Negative Voltage Source... - Page 1
Forum section: electronics / beginners

author: wcndave
author_rank: Newbie
timestamp: March 23, 2024, 06:24:02 pm
content: I dabble in electronics and have built quite a few things, including with arduinos and other microcontrollers, and have aStamos soldering/SMD stationwith built in power supply.My son came back from school for the weekend and needs to do a project with an op-amp that requires -15 and +15 voltage rails....He said at school they connect the positive of a source to the earth of another source.So I looked at my supply and figured I can't do anything like that. I have another supply, also without separate ground, but it feels like without a ground, I can't do much?I am thinking that with a supply like thisOwon SPE6103I would need to use two power supplies, and connect ground to positive on the Owon, and then the negative/ground from my Stamos to the negative/ground on the Owon also?Does that work, or is there an easier method?Many thanks! Dave

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 23, 2024, 06:52:10 pm
content: The Owan looks like it's output is floating (Not connected to protected Earth) but the Stamos may not be (Minus output possibly connected to protected Earth). So I'd connect your project like this:

author: tooki
author_rank: Super Contributor
timestamp: March 23, 2024, 07:00:53 pm
content: The issue is that you and your son are equating the negative and ground: the fact that the Owon supply has ground entirely separate should be a clue that they’re not the same thing. The ground (green) jack on a bench PSU is an actual earth ground (aka PE, protective earth), and 99.9% of the time you will not connect it at all. That includes this time. Leave the green jacks unconnected to anything.A bench power supply’s output is floating; it has no ground reference (circuit ground, i.e. your 0V reference). So you can (and must) place that ground reference wherever you want in the circuit. Here, you want it at the midpoint between your power supplies, so that you end up with: -15V — 0V — +15V.You do that by taking two 15V supplies and connecting them thusly:PS 1: negative jack = -15V, positive jack = 0VPS 2: negative jack = 0V, positive jack = +15VConnect the two 0V jacks together. That’s your circuit ground. Observe how in both cases, the red positive jack is 15V higher than the black negative jack. And that the difference between PS1’s negative jack and PS2’s positive jack is 30V.If this doesn’t make sense yet, you need to go back to the absolute basics of voltage. Take two AA batteries in series and a multimeter, and start probing around every combination of terminals. You need to understand that a voltage measurement is always relative — it’s thedifferencein voltage between two points.P.S. As Kim says, just in case the Stamos is not, in fact, a proper floating bench supply, make sure to wire them the way they show, using the Owon as PS1 and the Stamos as PS2.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 23, 2024, 10:07:39 pm
content: Dave, This is what triple power supplies were made for.They are typically two 0 to 30 volt supplies (around that voltage) and a 5 volt supply. They are "Floating"The 5 volt is for TTL, Transistor to Transistor logic devices. Very common devices.Perhaps getting one of these triple supplies will help your son in his endeavors as he progresses. The suggestions above will work butif you are going to purchase something, I would buy a triple supply. Just my opinion.

author: DLXXV
author_rank: Newbie
timestamp: March 24, 2024, 11:54:57 am
content: I would just throw an ICL7660 in the circuit and make it negate the 15V. Or use another method to negate the voltage. It’s much easier than buying a new power supply and all that. Not to mention it makes the entire circuit much simpler.

author: wcndave
author_rank: Newbie
timestamp: March 27, 2024, 08:31:46 pm
content: Quote from: Kim Christensen on March 23, 2024, 06:52:10 pmThat is what I was thinking, butonlyif I was correct in my assumption that a power supply with a specific separate ground terminal will always be floating.Quote from: tooki on March 23, 2024, 07:00:53 pmThat is almost precisely what I was avoiding, and hence asking...Quote from: tooki on March 23, 2024, 07:00:53 pmYes, that's what led me to think that a supply without a separate ground will have a problem, because connecting two of them in this way will short circuit them, assuming that a 2 output supply does in fact have the negative grounded, which if it isn't seemed rather dangerous...Quote from: tooki on March 23, 2024, 07:00:53 pmTalking of which, is it not dangerous to have a lot of ungrounded equipment like this?Quote from: tooki on March 23, 2024, 07:00:53 pmYes, the idea that it's a relative potential difference is clear. I was wondering why could one not use a single power supply with 30v. However having tried that, it didn't work. I imagine it's that one can't divide the resistive load so it just doesn't work, you get for example 10v across negative to 0, and 20 across 0 to positive...Quote from: tooki on March 23, 2024, 07:00:53 pmThis seems to imply that all "proper" supplies should be floating. As above, isn't having a lot of unearthed equipment dangerous, that's why we have earths!Quote from: Wallace Gasiewicz on March 23, 2024, 10:07:39 pmI agree, however triple supplies seem to start around €500, whereas the Owon is €100.... I'm not spending 500 on a power supply!

