What's the 'best' way to get bipolar power from a wall-wart?

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plushterry
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What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Thu Feb 04, 2021 5:45 pm

I'm planning a standalone filter box (not part of a modular) that I'd like to power from a wall-wart. I've been looking at various ways to get a negative power rail . it seems like there are a few ways to do it

2 equal resistors between the power rails, buffered, to create a mid point
some sort of switching device such as ICL7660 or MAX1044
use an AC wall-wart and a half-bridge circuit (MFOS wall-wart PSU style)

My rough estimate is I'm going to need about 100mA in total for the -ve rail, 200mA for the +ve rail. Preferably 15V.

I played around with some ICL7660s last night and I needed 5 in parallel to get close to the that sort of current and it looked horrible on the scope, even with quite a lot of capacitance between it and ground. Also, they only reliably go to 12V and ideally, I'd like 15V if I can. I've not used the MAX1044 before, but it looks like a very similar sort of thing.

Are there other switching devices that people could recommend me having a look at?

I haven't tried the buffered voltage divider yet, but I'm guessing that the amount of current the buffer can provide is going to be an issue here? not sure how to work that out. I might try this later.

I'd rather not use an AC wall-wart unless I have to because they're pretty uncommon here in the UK.

I'm happy to consider alternatives if anyone has got any suggestions. Any advice gratefully received!

Thanks!

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by Mungo » Thu Feb 04, 2021 6:05 pm

All the major semiconductor manufacturers have similar application notes:
https://www.analog.com/media/en/referen ... 1021fa.pdf
This is the circuit arrangement commonly used in small power supplies for eurorack.

There are also specific switching regulators for positive to negative conversion, but they are less common/more expensive.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by MikeDB » Thu Feb 04, 2021 6:23 pm

plushterry wrote:
Thu Feb 04, 2021 5:45 pm
I'm planning a standalone filter box (not part of a modular) that I'd like to power from a wall-wart. I've been looking at various ways to get a negative power rail .
If you're happy to use a 5V wallwart (e.g. some phone chargers, Pi PSU, some CCTV PSU etc) then the TI TPS6513x devices are very good. Can produce +/- 15V 300mA from +5V.

https://www.ti.com/product/TPS65130


Available as a module from China if you prefer.
e.g. https://www.aliexpress.com/item/1005001869133587.html

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Thu Feb 04, 2021 6:50 pm

ahh brilliant, thank you gentlemen! they both look really good, i'll do a bit of research on them. nice one.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Thu Feb 04, 2021 7:53 pm

MikeDB wrote:
Thu Feb 04, 2021 6:23 pm
https://www.ti.com/product/TPS65130

Available as a module from China if you prefer.
e.g. https://www.aliexpress.com/item/1005001869133587.html

these look great. have you used them? is it worth following them with a pair of LDOs would you say?

i did get a couple of the modules to check em out. will probably a layout myself as they give an example in the datasheet and it'd be a handy one to have in the arsenal.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by Haralds:Werk » Fri Feb 05, 2021 6:06 am

I have build something like this for a standalone device with +/-15V output. Look here as a starting point:
https://www.haraldswerk.de/PSU/PSU_Sing ... ouble.html
Completely analog.
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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by MikeDB » Fri Feb 05, 2021 8:02 am

plushterry wrote:
Thu Feb 04, 2021 7:53 pm
MikeDB wrote:
Thu Feb 04, 2021 6:23 pm
https://www.ti.com/product/TPS65130

Available as a module from China if you prefer.
e.g. https://www.aliexpress.com/item/1005001869133587.html

these look great. have you used them? is it worth following them with a pair of LDOs would you say?

i did get a couple of the modules to check em out. will probably a layout myself as they give an example in the datasheet and it'd be a handy one to have in the arsenal.
I haven't used the modules, but have used the TI device on my own layouts. You usually find these modules are just following the datasheets exactly anyway.

Needing LDOs depends on your design ? If it's purely op-amps referenced to ground then they reject stuff on the power lines well enough. But if you're referencing anything to one of the power lines then either filter well or add an LDO or two. But for one-offs either solution is so cheap you may as well just do it.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Fri Feb 05, 2021 2:44 pm

Haralds:Werk wrote:
Fri Feb 05, 2021 6:06 am
I have build something like this for a standalone device with +/-15V output. Look here as a starting point:
https://www.haraldswerk.de/PSU/PSU_Sing ... ouble.html
Completely analog.
many thanks Harald. I always enjoy looking at your designs!

