Powering a circuit with reverse polarity - Page 1
Forum section: electronics / projects

author: circuithead
author_rank: Contributor
timestamp: April 01, 2015, 01:55:55 am
content: Hi,I'm trying to get my head around a problem that I'm in two minds about now.. as it seems wrong to me.. yet right.. ?!If I've got a transformer on the input and output of an amplifier.. and it uses one +20VDC rail for power.. and the ground is tied to the chassis..Can I connect a -20V rail to the ground of the circuit (lifted from the chassis of course).. Put the +V of the circuit to the ground/chassis of the power supply.. And bob's my uncle?I'm thinking that the circuit is going to see +20VDC as per usual and testing with a multimeter would read exactly the same as normal.. However the other part of me feels wrong about it..

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 01, 2015, 02:12:02 am
content: Probably a bit slow today, but just trying to make sense of your problem, let me break it down (if I can)Quoteright, so circuitry of the amplifier is isolated form input and output circuitryQuoteok so the amplifier has a negative 'ground' and a positive supply.Quoteif by 'circuit' you mean the 'amplifier' mentioned earlier, and the amplifier 'chassis' is not electrically connected to any other circuitry involved then this would be correct.

author: circuithead
author_rank: Contributor
timestamp: April 01, 2015, 02:35:45 am
content: Yep exactly as you say, yes.. isolated from the outside world.. transformer balanced in & out..Normally it has a ground that ties to earth and a +20VDC supply yes.Great. I thought it was right but the idea of having ground where the +20VDC goes was tripping me up. I need to visualise better that current will flow from ground to a more negative potential just as easy as a +20VDC rail would flow to ground. (Is this correct?)If I may be so bold to ask a couple of follow up questions..Would there be downsides to be wary of.. A floating ground can be susceptible to interference I guess.. but if the positive rail is now grounded and bonded to the chassis.. does this give the circuit the "groundedness" it needs? (apologies for my lack of vocabulary!) The chassis can still be grounded I suppose so no interference can get in or anything..And lastly.. to help with my visualisation of this.. I normally think of earth or ground as the end of a circuit.. the electrons have finally reached their end destination.. but this dilemma would make me reconsider.. so are the electrons coming back out of the earth to get to the -20V rail where they want to be instead as it's a lower potential. (Not sure I've got the right end of the stick at all here!)PS. Thanks for the help. I'm self learning electronics at the moment as I've fallen in love with it for the last few years of tinkering.. but now going through a plethora of books from the beginning to learn properly!

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 01, 2015, 02:47:39 am
content: just to be clear you have something like this

author: circuithead
author_rank: Contributor
timestamp: April 01, 2015, 02:50:36 am
content: Yes.. with the fellow in the middle being the amp in question

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 01, 2015, 03:00:39 am
content: Quotewell this can depend on the application, but often there will be enough grounding for interference via the battery/power supply if this is a concern a capacitor between the 'floating ground' and the 'nominal' ground can be added.Quoteground only becomes the end of the circuit, because it is connected to the other side of the power source, in the case of MEN AC mains the Neutral line is connected to Earth at multiple points (usually at the switchboard where power enters each residence) hence the name MEN or 'Multiple Earthed Neutral'.My diagram I did not show any Ground as for the purposes of visualising current flow it was not significant, although of course if the three amplifiers had their chassis tied to Positive, then negative, then positive ground and you connected those three chassis together that would short everything out. (a ground symbol can be, and often is used in a schematic to avoid have a ground line drawn all over the place)

author: IanB
author_rank: Super Contributor
timestamp: April 01, 2015, 03:11:15 am
content: Quote from: circuithead on April 01, 2015, 01:55:55 amThis is your problem. DC circuits don't have a ground. They have a positive supply rail and a negative supply rail. Whether you choose to call one or other of the supply rails "ground" is up to you. You can make things easier by replacing the word "ground" by "common reference point" or "shared connection point". The chassis can be connected to any point in the circuit you like just as long as it doesn't cause a short circuit to other points in the circuit that it is also connected to.Where these common reference points start to matter is when the common reference points of different modules are connected together, or if they share the same power supply. In such cases the common reference point needs to be the same for all the connected modules or currents will flow where they shouldn't and problems will happen.

author: circuithead
author_rank: Contributor
timestamp: April 01, 2015, 03:38:34 am
content: Ok thanks very much for your help.. I'll stop thinking of ground as something magical in itself and rather think of it as the negative supply rail. The diagram with the battery certainly helped with this yes. If I powered it by a battery I wouldn't even give it a second thought..Ok.. back to the books I go!

author: codeboy2k
author_rank: Super Contributor
timestamp: April 01, 2015, 04:51:10 am
content: Quote from: circuithead on April 01, 2015, 02:35:45 amYes, this is a good way of thinking about it. Think of voltage only as the potential difference between two points, and that (conventional) current will flow like water flows downhill, from the higher potential to the lower potential.Also what Ian said about DC circuits not having a "GROUND". That's just a convention we use for a common circuit tie point, just like Vcc or the positive rail is also a common circuit tie point. (if you look on a schematic, you see multiple connection points to multiple positive rails, but usually only one ground symbol, because all the multiple negative rails have been tied to that one common point). Some schematics with isolated sections use multiple different isolated GROUND points and symbols, or GNDA, GNDB, DGND, AGND (digital and analog grounds)The common "GROUND" point is the reference point from which all other voltages are measured. The difference in potential between the two points is displayed on your multimeter.

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 01, 2015, 12:05:29 pm
content: Quote from: circuithead on April 01, 2015, 03:38:34 amah, but when you start playing with Op-amps you will find you often have three (or sometimes more) rails; a positive rail, a zero volt rail (also often the ground) and then a negative rail.In this case the 0V Rail is often tied to, or becomes ground.....as per the attached picture - although note this would be a block diagram as you do not usually attach the 0V rail directly to the Op-amp, although it is used by the voltage dividers involved in setting feedback and gain and some other amplifiers may require the 0V rail to be connected to them.