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My brain seems to be stuck on this very basic circuit design question. I want to power a 24VAC load (solenoid valve) with the help of a switch (triac). This switch, however, creates a voltage drop of approx 1V. In order to power this circuit, should I get a 25VAC power supply or should I just ignore this small drop?
I assume the load will work just fine, but I would like to know how this would be solved in a proper way.

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    \$\begingroup\$ just ignore it, that's typical for all Triacs due to the two junction drops (Vbe+Vce) \$\endgroup\$ – Sunnyskyguy EE75 Apr 13 at 20:37
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You can check the datasheet but voltage ratings for devices like that are nominal. Triac switching would be common as they are used in PLCs or SSR (solid state relay) switching as is standard in industry so a 1 V drop in voltage should never be an issue.

In addition, the solenoid will have a range of voltages over which it will operate. I would be surprised if it is not rated for 20 to 28 V or so as the 24 V AC supply voltage could vary with mains fluctuations and with variations in loading of the 24 V transformer. The other consideration is that the solenoid may be a long distance from the transformer so voltage will be reduced in those cases too.

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Things like relays and solenoid valves tend to be quite tolerant of deviations in their actual applied voltages and currents and the specified values.

So, without knowing a whole lot of detail -- the valve will be a bit slower and wimpier at 23V, but probably not that bad. Whether that makes a difference to you depends on whether you're designing your system so that the valve is riding on the hairy edge of failure (which is not a robust way to design a system), or whether you're designing your system with plenty of margin.

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