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My understanding is that "off the line" means "not on the line". For example an internet user is offline, meaning "not connected". However "Off-line power supplies" are directly connected to the grid (without line frequency transformer). Who coined this strange term?

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    \$\begingroup\$ There is a difference between offline and off-line. It's perfect English. \$\endgroup\$
    – DerStrom8
    Jun 21, 2016 at 18:00
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    \$\begingroup\$ Show the context you see this term used in. \$\endgroup\$ Jun 21, 2016 at 18:00
  • \$\begingroup\$ In terms of computers, it means a PC or network is down, or off-line. For power supplies and certain IC's, it means the power is taken off the mains AC power line. Please don't trip over you own brain on the different context possible with that expression. In this case 'off-line' is a contraction of several words. \$\endgroup\$
    – user105652
    Jun 21, 2016 at 18:08
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    \$\begingroup\$ OK, I get it, "off-line" (power supply) is actually short for "directly off the line". Thank you! \$\endgroup\$
    – Cubrilo
    Jun 21, 2016 at 19:45
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    \$\begingroup\$ This is simply the multiple meanings of the "off" preposition. "off" means "not on" (get off my lawn!); "from" (I got that off E-bay); "from a level or departure point" (50% off sale; 5 millmeters off centre); "from a source" (you can copy off me, if you can't solve it by yourself); "from a direction" (ship veering off course), "away from/toward" (I'm off to the mall); "attached to something, emanating from something" (leaves shooting off the vine; devices hanging off a network) and others: not to mention various phrasal verbs involving "off". \$\endgroup\$
    – Kaz
    Jun 21, 2016 at 20:02

3 Answers 3

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For me, who are not native English speakers, the terminology is a bit mysterious. However here I found this definition:

Off-line power supply: a PSU that processes electric power from AC line without using line frequency transformer. Such supply can still be isolating if it uses high-frequency transformer in one of its power conversion stages. A typical off-line SMPS rectifies input AC line voltage, produces high-frequency pulses by using semiconductor power switches, changes the voltage level by using inductors and/or transformers, then rectifies it again and filters for DC output. An example of isolating off-line PSU is a switching power supply of a computer.

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    \$\begingroup\$ Well, -1, but can't downvote myself: the question had nothing to do with UPS-es (uninterruptible power supplies). Instead, given the generic context, OP must have been concerned with this classification of topologies instead. Quote from there: 'This ... terminology has no relationship to the use of "online and offline" for computers and networking, and no relationship with uninterruptible power supplies that provide power while disconnected from the electrical grid.' \$\endgroup\$
    – Sz.
    Jul 17, 2018 at 14:41
  • \$\begingroup\$ @Sz. you are right. I read uninterruptible but i don't know where... :-) I modified the answer. \$\endgroup\$
    – Antonio
    Jul 18, 2018 at 16:18
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Offline UPS and Offline Power Supplies both are different things. A switcher which can work directly on high voltage without a line transformer is offline power supply. A common controller for offline design is NCP1126. But the question here is who coined the term, i dont know the answer but the above answers are misleading they are confusing with online UPS.

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  • \$\begingroup\$ +1 for "Offline UPS and Offline Power Supplies both are different things.", compensating the gross mistake of the most upvoted answer. \$\endgroup\$
    – Sz.
    Jul 17, 2018 at 14:46
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From a tutorial:

In off-line converters the ac voltage is rectified to dc directly off the ac power line and filtered with no isolation transformer and then processed with a dc-dc converter that provides isolation at the switching frequency. Since the switching frequency is much higher than the line frequency the isolation transformer and output filter are greatly reduced in size and weight. The switching frequency is usually 20 kHz or higher to place any audio noise from the switching beyond the range of human hearing. Regulation of output voltage, current, or power is assumed, because that's where the fun begins. Also the rectification process may or may not include power factor correction or harmonic current suppression techniques.

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    \$\begingroup\$ This does not answer the question of why offline supplies are called so. This explains what they are - but not why they are called so. \$\endgroup\$
    – efox29
    Jun 21, 2016 at 20:40
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    \$\begingroup\$ ... off the ac power line ... \$\endgroup\$
    – Mario
    Jun 21, 2016 at 21:01

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