Miscellaneous desk
< March 4	<< Feb \| March \| Apr >>	Current desk >

Welcome to the Wikipedia Miscellaneous Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

March 5

solid state relay

I came across this[1] affordable, high-current solid state relay but hmm, it says it only switches AC loads: ok fine, it has a thyristor inside. I'd like to switch DC loads, so I typed "mosfet solid state relay" into a search engine. All the ones I could find, though, even on places like ebay and even at lower current like 5-10 amps, were quite expensive, like $100+. Power mosfets themselves don't cost that much, like $1 retail for a 20 amp one right there on sparkfun, which is a hobby site. Is something going on here that I'm missing? I figure that a mosfet SSR would just have an optoisolator, a couple of power mosfets, and maybe a few small analog parts. So I don't understand why they are so much more expensive than thryistor SSR's. My application is a battery protection system, to disconnect a rechargeable battery pack from its load once the pack voltage gets below some level. Thanks for any thoughts. 2602:24A:DE47:B270:A096:24F4:F986:C62A (talk) 03:01, 5 March 2020 (UTC)[reply]

Some thoughts:

A mechanical Relay is lower tech that is cheap and robust. You could supply the load through a normally-open pair of DPDT contacts which ensure zero voltage loss when closed and complete isolation when open.
Beware inductive loads such as motors which impose a high reverse transient voltage on the relay (whether electromagnetic or solid-state) when it opens. It can be protected by diode(s) connected across the contacts or mosfet(s).
Your battery pack voltage will rise after the load is disconnected. This means that a simple voltage sensor arrangement will oscillate or chatter when the pack voltage reaches minimum. Solutions to this problem are A) a latching circuit that has a manual reset button, or B) a voltage sensor with hysteresis. DroneB (talk) 12:54, 5 March 2020 (UTC)[reply]

An SSR of that type (and built down to the horribly low standards of current Chinese retail SSRs) will work fine for switching on a DC load. However it relies on a simple zero crossing circuit to turn off again. Fine for AC, but if you use it on DC it will latch on and not go off again! Also don't believe the current ratings.

If you're switching DC batteries, you might also want a lower voltage drop across the switch, because you have such a low voltage to start with. Although relays have a significant current consumption to hold them engaged, which might be an issue in a low-power system, they do have zero voltage drop and aren't fussy what they're switching. Andy Dingley (talk) 20:49, 5 March 2020 (UTC)[reply]

Thanks all. I assume the AC-only SSR's use silicon-controlled rectifiers (thyristors) as the switching element, so they switch by zero crossing as Andy mentions; but for DC, I thought MOSFETs would have no voltage drop except that across some very low resistance, and switch by external control. Depending on the load (I have a few different ones in mind) I might not mind the coil current for a mechanical relay. It's just that... mechanical.. ugh ;). Good point about the battery pack voltage increasing the load is removed. Yes I'll have to account for that (in fact I probably want to monitor the voltage of individual cells in the pack, but that gets away from the original question). What I really wondered though, was what is going on in MOSFET SSR's that make them so much more expensive than the handful of parts that I'd expect to see inside, while that doesn't happen with thryristor SSR's. I do figure there is some hysteresis in both. 2602:24A:DE47:B270:A096:24F4:F986:C62A (talk) 02:32, 6 March 2020 (UTC)[reply]

Upper fourth

In British secondary schools the "sixth form" is often divided into upper and lower, for reasons (almost) explained in the relevant article. However in Enid Blyton's Malory Towers series there is an upper fourth. Why is this? All the best: Rich Farmbrough (the apparently calm and reasonable) 13:49, 5 March 2020 (UTC).[reply]

In other similar English secondary schools (such as the one I attended) the form between 4th and 5th was/is called "the Remove": thus the full sequence ran 2nd, 3rd, 4th, Remove (or Upper 4th), 5th, First Year (or Lower) 6th, Second Year (or Upper) 6th: other schools had variations on which years were called what. This nomenclature was/is most often used in English Public schools (as opposed to State-run schools), which are more likely to conform to that shown at Secondary school#Terminology: descriptions of cohorts.

