We have a really weird issue which we just cannot get our head around.
In high level:
We have a unit which contains some electronics, we can access it with 4 cables 2 for power and 2 for communications. The power is 24V DC and communications is RS485. We could not establish communication using a 10m test lead, it has been noticed that when we moved the cable the device functioned sometimes. The conclusion we drew was that the cable was broken somewhere. The cable was cut shorter to around 1m long and worked like a charm.
We tried to use two other long cables as we still needed to be sure that the system works with the required cable length. It did not work.
This is when we started to do some more testing and try to rule out the possible faults:
- Correct length power and comms cable works fine with test apparatus
- Short power and comms cable works fine for our actual apparatus
- Short power and long com works fine for our actual apparatus
- Long power short comms does not work with out actual apparatus
- Long power and long comms does not work with our actual apparatus
We have concluded that the issue is with the power cable. The only variable that seems to have an effect is length of the power cable. We have experimented with three different long cables of different qualities, none of which have any effect.
We have one of the electronic components(comms and power as well) as a spare unit which we have tested with the same long cable and it does work. In the unit it self which we are trying to fix we have a power converter (in 18-75V out 24V) which we have no spares for. We have called the manufacturer and asked if they have encountered anything similar but they said they have not.
We have hooked up a multimeter to check the current draw, at short cable we get 0.6A while at long cable we pretty much have nothing.
There is a really small voltage drop at the end of the cable. The power supply we use is 24-28V 15A we have set the power supply to around 25.6V at the end of the cable we had 25.4V.
Currently we cannot access to any electronic as they are in an enclosure, we could open it up but we would like to avoid that if we can solve the issue without it.
In short, how could the length of the power cable effect the current draw from the system, why it would not draw anything when the cable hits a certain length?
Thank you in advance for any ideas you may share to help solve this problem.