old schematics - positive rail at the bottom? - Page 1
Forum section: electronics / projects

author: ledtester
author_rank: Super Contributor
timestamp: February 15, 2016, 12:20:15 am
content: I was wondering why older schematics were often drawn with the positive rail at the bottom, for example like the attachment below.Nowadays the convention seems to be to place the positive voltage rail at the top. Was there a good reason for the original convention? And how and when the "switch-over" occur? The attached schematic is only from 1960.Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: February 15, 2016, 12:21:27 am
content: That's because the active device is a PNP transistor. Such circuits would typically ground the negative rail, if ground there be.

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2016, 01:12:59 am
content: Thermionic valve schematics were drawn with the positive rail at the top. When the Germanium transistor was first commercially introduced, back in the 1950's they were all PNP and as it was desirable for engineers to recognise familiar circuit configurations the convention of putting the negative rail at the top was introduced so the layout resembled a valve schematic. Later, NPN transistors became available, and with the introduction of the Silicon transistor, common, and the original convention of positive at the top was restored.Rule of thumb: If the majority of the active devices in the circuit are Germanium PNP transistors, the schematic will usually have the negative rail at the top.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2016, 03:33:39 am
content: What do you mean? Power's on top, emitter's on the bottom, signal's from left to right (well, it's an oscillator, so nevermind that)...An excellent reminder that ground is an arbitrary definition, and it makes more sense to use different polarities with different circuits.It's a common sight in oscilloscope schematics to see the signal path from left to right, with one phase on top, inverse phase on bottom. Which means the bottom half is all upside down, but you get used to it because it's almost always mirrored from the top side (and anything that's going between, like common mode terminations, bias, etc. tends to stand out).Tim

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: February 15, 2016, 03:35:36 am
content: It is just wrong, wrong, wrong, wrong!

author: Len
author_rank: Frequent Contributor
timestamp: February 15, 2016, 03:17:17 pm
content: Quote from: Alexei.Polkhanov on February 15, 2016, 03:35:36 amThe wrongest thing is a grid dip meter with no grid!

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 15, 2016, 03:31:36 pm
content: Well I guess it dosent truly matter- but good question. To add to your confusionI have seen amp circuits with the input on the right side and the positive power rail (like with what your saying) on the bottom. I HAVE to flip these in photoshop or a editing program because its just SOO confusing!I would be interested if there is actually a reason..

author: AF6LJ
author_rank: Unknown
timestamp: February 15, 2016, 03:38:19 pm
content: Quote from: T3sl4co1l on February 15, 2016, 03:33:39 amMany things get drawn that way, I am sure the case is true with you as well as I; After many years in this hobby / business we are not bothered by departures from convention.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 15, 2016, 04:25:36 pm
content: Quote from: AF6LJ on February 15, 2016, 03:38:19 pmIn the past we had decent schematics, whether hierarchical or not, with neat drawn signal lines, buses and proper page references. Today we get a stack of sheets with nummeric lists of p-t-p connections. Looks we are going back in tiime when we programmed a 256 bytes fuse prom bit by bit by hand...

author: AF6LJ
author_rank: Unknown
timestamp: February 15, 2016, 05:45:08 pm
content: The human drawn diagrams were always nice.Well mostly.......

author: German_EE
author_rank: Super Contributor
timestamp: February 15, 2016, 05:56:19 pm
content: The first electronics book I ever had was 'The Pegasus Book of Radio Experiments' by FG Rayer G3OGR and it was a mixture of tube and transistor circuits. All of the transistor designs used PNP devices like the OC71 with positive ground at the bottom so changing over to negative ground BC108 circuits was a bit of a challenge.