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Old December 7th, 2017, 01:54 PM   #31
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keithkyll is offline
Join Date: Jul 2005
Location: Heath, TX
Posts: 2,026
With relay outputs, I might go 4007. The forward conduction doesn't matter much, so the 1000V rating would be better than to 600 volts for 4004.
For PLCs with transistor or MOSFET outputs, there is a diode junction across the output. It's forward conduction will likely be about .7 volts or lower. This is the nominal voltage for a silicon junction.
If the forward conduction voltage is higher with the diode on the coil, it might not conduct at all, because the PLC output will conduct instead. This causes the collapsing voltage to dissipate in the coil as well as the leads running from the PLC to coil.
In most cases, this probably isn't an issue, but can generate noise that would be hard to troubleshoot.
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