Creating negative voltage with two bench supplies - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 23, 2012, 08:35:50 am
content: G'day,I have two chinese ps305d bench supplies and I want to create +/-15v and gnd.Am I right in thinking, I:Connect negative on PSU one to ground on PSU oneConnect negative on PSU two to ground on PSU oneConnect positive on PSU one to positive on my itemConnect positive on PSU two to negative on my itemCan I connect ground on the psu to ground on my item?Obviously I will measure it first.CheersRichard

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2012, 08:39:10 am
content: fisrt things first, are the outputs isolated? if yes, thenyes you can link ones negative to the others positive and use the center junction as your circuits ground, with + and - around it,

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 23, 2012, 09:00:55 am
content: Thanks rerouter,How do I confirm they are isolated?If I can confirm isolation, can you break down the connection required to make +/-/groundPsu1+ connects to ?- connects to + on psu2Gnd connects to ?Psu2+ connects to - on psu1- connects to ?Gnd connects to ?What is a centre junction, do I need extra cables/connectors?A diagram would be amazing, if someone can chime inThanks.Richard

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2012, 09:05:33 am
content: ok, if each supply has a ground terminal (probably green) seperate from negative then it is probably safe to assume its isolated,you would leave those ground terminals disconnected on both,PSU1:+ goes to your circuit +- goes to your circuit groundPSU2:+ ties to PSU1 -,- goes to your circuits -

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 23, 2012, 09:08:38 am
content: To see if they're isolated, check resistance between the ground socket on the power supply and its earth pin on its wall plug, or just plug them both in and check between the grounds. If it is isolated there will be no connection between them, you may also want to check while they are on (between their grounds) just incase. If it has green earth terminals it is probably isolated also.if they are isolated;psu 1 psu2(+)(-)---------(+)(-)v+ 0v 0v v-

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 23, 2012, 09:17:43 am
content: Thanks boys, the psu has three terminals on the front, as a quick check can confirm continuity with my fluke between the gnd on the front and the earth pin on the IEC socket?

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2012, 09:22:30 am
content: he meant the negative, the green earth terminal should be tied to mains earth, but for an isolated supply the negative should not,

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 23, 2012, 09:47:45 am
content: Doing a continuity check between the earth pin on the power cable and negative on the front, no beep.Doing a continuity check between the earth pin on the power cable and ground on the front, beep.So it's floating.Thanks guys.

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 27, 2012, 01:26:55 am
content: Thanks guys, so I cabled it up, if I measure the voltage by clipping the + terminal on PSU1 and the - terminal on PSU2, I get 30v is that correct, come to think of it I am basically measuring it in series that way?Which terminals would I measure to see -15v, or is that not possible?I just want to confirm it's ok before hooking it up to the board?Thanks again.Richard

author: Rerouter
author_rank: Super Contributor
timestamp: October 27, 2012, 01:34:07 am
content: + of your meter to where the 2 supplies are joined, and - to the negative of PSU2

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 27, 2012, 01:47:51 am
content: Thanks rerouterSo I cabled it like you said:PSU1:+ goes to your circuit +- goes to your circuit ground- goes to + on PSU2PSU2:+ ties to PSU1 -- goes to your circuits -So in the above config - on PSU2 to + on my fluke and - on PSU1 to - on my fluke gives me -15vSo to confirm, should I be good to go:PSU1 + to circuit +15vPSU1 - to circuit gndPSU2 - to circuit -15vRichard

author: IanB
author_rank: Super Contributor
timestamp: October 27, 2012, 01:53:34 am
content: Quote from: rthorntn on October 27, 2012, 01:26:55 amAny time you want to see -15 V instead of +15 V just swap over the positions of the red and black test probes when you are measuring. Which is another way of saying -15 V and +15 V is just what you make of it.For your test setup you will put the two power supplies in series, so that the red terminal of #1 is connected to the black terminal of #2 (this connection is your common, or 0 V point in the circuit). To measure voltages, place the black meter probe at this middle point, then while leaving it there move the red probe from one side (black of #1) to the other (red of #2).