AC / DC Question - Page 1
Forum section: electronics / projects

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 11:09:59 am
content: I'm designing a device that can be powered by either a 24V DC supply, or 24V AC supply. As this is not mains AC compatible, am I right in thinking that the input to the rectifier only needs to be a 2 way connector with an AC/DC select jumper?My other thought was to use a 3 way input connector for a dedicated high side, low side and Ground terminal.Thoughts?

author: ruffy91
author_rank: Regular Contributor
timestamp: July 20, 2016, 11:20:57 am
content: You don't even need a jumper. Just let the DC through the rectifier. You loose a bit efficiency but it's polarity independent.

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 11:43:42 am
content: Thats what Im planning to do, but was thinking that I would need to provide the actual GND reference for the circuit in DC mode. Surely connecting the GND reference to the low side input on the rectifier will screw up the GND reference for the circuit post rectifier??

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 20, 2016, 11:47:44 am
content: Since you have not revealed the larger picture, we have no idea whether "provide the actual GND reference" is necessary or even desirable (or dangerous, for that matter). You are asking questions about a system which only you know the necessary details.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 11:55:43 am
content: Quote from: MyElectronsFellOut on July 20, 2016, 11:43:42 amIt might, but it depends on what the rest of the thing is, which you DON'T seem to have supplied any details about.

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 12:05:00 pm
content: Well here's the idea of what I'm working on.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 12:25:01 pm
content: Did you draw that out yourself ?The diodes seem to be incorrectly connected up (the wrong way round in some cases).

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 12:29:22 pm
content: Ah yea may not have been paying attention there.Lolling at how amazingly wrong I got those diodes lol. anyway, going to be using a rectifier IC, the diodes were merely for illustration.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 12:39:40 pm
content: Everyone in this thread, has promised to sign the official secrets act, and sign three none disclosure agreements.What is it powering, and is the ground used for some of the inputs and/or outputs ?Or is it essentially floating, relative to the power supply source(s) ?What exactly is this external ground thingie ?Labeled "DC Gnd" ?

author: Fungus
author_rank: Super Contributor
timestamp: July 20, 2016, 12:40:55 pm
content: It'smucheasier/cheaper/safer to just feed the DC into the rectifier using the same connector as the AC.Think what would happen if the jumper is in the "DC" position and somebody connects an AC adapter....

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 12:46:30 pm
content: Quote from: Fungus on July 20, 2016, 12:40:55 pmThat's a good way with many things.But if the DC supply, also supplies other stuff, which will be inputs and so on, with this unit. The diodes would introduce offset voltages, which may upset the input's readings and outputs etc.It's a fine method as long as the supply(s) have nothing to do (electrically) with anything else associated with this, whatever it is ?

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 01:01:05 pm
content: Quote from: MK14 on July 20, 2016, 12:46:30 pmWhy in the world would the power supply somehow have a direct relationship with the "signal" part of things? That would be a very particular, unlikely application.Agreed here, just feed the DC into the same 2 terminals as AC, no switching nor dual connectors.

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 01:05:53 pm
content: Ok here we go...DC GND is exactly what it says on the tin. 0 Volts! referenced to +24V at the input. It's not floating and merely represents a DC level of 0 volts. It is run of the mill Ground reference.Fungus, my thoughts exactly hence why I'm leaning away from that idea. Don't want anyone fiddling with it and co*king it up. With that in mind, If I connect +24v DC to the one input of the rectifier, and 0V (DC GND) to the other, (bearing in mind i'm feeding into a 15V Voltage regulator), what will the negative input at the regulator see?The regulator negative input needs to be at the same potential as the circuit ground IE 0V.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 01:11:38 pm
content: Quote from: Kilrah on July 20, 2016, 01:01:05 pmPower supplies cost money or there may be one big battery.So some things use the same power source for many of the electronic items connected up to it. It also helps, because one switch can turn everything off at the same time.It could be a vehicle, boat or something.The OP is being very secretive, so far.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 01:16:39 pm
content: Quote from: MyElectronsFellOut on July 20, 2016, 01:05:53 pmWhy ?You are still NOT explaining WHY ?Does a diode drop (0.7 volts or so), matter ?

