+/-/0v DC circuit, from DC +/0v ---- something simple to build?

Hey all! Please forgive my ignorance if this is something very easy to build/explained elsewhere on the site.

I have some old Schlumberger modules that requite + and - 15vDC and 0v. Normally I am powering everything from 24vDC source, so ideally, I'd like to have a step-down to 15vDC and output +/-/0v, but 15vDC>15vDC is also fine.

I bought this buck converter on Ebay -- https://www.ebay.de/itm/20W-5V-to-24-Positive-Negative-Dual-Output-Power-Supply-Boost-ConverterL-BJ/333676574872?ssPageName=STRK%3AMEBIDX%3AIT&var=542767309584&_trksid=p2057872.m2749.l2649 -- but it exhbits too much parasitic noise to be useful, but it does work.

Is there a simple circuit I can built to at least get +/-/0v from a 15vDC source? I could place a step down in front that I know to be clean.

Thanks for your help!!

adamasnan said:

I bought this buck converter on Ebay -- https://www.ebay.de/itm/20W-5V-to-24-Positive-Negative-Dual-Output-Power-Supply-Boost-ConverterL-BJ/333676574872?ssPageName=STRK%3AMEBIDX%3AIT&var=542767309584&_trksid=p2057872.m2749.l2649 -- but it exhbits too much parasitic noise to be useful, but it does work.

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This type of converter should be loaded enough, at least 20% of its rating.
The outputs should be passed through a CLC filter.

OK, thanks - I can try that, but never made a CLC filter before.. Could you estimate the approx values for cap, resistor and inductor for usage? I do not have the consumption specs for the modules, but I have 2 buck converters, so I can just use both, and draw would be consistent to each module.

Still, out of curiosity, is there a simple circuit to generate +/-/0v from a +/0v dc source?

Yes, there are rail splitter chips like this one: https://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1199.html

But you will only get +/-12v from +24v

That DC converter should work pretty well I would think. The China ones can break pretty easily, the builds are sloppy and the performance is mediocre compared to something like MeanWell ones. But they're super cheap and if they work for longer than a week they tend to keep working.

But I don't know how you could use a CLC filter. It wouldn't pass any DC! It would have to be RLC. First pick your inductor. You have two options. One is to make it just big enough to get the switching noise. This is technically all you really need. But bigger inductors are available especially if you're not putting to much current through it in which case you could go way down into the audio range.

But let's say you just want to get the switching noise so you pick a 100uH inductor. Punch this into an online calculator like here:

  http://sim.okawa-denshi.jp/en/RLClowkeisan.htm

and using 10 ohms, 100uH and 1u, that's a Q=1 at ~16kHz. That should completely nix any high frequency hash.

Alternatively you could buy two MeanWell 24V, use said RLC and regulate down to +-15. That would be the "pro" solution IMO.

adamasnan said:

I have some old Schlumberger modules that requite + and - 15vDC and 0v.

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Can't you do a dedicated PSU for those modules?

A + and - 15VDC PSU it's really simple to build

So, I made the RLC filter and the noise persists, albeit maybe less so. It's a weird noise, it's variable, up and down like sine sweep and squiggly. I suspect it's the pcb.

What other options might I have?

Ideally I'd like to keep these as mobile as possible, but a dual 15v PSU could be fine, just a bit annoying to have extra plugs. Actually I prefer to use battery supplies for the vast majority of my kit, hence the need for dcdc conversion.

squarewave said:

But I don't know how you could use a CLC filter. It wouldn't pass any DC!

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The
C's are in parallel with the output, and the L is in series.  Why wouldn't this pass any DC?  Or did I miss something obvious?

Matador said:

The
C's are in parallel with the output, and the L is in series.  Why wouldn't this pass any DC?  Or did I miss something obvious?

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True. I was thinking the first element is in series but of course it could be shunt > series > shunt as you state.

adamasnan said:

So, I made the RLC filter and the noise persists, albeit maybe less so. It's a weird noise, it's variable, up and down like sine sweep and squiggly.

