What does "Back to Back Diodes" actually mean

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jlawley97

Joined Oct 5, 2019
22
working on a simple project(apex sa303 for motor driving) that ask to have back to back diodes connecting the two ground planes (see image). Now for the last hour or so I have been trying to figure out what EXACTLY back to back means because I have been seeing it described in every single configuration you can have for 2 diodes which seem to serve completely different functions.
 

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Ian0

Joined Aug 7, 2020
5,103
Two diodes in parallel, but with one of them reversed. The cathode of one connects to the anode of the other, and vice versa.
(Note that if it said "back-to-back zeners" then it would be different - it would imply two zeners connected in series, with both cathodes connected together)

In both cases the circuit limits the voltage between two points - either to 0.6V for two diodes, or Vzener+0.6 for two zeners.
It is also knows as "two diodes in anti-parallel" or "two zeners in anti-series".
 

MisterBill2

Joined Jan 23, 2018
11,877
Back to back is really incorrect, they are in parallel but inverse parallel. Actual "back-to-back" only works with breakdown diodes that will conduct in both directions, such as zener diodes.
 

Ian0

Joined Aug 7, 2020
5,103
I'd say that "inverse parallel" was the most correct term that has been mentioned. Even "antiparallel" has its problems - "anti" means "opposite" so it suggests that they would be the opposite of being in parallel (which is perhaps being in series). "back to back zeners" is accurate, but they work just as well cathode-to-cathode as they do anode-to-anode so maybe they could also be "front to front zeners".
 

Ian0

Joined Aug 7, 2020
5,103
Are you sure? Back to Back is always in series.
But it has to be a misnomer, as two rectifier (I.e. non zener) diodes in series, one reversed, isn’t going to do anything useful.
The application is to connect them between the analogue and digital halves of a ground plane, so, obviously, the purpose must be to prevent a large voltage developing in either direction between the two partial ground planes, and that can only mean two diodes in parallel, one reversed, however badly chosen the original wording may be.
 

MisterBill2

Joined Jan 23, 2018
11,877
But it has to be a misnomer, as two rectifier (I.e. non zener) diodes in series, one reversed, isn’t going to do anything useful.
The application is to connect them between the analogue and digital halves of a ground plane, so, obviously, the purpose must be to prevent a large voltage developing in either direction between the two partial ground planes, and that can only mean two diodes in parallel, one reversed, however badly chosen the original wording may be.
In other words, "Back to back" is an incorrect speech thing, where the original documentation writer means anti-parallel but is unable to think of that word . It is quite illiterate, and very confusing to those not aware of the shortcut.
(My harsh sounding comment is removed)
 
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Jon Chandler

Joined Jun 12, 2008
190
already killed one small doggie by accident and it tore the leg off another small doggie.
In other words, "Back to back"is a lazy speech thing, where the speaker probably means anti-parallel but is unable to think of that word, or else it is too complex for them. Sort of like the word "swamp" meaning to absorb excess power for an application. Used by those lacking the ability to use the correct phrase. It is quite illiterate, and very confusing to those not aware of the lazy shortcut.
Actually, "back to back diodes" came out of the documentation the TS quoted, so your "quite illiterate" is somewhat out of line. Or may no one ask a question if they don't know the proper terminology?
 
Actually, "back to back diodes" came out of the documentation the TS quoted, so your "quite illiterate" is somewhat out of line. Or may no one ask a question if they don't know the proper terminology?
The author of the documentation was clearly the guilty party, which was my point.
No criticism of the posting party was intended, which I had thought was clear, since they were asking about how such an arrangement could work in the application referenced. And the TS was asking about how such could work, making it totally clear that they, the TS, understood what was stated and found it confusing, as it certainly was.
 

sagor

Joined Mar 10, 2019
695
Back to back diodes are often used across current meters. An example is a mA meter in a radio amplifier. It is used to protect the meter in the even of a high voltage flash over somewhere, that reaches the meter. Back to back diodes will conduct the high voltage, limiting the current through the meter and thus protecting it. But, the diodes do not affect the regular current measurement function as the voltage across the meter is less than the 0.6V required to trigger the diodes into conduction.
 

Ian0

Joined Aug 7, 2020
5,103
Back to back diodes are often used across current meters. An example is a mA meter in a radio amplifier. It is used to protect the meter in the even of a high voltage flash over somewhere, that reaches the meter. Back to back diodes will conduct the high voltage, limiting the current through the meter and thus protecting it. But, the diodes do not affect the regular current measurement function as the voltage across the meter is less than the 0.6V required to trigger the diodes into conduction.
Don’t you mean antiparallel diodes?
Back-to-back zeners could protect a voltmeter.
 
Well, the term back to back in meter protection is "anti-parallel" to some people I guess, but I've never heard "anti-parallel" as a common term before. Some new invented word? Basically, the anode of one diode goes to the cathode of the other, at both ends. This has been called back to back since I recall for radio meter protection, back to the 1970's as I recall.
Maybe they should call it a "69" arrangement?
 
'Back to back' implies the diodes are in series and facing opposite directions. Hence back TO back. Whether the anode or cathode is the back, is unclear.

'Anti-parallel' implies the diodes are in parallel and facing opposite directions.

Classic case of jargon gone wild. To quote my mentors, "Schematics are the language of electronics". It would be fitting if someone posted schematics of diodes 'back to back' vs 'anti-parallel' as they exist in the books. In my opinion, unless someone provides a proof, these discussions will continue to confuse the uninitiated.
 
Never came across "back to back" referencing protective diodes unless they are some sort of breakdown devices, such as Zener diodes. So once again, it is an incorrect term applied without thinking of what it means. It happens that the meaning of expressions is sometimes important. This is one of those instances.
 

sagor

Joined Mar 10, 2019
695
Never came across "back to back" referencing protective diodes unless they are some sort of breakdown devices, such as Zener diodes. So once again, it is an incorrect term applied without thinking of what it means. It happens that the meaning of expressions is sometimes important. This is one of those instances.
Back to back is a common term, albeit maybe incorrect. It may not have been meant to be taken literally. I guess it may depend in what era you used this term. If you search "back to back", you will get different answers as to what it means:
https://www.electronicspoint.com/forums/threads/meter-protecting-diodes.108773/
https://www.antiqueradios.com/forums/viewtopic.php?t=93774
and then "cross-connected", which is actually more descriptive than anti-parallel.
Best description for meter protection is describing how to wire the diodes, each in different direction in parallel.
Oh well, unless the original question puts a reference as to what the diodes are to do, hard to interpret what "back to back" diodes really means. In a literal sense, it would never work if cathode was to cathode, or anode to anode. Nothing (current) would flow, so that interpretation is redundant.
 
"Back to back" are the first three words of an obscene fraternity chant that I came across in New Orleans during the MardiGras, back in 1965. Not really part of electronics. The rest will not be repeated here.
 
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