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 01:19:28 pm
content: MK there is no need for me to post lots of info about the device here, all i'm interested in is how best to implement an AC/DC compatible input to a voltage regulator. And if simply passing 24V dc through the rectifier will mean that the negative input terminal on regulator will be at 0V?I have tried doing a node analysis and it says 0V, but I would like a second opinion.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 01:26:07 pm
content: Quote from: MyElectronsFellOut on July 20, 2016, 01:19:28 pmDefinitely the AC and DC going via the same bridge rectifier, using NO switches or different sockets is the PREFERRED solution. As other(s) mentioned above it is the easiest and safest way. Many things use that method. It is safe against user abuse.But there will be small voltage drops across those diodes, which for most circuits doesn't matter. Because usually the batteries and/or power supply are separate.If you need a DC Gnd, why not use the 0V connection on the regulator output (common pin) ?

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 01:52:46 pm
content: Quote from: Kilrah on July 20, 2016, 01:01:05 pmSorry, I was mixed up, and was misunderstanding the OP.I thought that he meant that the DC power supply, also went to other equipment, which he was not telling us about.So that it was essential that the regulator ground pin is at exactly 0V, without any diode drops and stuff.Apparently I was WRONG, because I misunderstood the little details that the OP has given us.

author: Monkeh
author_rank: Super Contributor
timestamp: July 20, 2016, 01:54:59 pm
content: Quote from: MyElectronsFellOut on July 20, 2016, 01:19:28 pmIf your DC supply is ground referenced and you want to ensure people can't cock things up (as they surely will with that jumper idea..), then provide a separate ground terminal.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 20, 2016, 03:15:45 pm
content: I give up. @MyElectronsFellOut thinks there is no reason to reveal critical parts of the puzzle. He doesn't answer direct questions, and he thinks that we will mess with his design. So it is not even clear why he is here and asking this question.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 03:18:34 pm
content: Quote from: Richard Crowley on July 20, 2016, 03:15:45 pmI agree with you.I am still very unclear as to what is going on with the grounds.

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 20, 2016, 03:46:34 pm
content: Richard, I can get snide remarks from my little sister thanks.For now, everything past the voltage regulator is a 'dont care black box' it's simply doing a little analogue signal conditioning. Nothing exciting. I've fixed the original diagram I posted and removed the 'GND's as they seemed to be confusing / annoying people?!? It also describes the three options I was looking at/ considering.

author: Monkeh
author_rank: Super Contributor
timestamp: July 20, 2016, 03:51:32 pm
content: Quote from: MyElectronsFellOut on July 20, 2016, 03:46:34 pmNo, it is your lack of clarity which is confusing people.You are deeply concerned that your regulator not be at a different potential to the DC negative lead.Why?Is it an earth referenced supply and you have concerns about lifting the ground? Are you supplying multiple modules and need to ensure their ground potentials meet? (this is equally a problem with a common AC supply!)In either case, if you're really worried about making sure 'ground' is always the same potential between devices, do not use the power supply leads to provide this: Connect all device grounds to a star ground or common chassis firmly, no matter whether you're using floating supplies or earth referenced, AC or DC.

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 03:58:31 pm
content: Quote from: MyElectronsFellOut on July 20, 2016, 03:46:34 pmOnly knowing what is in the "don't care black box" can allow people to determine whether the diode drop of option 3 is acceptable or not.Only knowing about both the supply, the "don't care black box" and other devices the system may be part of can allow people to determine whether option 2 is appropriate or not.So you're asking people about 3 options, but not giving them the info needed to advise you. You need to either describe all relevant aspects of the system, or figure out requirements yourself and mention them. If you say "a diode drop is NOT acceptable" then we know you thought of that and excluded it. If you don't then we don't know if you considered and excluded it, or if you just didn't think of it and it could/should be investigated, with the appropriate info.

author: MK14
author_rank: Super Contributor
timestamp: July 20, 2016, 04:24:52 pm
content: Quote from: MyElectronsFellOut on July 20, 2016, 03:46:34 pmYou don't necessarily have to tell people everything, and give all the tiny details.But on the other hand you don't say "There is a Black box, do I go for options 1, 2 or 3 to best/optimally connect it up, with 100% safety ?"E.g.I have half a dozen analogue sensor boards, with op-amps, and the 15V rail stuff supplies all those boards. It can run off an AC adapter, or a large battery.Just summarize what the stuff is and what is going on, as much as you want, to hide the reality of what you are doing.But BLACK BOX is going too far, if you want people to happily help you with stuff like this.Question:What exactly is the alternative (non-AC) DC power source ?Is it batteries or something ?When and how often is it used ?Does the right hand 0V lines (coming out of the regulator), need to be electrically (directly) connected to the left hand 0V lines, coming in from the DC ?Or is it just the way you drew out the diagram ?If you CAN'T/WON'T answer any of these (or anybody else's) questions.I go for Option4, which is where the AI inside the black box, creates its own power absorbing force field, hence no longer needing any external power.