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Fri Feb 05, 2021 2:46 pm

MikeDB wrote:
Fri Feb 05, 2021 8:02 am
Needing LDOs depends on your design ? If it's purely op-amps referenced to ground then they reject stuff on the power lines well enough. But if you're referencing anything to one of the power lines then either filter well or add an LDO or two. But for one-offs either solution is so cheap you may as well just do it.
thanks Mike, much appreciated! I figured that for a postage-stamp sized piece of PCB real-estate and a quid's worth of components, it's probably worth doing

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by EATyourGUITAR » Fri Feb 05, 2021 3:05 pm

You want your charge pumps on a completely different PCB for a very low noise result. Integrating the charge pump into the same PCB while also achieving good noise isolation is something that is possible but it is for expert designers so we can say that it is beyond the scope of this thread. I have some books on the subject thicker than a phone book.
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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Fri Feb 05, 2021 3:52 pm

EATyourGUITAR wrote:
Fri Feb 05, 2021 3:05 pm
You want your charge pumps on a completely different PCB for a very low noise result. Integrating the charge pump into the same PCB while also achieving good noise isolation is something that is possible but it is for expert designers so we can say that it is beyond the scope of this thread. I have some books on the subject thicker than a phone book.
Ahh, so you'd recommend putting the LDOs on the 'mainboard' and have the 'psu board' only with the charge pumps and nothing else? that makes sense.


I wasn't planning on having the switcher on the mainboard, rather to make a handy little PSU board (like the one MikeDB posted from AliExpress, but i'll put some mounting holes on mine). I'm competent enough on kicad to be able to copy their example in the datasheet, I wouldn't attempt to do my own design yet :hihi:

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by MikeDB » Fri Feb 05, 2021 4:29 pm

If you're going to put the LDOs separate from the switcher then use thick wire between them, not IDC ribbon cable, especially for the ground. If you add a long thin cable between the switcher and LDOs you'll be radiating shit everywhere and possibly undoing any good the LDOs can do. How big is your PSU board going to be ? If a reasonable size, just put the LDOs on the opposite edge of it and run thick tracks across.

I always put mine hard against the final capacitors of the switcher but as Guitarguy says, layout becomes very critical then to make sure ground currents are circulating where you want them to be, not wandering off where they want to go :-)

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by EATyourGUITAR » Fri Feb 05, 2021 5:01 pm

plushterry wrote:
Fri Feb 05, 2021 3:52 pm
Ahh, so you'd recommend putting the LDOs on the 'mainboard' and have the 'psu board' only with the charge pumps and nothing else? that makes sense.
you can put your charge pumps on the same pcb as the other power stuff but you don't want to put your charge pumps on the same PCB as the audio circuit. you said this is about putting a filter in a stand alone box. You still need to take some things into consideration when designing the power PCB if your wallwart is a noisy DC 5v or some AC transformer. almost all your DC power bricks are switching supplies that have switch noise between 60KHz and 100KHz.
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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by soup » Fri Feb 05, 2021 6:53 pm

I used to build a lot of stand alone stuff. I built quite a few power supplies on stripboard using a recom rs3-1215d similar to the lower schematic...
klee_dc_dc_132.png
stolen from this page...
https://electro-music.com/forum/post-255936.html

For larger power supplies there's a lot of parts that will fit the frogleg synthesis beetle pcb too.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by FactoryDefault » Sat Feb 06, 2021 1:18 pm

I have this bipolar supply in my “to build” pile:
https://obsoletetechnology.wordpress.co ... ct-pedals/
I can’t promise it’s the best but it might be the smallest.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Tue Feb 09, 2021 2:18 pm

right, so I've come up with this:

the maximum voltage of the TPS65130 is 6V so I put a pair of parallel 5V6 zeners to shave off anything extra that might come out of the 5V wall-wart. PMOS for reverse voltage protection. Basically everything else is datasheet stuff.

I'm planning on running the TPS65130 to get +/-15V output, then drop a volt across the each of the LDOs to get +/-14V regulated at around 150mA per rail (haven't put these values on the schematic yet R3, R4, R6, R8, R9, R10, C10). So pretty close to my original target, I'd be happy with that.
mainschemo.png
chargepumpschemo.png
regulatorsschemo.png

the question I have is: the outputs of the switching converter have 22U caps on the outputs, and the LDOs have 10U caps on the inputs (both as recommended in the datasheets). Would it be a good idea to combine these? possibly get rid of the 10U? or do I still need both?

if anyone can spot anything stupid I've done/talk some sense into me, I would be most grateful.


