[thingstodo]

It should come as no surprise to anyone who's read this far that things did not go as planned.

Attached is a simple list of the connections that I began with. The power is routed from the battery pack, skips the PLC output for now, goes through the load, a 5A self-resetting fuse, the shunt resistor, an ammeter that I'm using to determine how accurate the shunt is, and finally back to the battery pack. So the PLC is not involved in driving the motor and is not measuring the voltage or current from the battery pack.

The idea is to check if my 5.5A maximum is still the same, now that I have the resistors in series. A simple check of the rest of the circuit was done with an ohmeter and all appears to be good.

Connect Power from the battery pack. Nothing appears to happen. The load does not rotate. I check the temperature of the shunt resistor - warm. Why would that be warm already? Check the maximum current on the clamp-on meter (on one leg of the load - not on the list of terminals since it does not connect to a terminal). 5.1A seems reasonable. Touch the self-resetting fuse ... it's HOT. Disconnect the battery pack. Check the fuse with a meter and it is open circuit (or above 100 Mega Ohms, at least) A 5A self-resetting fuse should not trip at 5.1A for a short time.

A self-resetting fuse looks like an automotive fuse, at least this one does. It has two leads, and it does not matter which way current flows through it. There is a strip inside that heats up and changes it's resistance quickly when more than rated current is put through it. This does not really 'turn off' the current, but it is reduced by a whole lot. Enough voltage is dropped across the fuse that the rest of the circuit basically does not function. This particular self-resetting fuse is stamped as 5A. I don't expect it to be super accurate, but a 5A fuse would normally run for several minutes at 5.5A and perhaps a minute at 7A and a few seconds at 10A. The specification sheet shows a curve of input current versus resistance.

I think that this would need to be a 0.5A self-resetting fuse to reduce the current so quickly without the load (my grinder) actually turning.

So I tried a different one. The grinder actually turned this time, but not enough to tell me anything more than the load DID get some current ... the peak current was captured at 5.0A. Something is very wrong here.

Remove the battery pack. Check each connection with an ohmeter. The connections check out. Check again after connecting the negative battery pack. There is a large drop in the resistance. It appears that by attaching a jumper cord to the terminal strip, the terminal strip rotates and puts some pressure on a poor wire connection, which then bridges across the load. The screw on the terminal strip was tight. I must have neglected to pull on the wire after tightening to verify that it was grasped well by the terminal ... I guess. It is difficult to see, but there appears to be a 'whisker' of copper wire that is not in the terminal and can touch the adjacent wire if the terminal block is rotated a bit. The first trial did NOT turn the load at all. The current seen by the clamp-on is a bit of a mystery. The rotation of the grinder in the second trial shows that the bridge is intermittent. I have no idea how much current went through the fuse. It DID do it's JOB .. in both cases.

To me, this again illustrates that a short cut is indeed the longest distance between two points. I saved maybe 20 minutes on the wiring by jamming things together. And I wasted an evening tracking down the problems.

The fix is to spread out the wiring, putting spare terminal blocks between those used, using wires on the terminal blocks, even the ones that I'll be connecting to alligator clips, so that the wires fit snugly and you can actually see where the wires go and what they do.

There are a couple more things that I take away from this:
- in NO CASE should you skip the FUSES! This limited the current and the result was NO DAMAGE.
- the sensors should be set up to handle worst case. In this case, 24V was dropped across the 1.6 ohms of shunt resistor. The voltage across the shunt was 24V. If the PLC analog input had been connected, as designed, that would have failed the PLC analog input, which can only deal with 18V maximum. So I need to add some protection to the PLC's analog input so that I don't blow up any equipment with silly wiring problems.

The wiring is a bit tedious. But it's important. I'll get things cleaned up and try again.