Yet another Denford for the collection (Triac Retrofit thread)

[m_c]

Now basic machine movement was possible, the next step was the tool changer.

First thing was to wire up the various sensors and relays, which brings me onto the subject of the possible methods for wiring up such things.
Do you switch the positive, or negative side of relays/solenoids?

As I'm using a Konnect board which has individual opto-isolated outputs cable of switching a fairly substantial load for an interface board (250mA @ 30V max), it gives a good amount of flexibility.
However, some controllers will have output banks grouped, and sharing positive supplies (likely to be described as PNP outputs), or sharing a negative supply (NPN style outputs).

Either will work, but switching the positive feed does usually provided an extra layer of safety. Should a wire short out to ground i.e. chaffs through on the chassis anywhere, then the output should just stop working. It may damage the output chip/transistor/FET on the board due to overloading the output, but at least whatever that output controls should stop working.
The flip side is if you were to switch the negative side, and the wire was to short out, whatever that output controlled would remain active.

And as we're on outputs, I'll cover inductive loads. An inductive load is pretty much anything that involves some form of electromagnet, which covers solenoids, relays, and motors.
These all provide a couple problems for controlling them. First, they will usually cause a surge of current on activation, usually followed by a voltage spike on deactivation.
A typical 250mA output will usually quite safely handle a standard icecube sized relay (IIRC they have a constant coil consumption of around 70mA at 24VDC). The switch on spike will be multiple times that, but for CNC use, this is the size of relay expected to be used for interfacing purposes.

However the bigger problem is during disconnection. When you disconnect a relay coil, the magnetic field not only quickly collapses, but as the solenoid moves back (remember a relay is essentially a solenoid operating a spring loaded switch), it also affects the magnetic field. Uncontrolled, this sudden change in magnetic field will cause a voltage peak of several times the rated relay coil voltage, with a reverse polarity of the voltage that was previously applied.
This surge can very quickly kill electronic outputs, but not only that, it will produce a spike of electrical noise.
The solution is simple. You add a reverse fly back protection diode to the relay coil terminals. This doesn't have to be anything fancy (I use 1N4007 - 1A 1000V rating as that's what I usually have in stock), and you connect it so that the cathode (end marked with a ring) is on the positive side of the coil. It should also be mounted as closely to the relay as possible, which is why they are rarely incorporated into the interface board.
During normal power, the diode does nothing, but when power is removed, and the reverse voltage spike starts to build up, the diode causes the energy spike to recirculate through the relay coil, where it will safely dissipate.