I had a call the other day from a customer who couldn't work out why his MMi99 outputs blew up when driving a 24V 500mA water valve coil. I've seen this kind of thing before (there's not much I haven't seen in the "traps for young players" department), so I asked him exactly what the printed ratings were on the coil. Here are the ratings:
24V - 12VA - 2W - 60Hz
Ahah! If the coil has a VA rating or a frequency then it's an AC coil. Coils ain't coils, and a coil designed for AC is going to draw huge currents if run on DC. Other clues would have been "24VAC" or "PF=0.4", i.e. anything else indicative of AC operation. Of course, if it had said "24VDC" then I'd have to look elsewhere.
Now a coil can be considered to consist of a pure inductor L in series with a resistor R:
R L
___ ___
o-|___|---UUU---o
If an AC voltage is applied across the coil, the current will be limited by the inductive reactance 2*Pi*f*L in series with the resistance R. If a DC voltage is applied, only the resistance will affect the steady state current.
Let's look at the ratings 24V, 12VA, 2W, 60Hz.
From those figure I deduce that on 60Hz AC the coil will draw 12VA. VA means volt-amps, the product of voltage and current. The 2W means it will dissipate 2W of power (resulting in heat). The ratio 2W/12VA is called power factor but that's incidental to this discussion.
Now, if we have 12VA and 24V then the current must be 0.5A because 24V * 0.5A = 12VA. We also know from the nameplate information that with 0.5A through it the coil will dissipate 2W of power. If we invoke the equation P=I*I*R we can calculate R=2/(0.5*0.5) = 8ohms.
If an 8ohm load is driven from 24VDC the current will be 24V/8ohm = 3A. That is why it was blowing up a SPLat output that's rated 0.5A maximum! Clearly it is the inductance that is mainly responsible for limiting the current when AC is applied. And by the way, if the coil were left with 24VDC across it it would get pretty hot. It has been designed to dissipate 2W but with DC it would be dissipating 24V * 3A = 72W, 36 times what it was designed for.
If you are in the slightest doubt about a coil or other wound component (say a motor), then measure its DC resistance using a multimeter and calculate its current draw from Ohm's law.
And remember: Coils ain't coils!