Using positive output as ground. - Page 1
Forum section: electronics / beginners

author: TNb
author_rank: Regular Contributor
timestamp: October 07, 2013, 08:27:02 am
content: Hi! I recently got a new power supply and it has a separate ground output that is considered to be connected with negative terminal if needed. I have a stupid beginners question - if I will connect positive(!) output of power supply to ground - will it work the same way or it's a bad thing to do? I have no practical purpose for doing it, just curious.Thanks!

author: kizzap
author_rank: Unknown
timestamp: October 07, 2013, 09:47:09 am
content: How do you think negative voltages work?-kizzap

author: deth502
author_rank: Frequent Contributor
timestamp: October 07, 2013, 10:37:04 am
content: you want to connect the pos output to the gnd output? NO!you want to use the pos as "gnd" and the neg terminal as a neg voltage on a circuit? sure.

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2013, 11:14:45 am
content: Quote from: TNb on October 07, 2013, 08:27:02 amDepends on the supply. If it is a floating or isolated output (as most lab supplies are), then yes, you can connect the ground terminal to ground when you want a negative supply for your circuit.

author: Monkeh
author_rank: Super Contributor
timestamp: October 07, 2013, 12:38:07 pm
content: Quote from: deth502 on October 07, 2013, 10:37:04 amPlease do explain why not, because I have lots of equipment with negative voltages relative to ground.

author: geo_leeman
author_rank: Regular Contributor
timestamp: October 07, 2013, 12:43:42 pm
content: On an isolated supply, ground is just some arbitrary reference point. It could be 50V! Let's say you do that. Voltage is the difference in potential right? So then lets say 50V is your ground and then 100V would be a +50V difference or supply. 0 Volts to your new floating ground would then be a -50V supply. This is similar to using 2 9V batteries to make a +/- supply for Op-amps and the like. Where you can get in trouble is using mains (earth) referenced equipment. Then ground is tied to the building ground and connecting 50V will result in a large current flow and bad things... Dave actually talked about this in his "How now to fry your scope" video.

author: Dave
author_rank: Super Contributor
timestamp: October 07, 2013, 03:27:19 pm
content: Quote from: deth502 on October 07, 2013, 10:37:04 amYou can easily connect the positive output to mains earth ground, given that the power supply is completely isolated.If you meant that he shouldn't connect the positive output to the negative output (shorting the output), you would kinda be right. It's not that big of a deal, really, since power supplies usually have current limiting and/or thermal shutdown.Well, technically you could burn down your house, but you would kinda be asking for it, if you were messing with a power supply with no protection whatsoever.

author: deth502
author_rank: Frequent Contributor
timestamp: October 07, 2013, 07:55:23 pm
content: Quote from: Monkeh on October 07, 2013, 12:38:07 pmrelative to ground, yes, not the same as shorting your positive output directly to your gnd/neg output.like i said, he said "if I will connect positive(!) output of power supply to ground". without specifying earth ground from the outlet or the gnd of the power supply. so i reiterate, pos output to the gnd output? NO. if you want to use the pos output as a "gnd" on a circuit, or even connect the pos to an earth, as long as the supply is isolated, fine.

author: Monkeh
author_rank: Super Contributor
timestamp: October 07, 2013, 08:03:01 pm
content: Quote from: deth502 on October 07, 2013, 07:55:23 pmHe made it pretty clear he has a floating power supply with three terminals.

author: IanB
author_rank: Super Contributor
timestamp: October 07, 2013, 08:03:19 pm
content: Quote from: deth502 on October 07, 2013, 07:55:23 pmYou are confusing two concepts by somehow assuming "negative" and "ground" are the same thing. But they are not. There is no such thing as a "gnd/neg" output.A bench DC power supply usually has two output terminals: a red one (positive) and a black one (negative). Some power supplies may also have a green terminal (earth/ground reference). If such a terminal exists, it is perfectly OK to connect either the red terminal or the black terminal to the green terminal. It makes no difference to the operation of the power supply either way.Under no circumstances is the black negative terminal a "ground" terminal(*). It is completely floating, just like the red terminal. Calling it ground will confuse both yourself and others.(*) Except in the case of an ATX computer power supply. But this is not a laboratory DC power supply and should never be confused with one.

author: Lupin III.
author_rank: Contributor
timestamp: October 07, 2013, 11:48:13 pm
content: In a schematic "ground" is often just the definition of the point of zero volts. This does not necessarily mean it's the same as earth ground. On your powersupply on the other hand "ground" is a connection to earth ground of the wall outlet.If you have isolated outputs you will have to touch + and - at the same time to get a shock (assuming your powersupply is set to a high enough voltage). Touching only one of them wont get you shocked. If you now connect one of the outputs to ground (to answer your question: it doesn't matter if it's + or -), the other output is now the set voltage higher/lower than ground. This means you can get shocked by only touching the one lead that's not connected to ground.

author: geraldjhg
author_rank: Regular Contributor
timestamp: October 15, 2013, 01:38:00 am
content: if its the only test equipment you operate theres no big problemBUT some instruments, notably those with BNC inputs as scopesand rf instruments are ALWAYS connected to ground and thenhaving your power supply positive grounded gets one confusedas GENERALLY power supply negative is near ground potential.you forget and clip scopes probe to something and then power it and POOF

author: Stonent
author_rank: Super Contributor
timestamp: October 15, 2013, 02:02:49 am
content: Think of it this way. You have 2 9V batteries in series which gives you a total of 18V. If you connect your - lead to the bar that connects the two batteries together, you have +9 on one side and -9 on the other.(Ignore the opamp part, I just googled 9v batteries in series and that's what I found)But remember you can only do stuff like this if the two are electrically isolated. 2 batteries are isolated from each other. A battery and a power supply are also isolated. Two power supplies maybe isolated or may not be. A power supply and a - pin on a USB port might be isolated or might not be. If both devices plug into a wall outlet there's always the potential of causing a short so you have to be very careful. Dave's video on how to not blow up your scope explains this fairly well.

author: Jebnor
author_rank: Regular Contributor
timestamp: October 15, 2013, 03:09:04 am
content: Quote from: TNb on October 07, 2013, 08:27:02 amAre you talking about a Ground earth Reference, an additional binding post, usually green?