Click to expand...

It's typical of a smps in hiccup mode because it is not enough loaded.

I'm little bit confused how this circuit works. Is it possible to get +/- voltage using three single coils?
Schematic from manufacture and picture from eBay attached.
Seller on eBay also said in notes:
1 In order to obtain a more stable voltage, it is recommended that the output is greater than 15MA
2 V- cannot be used alone, or no voltage output.But you can just use V+
3 The startup current of the module is 2.5-3 times the operating current.
4 Increasing the 1UF-10UF MLCC capacitor reduces the startup current

Ah, I see - I could only assume the Schlumberger module was 15ma or more, but perhaps not - I don't have any specs.

So, let's assume this PCB won't work properly for the application, what are my other options do you think?

Did you hang 680 ohms on the output and then test for noise?

adamasnan said:

So, let's assume this PCB won't work properly for the application, what are my other options do you think?

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I believe that converter you bought on eBay will not work properly at all. All coils must be wound on one core as indicated in the diagram and not each separately.
I would try this one,
https://www.meanwell.com/webapp/product/search.aspx?prod=DKMW15
https://hr.mouser.com/ProductDetail/MEAN-WELL/DKMW15G-15?qs=hWgE7mdIu5SxQYGTL7W5AA%3D%3D

moamps said:

I would try this one,
https://www.meanwell.com/webapp/product/search.aspx?prod=DKMW15

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Wow. That looks like a great part! 500mA of +-15 from a 1 square inch metal can with only 50 mVpp of 300kHz ripple. The single output ones are trimmable.

Requires a few extra parts though and looks brand-spankin' new. Mouser shows November availability. If they have or come out with some higher voltages like 24V or 48V, that would make building a supply really easy for just about anything minus tubes.

moamps said:

I believe that converter you bought on eBay will not work properly at all. All coils must be wound on one core as indicated in the diagram and not each separately.

Click to expand...

The schematic attached to this one is the generic schemo from the XL6019 chip. I think it is not based on the same principle. It combines both inverting and non-inverting mode, so one of the inductors is the tank and the two others are just filters. Just guessing.
I know the chinese can't always be trusted but I believe this part works somewhat.

I would try this one,
https://www.meanwell.com/webapp/product/search.aspx?prod=DKMW15
https://hr.mouser.com/ProductDetail/MEAN-WELL/DKMW15G-15?qs=hWgE7mdIu5SxQYGTL7W5AA%3D%3D

Click to expand...

Indeed, I have never been disappointed with Meanwell products.
I would certainly make provision for additional LC filtering.

squarewave said:

If they have or come out with some higher voltages like 24V or 48V, that would make building a supply really easy for just about anything minus tubes.

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Well, they are non-isolated converters; you'd still need an AC/DC isolated converter in front.
And probably additional LC at the output.
Actually I prefer using 24V smps with additional linear regulators. I know it's not energy efficient, but not less than a typical linear PSU, and cheaper.

abbey road d enfer said:

...... It combines both inverting and non-inverting mode, so one of the inductors is the tank and the two others are just filters. Just guessing....

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I never saw dual SMPS design where only one active coil is used.  Could you please draw a functional schematic of it?

moamps said:

I never saw dual SMPS design where only one active coil is used.  Could you please draw a functional schematic of it?

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I'm not a specialist of smps, but I imagine where there's a square-wave, there's a way to produce rectified voltages of both polarities.

abbey road d enfer said:

I'm not a specialist of smps, but I imagine where there's a square-wave, there's a way to produce rectified voltages of both polarities.

Click to expand...

Ok, find me such a working schematic (single active coil, single voltage input, +/- output / same current) on www or anywhere else (any literature, credible source) and I will donate USD25 to maintain this site.
@adamasnan
can you please draw the smps schematic you bought, if it is essentially different from the schematic I posted in my replay 10 I will donate an additional USD25 to maintain this site