While I myself was a pupil, I assumed that the sequence started with the 2nd Form (for pupils aged 11-12) in order to make preceding primary schooling a notional "1st Form", though this was never explicitly stated, but I may have been mistaken. {The poster formerly known as 87.81.230.195} 2.122.3.11 (talk) 15:54, 5 March 2020 (UTC)[reply]

Wow. I vaguely remember reading Billy Bunter books when I was a kid, and seeing him referred to as "the fat owl of the Remove", and never having any real idea of what the Remove was. I thought it was some physical place within the school. Thanks. -- Jack of Oz ^{[pleasantries]} 20:15, 5 March 2020 (UTC)[reply]

Our school had the Third form, the Upper Fourths, the Divisions, Fifth, Lower and Upper Sixths. There was a Lower Fourth too, but not in the Autumn term, because it was for people who joined at Christmas or Easter. --ColinFine (talk) 22:38, 5 March 2020 (UTC)[reply]

Does anyone know the etymology/origin of the various terms for the years of the English schooling system (i.e. "form" for year or the term "Remove")? The term Remove is particularly perplexing to me. What is being removed? How did it come to refer to the 10th year of schooling? Us lowly Americans keep it fairly simple and just number the years "1st Grade" through "12th Grade". --Jayron 32 13:51, 6 March 2020 (UTC)[reply]

Form (education) says: "During the Victorian era a "form" was the bench upon which pupils sat to receive lessons. In some smaller schools the entire school would be educated in a single room, with different age groups sitting on different benches". This refers to the Monitorial System in which a single teacher would supervise a large schoolroom of 100 or more pupils, arranged in benches according to age group, each being supervised by a "monitor" who was one of the older pupils. Alansplodge (talk) 18:35, 6 March 2020 (UTC)[reply]

Thanks for that! Makes sense. What about "Remove"? What's being removed in the Remove Year/Form? --Jayron 32 18:58, 6 March 2020 (UTC)[reply]

I drew a bit of a blank on that, it seems to originate at Eton College which is full of arcane terms. Perhaps you had to pass an exam to be "removed"? Alansplodge (talk) 19:01, 6 March 2020 (UTC)[reply]

NED "Remove" 4 b Promotion, at school... c ...certain division of the school. Lists early usage. fiveby (talk) 20:14, 6 March 2020 (UTC)[reply]

Thanks. That kinda makes sense in concert with the first part of that definition "The act of transferring a person from one post to another" Thus the Remove year is a transitional year, maybe? --Jayron 32 20:22, 6 March 2020 (UTC)[reply]

[Edit Conflict] My understanding of the term is that it originally referred to being "removed" from the notionally junior half of a school to a notionally senior half, but I don't have a secure reference for this.

We have to remember that these nomenclatures arose in privately run (i.e. "Public"), British (mostly) boarding (mostly) schools some time in the eighteenth/nineteenth centuries, when educational organisation was less regulated and more diverse, and the age division betweens "primary" and "secondary" education was not necessarily at age 11, or indeed at any other consistently observed age. Some schools might have taken pupils from (for example) age 13 onwards, others from 9 or even younger as circumstances arose.

Like many others, my school for ages 11 and up had an associated junior Prep school for ages 7-10 (other schools' age split was at 13, not 11): its pupils did not necessarily go on to join our Senior school, the majority of whose pupils (like myself) came from other establishments.

Incidentally, the reason that the relative abundance of boarding schools in the UK (nearly all of which also take local "Day" pupils who do not board) arose and persisted is because of the relatively large proportion of parents who necessarily moved frequently and/or lived overseas due to Civil Service or Military careers. My own father was a career soldier, and having gone to 7 different schools in 3 different countries by the age of 10, I would have faced a potentially very disrupted secondary education if I had continued to live with my parents and attend a rapid succession of local schools, each potentially following a different curriculum, rather than being settled in a single boarding school for 7 years. {The poster formerly known as 87.81.230.195} 2.123.27.199 (talk) 20:38, 6 March 2020 (UTC)[reply]

And here you are, editing Wikipedia. I always said boarding school breeds criminals and degenerates. :) -- Jack of Oz ^{[pleasantries]} 22:39, 6 March 2020 (UTC) [reply]

In view of its early colonial population's origins, I suppose a denizen of Downunderia can be considered a first-hand expert in those fields. :) {The poster formerly known as 87.81.230.195} 90.202.210.25 (talk) 05:18, 7 March 2020 (UTC)[reply]

Note that since the 1980s, school classes are labelled according to year, even in most private schools. Children join the "Reception" class at the age of four, followed by "Year 1". Secondary education starts at 11 years-old in "Year 7" which would have been "First Form" under the old system. The title "Sixth Form" is the only survivor from that system, encompassing Years 12 and 13. See Education in England#Schools and stages. 12:28, 7 March 2020 (UTC)

