Almost all "relatively advanced and modern" hydraulic systems employ solenoid-operated valves with DC coils. AC coils used to be the common choice for fixed installations "back in the day", but nowadays DC is the universal king, and to be honest I can't see why one would go for an AC coil anymore, with its in-rush currents and the treacherous variable impedance. But that's debatable, and not what I want to talk about. What I want to talk about is the times when the diagnostics and/or application of DC coils is... "off a bit" to say the least, something I come actross quite often. So, in this post, I decided to list some bits of "DC coil-related wisdom", and maybe combine them with a few ways one can "get in trouble" when not aware of a couple of simple facts.
DC coils consume significantly more power when cold. This happens because copper, used in the coil winding, like all metals, has a positive temperature coefficient of resistance, and so a hotter coil will always have more resistance. Just look at these 12V and 24V DC coils operating hot and cold (the multi-meter shows their temperature in Cº):
As you can see, the 12V coil draws from 2.70 to 2.08 amps, and the 24 V coil - 1.34 to 0.94 amps. This is important to consider when you size the wires and switching elements, especially when multiple coils are switched together. In the case of the 12 V coil a 5A relay to switch two coils, for example, may seem like an adequate choice if you only consider the coil's nominal (hot) rating, but it's not enough to switch two cold coils. Of course, easily replaceable automotive relays, rated for high amps, would cause zero issues in such a case, but if you're designing a PCB with SMD solid-state relays, making such a mistake can be costly! This is why I always oversize the electrics. And measure coils' resistances when they are cold. Even when it's stated in the catalog.
In fact - the best advice for any hydraulic and electric installation has and will always be - "Never use conservative safety margins!". Use it wisely, my friends!
I see that a lot of people find the temperature that DC coils can reach during normal operation quite alarming. I've even had cases when our clients would replace their coils because they were "overheating so badly that they were too hot to touch".
Well, for normal power rating coils "too hot to touch" is nothing. Coils easily jump 60-80 C above the ambient temperature. This is perfectly normal. Just look at the pics above again. The coil that was mounted on the DCV only reached 85 degrees because the valve's massive body acted as a heat-dissipating element, but the isolated 24 V coil reached 125Cº after but 10 minutes, and the temperature kept on rising after that!
You can use the fact that the coils get pretty hot when energized to your advantage since you can confirm if a coil draws current by checking its temperature (even by hand). And, by the way, it takes some time for a coil to reach a stable temperature ceiling - about an hour under normal operating conditions.
Another interesting fact - the temperature of the copper winding inside the coil will be a good 15-30Cº higher than the coil surface temperature, so if you're measuring the coil's temperature at its insulation rating temperature, know that something's about to burn out!
Almost all on/off solenoid-operated valves will gladly work with voltage value in the range of 85% to 115% of nominal, even continuous duty. And for short periods even with more. Of course, aiming for 100% is always a safe choice, but in some situations, for example, when the valve is exposed to high ambient temperatures, you may need to purposefully reduce the supplied voltage to make sure the coil's insulation can withstand the thermal abuse. Usually, the ambient temperature becomes an issue above 80 - 90Cº (read engine compartments and the like).
In other words - if your coil is rated at 24VDC and the voltage across it drops to 22, don't sweat too much - the valve should switch problem-free.
A lot of DC coils have fly-back spike suppressors already embedded in them. Some may be bi-polar varistors, and some - simple diodes, which have polarity, and weld short if you connect them incorrectly. By the way, unlike the diodes, the varistors usually fail open, which leaves the coil operational, but without any spike suppression.
A funny thing about the fly-back spike - you can ask someone to hold a couple of stripped wires to the coil's leads with bare hands "just to test it" and then repeatedly touch a power source with the other ends of the stripped wires. Don't forget to tell the subject that it's perferctly safe because it's only 12 volts. Try it if you want to see how far a person can throw a solenoid coil when he or she is shocked without a warning!
Last but not least - when you test or troubleshoot solenoid circuits - check current, not voltage! Well, ideally you'd be checking both, but always give preference to the current! You wouldn't believe how many times I've seen people remove a valve's plug, insert the multimeter probes into the connector, and then proclaim - "There's a steady ... (insert voltage) volts here, so the electrical part is all good!"
No, no, no! Do not measure voltage, measure current! Under load! If the relay has burnt contacts, or one of the wires is loose (corroded) somewhere but is "barely touching" - it is very possible to read full voltage when you measure it without a load. Current does not lie!