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I have been looking at buying power supply units for a project, and I keep seeing supplies that output 27.6 V. It's an unusually specific number, so what they are used for, and why is it important that they produce 27.6 V?

It appears that these are designed for use in fire and security systems (and specified by regulation EN 54 within the European Union). Still, this only tells me who thought that 27.6 V was a good idea. It doesn't tell me why.

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    \$\begingroup\$ This reminds me a lot of "why is the standard for phantom-powered XLR microphones 48V?" -- because that's the voltage used for emergency lights at the Norwegian studio that Neumann was commissioned to build early transistorized microphones for, and that eventually turned into the DIN 45596 standard (though 24V is also supported, and preferred, by modern IEC 61938:2018). \$\endgroup\$ – Charles Duffy Jun 2 at 15:55
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27.6V is 2 x 13.8 V.

13.8 V is a common voltage for a 12-V Sealed lead-acid battery (SLA) while under float charge. In float charging the AC power supply maintains a constant voltage across the battery so that it keeps its charge.

Security systems commonly use lead-acid batteries for power so they can operate for a number of hours when AC power fails. The ones you are looking at are using two of those batteries in series.

Although it specifies 27.6 V, the security system will work over a very wide range of voltages, maybe 20-32 V or even wider range.

Float charging is where a constant voltage is applied to a battery to maintain its charge level for long periods. It is not enough to charge a battery quickly though, so after a power outage a security system may take a significant time to recharge the batteries.

12 V battery for security system

enter image description here

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  • \$\begingroup\$ Wouldn't rather 12V be the nominal voltage for the 12V battery. Hence 12V, or we would be calling it a 13.8V battery. And when fully charged, these can be some 14.4V iirc. \$\endgroup\$ – Lundin Jun 3 at 11:26
  • \$\begingroup\$ There is no mention that 13.8V is the float charge used ONLY on SLA batteries (-1) Whereas flooded acid cell batteries used in cars use 14.2V and some higher which only shortens the normal life but purges sulphation faster. We call it 12V to make it simple as flooded acid cells are 2V cells that vary in capacity voltage +/- 1/3V after resting or quick load after charging \$\endgroup\$ – Tony Stewart EE75 Jun 3 at 13:04
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    \$\begingroup\$ @TonyStewartEE75 - Maintenance-free (not SLA) batteries in cars don't usually use such high voltages. The Chevy maintenance manual I have says it uses 12.9-13.2V in fuel-economy mode but going to higher voltages for limited times. \$\endgroup\$ – Kevin White Jun 3 at 16:26
  • \$\begingroup\$ @KevinWhite I think you'll find your battery is not a flooded cell battery acdelco.com/parts/batteries/… "AGM (Absorbed Glass Mat) refers to a specific type of SLA/VRLA where the electrolyte is absorbed into separators between the plates " \$\endgroup\$ – Tony Stewart EE75 Jun 3 at 17:11
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    \$\begingroup\$ @TonyStewartEE75 - The charging regime goes up to ~15V on occasion (it even has an anti-sulfation cycle). It is typical of a modern car battery system. Rarely do car batteries require distilled water to be added these days. As a result the charging voltages tend to be lower. \$\endgroup\$ – Kevin White Jun 3 at 17:17
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This is simply the nominal voltage of lead acid batteries while charging. Think 2 x "12 volt" in series, where the nominal 12 volt is really 13.8 volt.

See for example What should the voltage of a fully charged lead acid battery be? here on Electronics Stack Exchange.

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Smallish transformers are commonly 10% over the rated voltage with no load. So that I*R losses bring the no load voltage down to the rated voltage at full load. This unusual specification happens to be 15% over 24 V, the most common standard for AC alarm and furnace systems.

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    \$\begingroup\$ 10% vs. 15% - what is the conclusion? Acceptable? Unacceptable? Happy coincidence? Or something else? \$\endgroup\$ – Peter Mortensen Jun 3 at 7:39
  • \$\begingroup\$ wide tolerance is probably the maximum and not nominal \$\endgroup\$ – Tony Stewart EE75 Jun 3 at 12:54
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Looking at the tolerances involved including line I would find that is acceptable for 24VAC. Has your meter been calibrated lately? Consider this: In industry, 30 volts is generally considered to be a conservative threshold value for dangerous voltage. The cautious person should regard any voltage above 30 volts as threatening, not relying on normal body resistance for protection against shock. 24V meets that requirement and gets a reasonable amount of wattage for the actuator.

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    \$\begingroup\$ Fire and security systems use DC battery backup, not AC. \$\endgroup\$ – Transistor Jun 4 at 7:06
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    \$\begingroup\$ OP looks at listings for power supplies stating 27.6 volt outputs. I doubt he took his meter to the store to measure them. \$\endgroup\$ – pipe Jun 4 at 12:18

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