author: radiolistener
author_rank: Super Contributor
timestamp: March 27, 2024, 08:38:47 pm
content: you can buy two Chinese 5 Amps / 15 Volts linear PSU like HY1505D, they cost about 70-80 USD.Then you can connect them in series and use as bipolar source by using middle point as a ground.So the total price will be about 150 USD.

author: shapirus
author_rank: Super Contributor
timestamp: March 27, 2024, 08:52:58 pm
content: Quote from: wcndave on March 23, 2024, 06:24:02 pmIn addition to what was suggested above, you can grab a bunch of these isolated DC/DC switching converters. Really handy stuff.2-in-1:https://www.aliexpress.com/item/1005006141013253.htmlsingle:https://www.aliexpress.com/item/33014381098.html

author: radiolistener
author_rank: Super Contributor
timestamp: March 27, 2024, 09:22:47 pm
content: Quote from: shapirus on March 27, 2024, 08:52:58 pmDC/DC produce a lot of noise which is problematic to filter and it is critical issue for analog circuits like op-amps. I don't recommend to power op-amp from DC/DC module or SMPS PSU.For example I have some Chinese module with DAC and buffer op-amp. And it uses switching capacitor negative voltage converter in order to get bipolar power for op-amp. And its noise leads to spikes on op-amp output which is higher than a half of DAC dynamic range...I also had experience with these DC/DC modules, I tried to filter it's output for analog circuit. But it's very hard, and probably just impossible. In addition to noise on power line it also produces a high magnetic field noise around this module, and any component or wire which is placed close than 10-15 cm to that module get a strong EMI noise. Metal shielding don't helps much.

author: wcndave
author_rank: Newbie
timestamp: March 27, 2024, 09:31:00 pm
content: Quote from: radiolistener on March 27, 2024, 08:38:47 pmYes, the suggestion about a triple supply was to avoid having two... I had thought having two might be a more economical course!

author: shapirus
author_rank: Super Contributor
timestamp: March 27, 2024, 09:41:01 pm
content: Quote from: radiolistener on March 27, 2024, 09:22:47 pmIt depends. For some applications it may be unacceptable at all, for some it can be easy to filter down to acceptable levels, and for some -- school projects may very likely be in this category -- it may be acceptable as is.Op amp's PSRR combined with RC filters (which don't even require R to be large given the frequencies in question) before the VCC/VEE pins can do a good job of supply noise cancellation.In demanding cases shielding can be used as well.Quote from: radiolistener on March 27, 2024, 09:22:47 pmThat may be either of, or a combination of: power supply noise, crosstalk between power and signal tracks, or an inappropriate probing technique.For example, in one case recently I had to use a piece of coax cable with a BNC connector on the output of a current transformer module that I powered from such a +/- 15V dc/dc unit to be able to observe output in the 10s of mV range. Otherwise, if a pair of plain wires was used, the scope showed a lot of noise on that output, and I wasn't sure if that was in the actual output signal or just EMI-induced noise.It turned out to be the latter: the noise that the scope was displaying was reduced by a factor of 10-15 with the switch to the coax cable. And even that little which remains may still be EMI leaking through the spots not covered by the coax shield (there are some).

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 27, 2024, 10:36:29 pm
content: Quote from: wcndave on March 27, 2024, 09:31:00 pmThe nice thing about dual output supplies is that they usually have something called "tracking mode". This means that you can flip a switch and then simultaneously control the voltage of both outputs with a single knob. Or you can flip the switch the other way and adjust them independently.