber capacitors as recommended by Joe, external 5v regulators, and as a bonus we've made up a soft start thingy with a 1 ohm resistor and 470000 uF capacitor on the power supply. This might not have any up side, but it sure doesn't have any downside, and was very easy, unlike modifying the pcbs. I managed to destroy one board getting the dead Arduionos out, so next step is to make some new boards

So far so good, no further dead Arduinos, but it's early days

I read this thread originally, but I have not had time just now to read through it to see if the point I am asking now has already been covered.

Isn't the problem caused in the first place by hanging long lengths of wiring directly on the logic inputs of the chip itself?

Normal practice is to have galvanic isolation (say with opto-isolators), Do you have anything in your layout?

Ian P

Ideally we would have opto isolators, but I have a 3 level signal 0v, 2.5v and 5v (all nominal). Actually short circuit, 1 k resistance, open circuit, and I haven't come across any analogue opto isolators. We were not starting from scratch, the idea was to make what we have (15 off) work. The system has been working for at least a couple of years before this spate of failures

If this doesn't fix it I'm going to try an emitter follower on the inputs, on the basis that a discrete transistor might be more robust than the Arduino input, but that is more difficult to engineer.

Edited By duncan webster on 03/10/2020 14:07:24

Duncan

Just reading this again, I see that you are reading the analogue voltage to determine the status. Is this just an instantaneous reading? If the problem is caused by transients, you could do multiple readings to in effect filter out transients using software. Of course, you might already be doing this!

Stuart

The majority of opto-solators are analog. Only those with comparators on the output are truly digital. The problem is that the transfer function varies between devices and with temperature. To counter this they are often used with on/off input signals and hard on output devices. The are bein used with digital signals but are analogue devices. For a simple 3 level system I don't see any problem using opto isolators.

A "soft-Start" circuit can have a downside depending on the power on reset circuit being used. If a simple power on reset is used with a slowly rising power supply the reset may finsh before the supply is stable enough for correct operation. This can lead to unpredictable operation.

Robert G8RPI.

Good point, but I believe Arduino's all have Brown-out detectors that hold the chip in reset until the power stabilises. Although soft-start shouldn't be a problem in Duncan's application, it's yet another potential gotcha. Amazing computers work at all!

Dave