Yes: hence my link to Secondary school#Terminology: descriptions of cohorts in the first response to the OP. "Keep up, lad; keep up!" {The poster formerly known as 87.81.230.195} 90.202.210.25 (talk) 20:40, 7 March 2020 (UTC)[reply]

“Yaroooh!" Alansplodge (talk) 23:30, 7 March 2020 (UTC)[reply]

Although most UK schools have abandoned the old naming system, it still survives in some Commonwealth countries. Not NZ, which also abandoned it, but Malaysia for one. In Malaysia, remove generally refers to the year between Standard 6 and Form 1, which many students from government funded "vernacular" primary schools are required to go through in government secondary schools, with the stated reason I think primarily being to help them adjust to the change of medium of instruction from Tamil or Chinese to Malay. (There are no government funded secondary schools which don't have Malay as their medium of instruction.) Those who do well enough in their UPSR BM can skip this year, although the requirement has changed over time. [2] (Those who do poorly but attended non-vernacular/Malay medium primary schools do not have to go through that year.) In Malay, the year is called peralihan which roughly translates to transition.

I personally find the Malaysian system less confusing than the US or modern UK one, as it's clear whether the student is in primary or secondary school, unlike when I watch US TV shows and when I see them talking about someone being grade 8 and I don't know if they've just started a new school or it's their final year in that school or what. Of course the Malaysian system only works well if there are clear boundaries between primary and secondary schooling, which there isn't always is, and some may suggest for pedagogical, social or other reasons there shouldn't be. In Malaysia, the UPSR means it doesn't make much sense for a primary school to have form 1 and 2 as an example.

And indeed formerly there was the PMR or SRP exams, and now the PT3 school based assessment which meant forms 1 to 3 have a certain structure to them so it doesn't make much sense to separate them. So do forms 4 and 5 with working towards the SPM.

Of course that also means these issues, and the other corresponding things like technical and vocational vs academic streaming coming from the SRP/PMR/PT3 is not something simply having Form 3 and Form 4 particularly conveys. Although I think most students continue from Form 3 to Form 4 in the same school.

Malaysia does maintain the Form 6 lower/upper distinction which is perhaps the main confusing part of their system to my mind. That said, Form 6 is mostly only taken by those hoping to get into government universities or a government scholarship, and even then mostly non bumiputera, due to the controversial Malaysian Matriculation Programme and some similar things mostly open only to bumiputera [3] [4]. Those who plan to attend university in other countries or in private universities in Malaysia, or otherwise get tertiary education via some other pathway, or get no tertiary education do not generally attend sixth form in government schools AFAIK. Therefore not all secondary schools offer it and more significantly, there's no way to convey the Form 6 situation simply.

Note also that somewhat separately, it's very common that Standard 1-3 and Remove along with Form 1 - 2 will be afternoon sessions of their schools; while Standard 4-6 and Form 3 - 6 will be morning sessions of their schools. There are continual talks since at least the 1990s of changing to a single session, but AFAIK, very limited progress. (To be clear, AFAIK teachers only ever teach one session, even administrators are mostly active for one session, so the primary barrier is classrooms and similar physical resources.)

BTW, before NZ followed the UK and others in abandoning Forms and Standards, NZ had a form 6 and 7 rather than the lower/upper distinction. That said, NZ also had and has various different possibilities of what a school may teach which I find confusing, and then the Primers 1-4 [5]

Nil Einne (talk) 17:49, 8 March 2020 (UTC)[reply]

The term "standard" comers from the numbered sign, resembling a Roman legionary standard, which stood at the end of each row of benches in a Lancastrian System schoolroom, denoting the year-group of those pupils. From May, Trevor (1994). The Victorian Schoolroom. Princes Risborough, Buckinghamshire: Shire Publications. p. 6. ISBN 978-0747802433. (sorry, no viewable link). Alansplodge (talk) 18:33, 8 March 2020 (UTC)[reply]

Lancasterian System, named after a chap called Lancaster, rather than a "Lancastrian System", named after residence in a county, thus the adjectival form. I presume the Lancastrian System teaches tripe preparation and clog dancing. Andy Dingley (talk) 15:59, 10 March 2020 (UTC)[reply]

Thank you for the correction Andy, I shall write it out a hundred times before playtime. Alansplodge (talk) 19:02, 12 March 2020 (UTC)[reply]