I've watched enough youtube videos now to frighten myself about SMPS PCB design, but hopefully I've cobbled together a layout something thats not going to cause too many problems. I've got to poke it about a bit more but will hopefully post it up this evening so you can all point and laugh :hihi:

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by neil.johnson » Tue Feb 09, 2021 3:18 pm

Hmmm ... is this for a DIY project or a commercial design? The TPS part comes in a very DIY unfriendly package with a bottom pad which will need soldering down to ground.

For a one-off DIY project I'd try to find something off the shelf, since component cost is tiny compared to your time spent on the project! For a commercial product anything with a switcher operating in the 100s of kHz is going to need attention to emissions (e.g., there is no RFI filter on the DC input).

If you prefer your solution, I would rotate D3 and D4 otherwise you'll get a nice pop when they blow!

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Tue Feb 09, 2021 3:33 pm

neil.johnson wrote:
Tue Feb 09, 2021 3:18 pm
Hmmm ... is this for a DIY project or a commercial design? The TPS part comes in a very DIY unfriendly package with a bottom pad which will need soldering down to ground.

For a one-off DIY project I'd try to find something off the shelf, since component cost is tiny compared to your time spent on the project! For a commercial product anything with a switcher operating in the 100s of kHz is going to need attention to emissions (e.g., there is no RFI filter on the DC input).

Hi Neil, it's just for DIY use. I was planning on getting a small run manufactured (10 or so) because they will be handy things for me to have around. Also it's a learning experience for me. Switchers are here to stay so the sooner I start learning about them, the better.

neil.johnson wrote:
Tue Feb 09, 2021 3:18 pm
(e.g., there is no RFI filter on the DC input).

If you prefer your solution, I would rotate D3 and D4 otherwise you'll get a nice pop when they blow!
:oops: thank you!

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by Portabella » Tue Feb 09, 2021 4:04 pm

double-post
Last edited by Portabella on Tue Feb 09, 2021 5:55 pm, edited 1 time in total.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by MikeDB » Tue Feb 09, 2021 4:36 pm

Yes you may as well combine the switcher output and LDO input caps, provided they are close up.

I know the TI datasheet says 4.7uF on the input but I always use 2 or 3 10uFs just to keep crap out of the input cable. Also the TPS65130 isn't spread spectrum so RFI inductors on the input would be a good idea as well.

Not sure on your loads, but check 10uF on the output is enough.

Put some large diameter vias and a large area of exposed copper under the ICs to get rid of any heat.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by EATyourGUITAR » Tue Feb 09, 2021 5:05 pm

If your 5.6v zeners are shunts then you need to consider there is a TDP and current limit before they pop. Then you would have no protection as a result of going over voltage. Do an internet search for power supply crowbar circuit. But this may not be needed if your 5v DC supply has some over voltage protection or some very predictable failure mode of catch fire and turn off.
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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by Portabella » Tue Feb 09, 2021 5:56 pm

I'm using some cheap boards from china and they do the job:

https://www.ebay.de/itm/AC-DC-to-5V-15V ... 2749.l2649

however those TO220 regulators resp. the heatsinks on them get kinda got.
I meassured 63 degree Celsius .. is that okay for a voltage regulator TO220?

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by MikeDB » Tue Feb 09, 2021 6:11 pm

Portabella wrote:
Tue Feb 09, 2021 5:56 pm
I'however those TO220 regulators resp. the heatsinks on them get kinda got.
I meassured 63 degree Celsius .. is that okay for a voltage regulator TO220?
Nowadays probably yes. When I graduated the rule was 20 deg above ambient max but semiconductors have got a lot better over the years. However a small £1 plastic fan screwed into the top of those two heatsinks could knock a good 20 degC off.

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by skee » Tue Feb 09, 2021 7:12 pm

I’m using an MFOS style psu with .5A 12 volt ac wall wart.The circuit consumes around .25A, about half of the wart’s rated output. Even so, it is pulling the unregulated voltage very low, from +/-17 volts off load down to -15 and +12.25 when on load. I’m getting a beefier 1.5A unit from RS in the next day or so and let you know if this improves things.

If I were starting on another build, I wouldn’t use this design for voltage sensitive circuits like VCOs

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Re: What's the 'best' way to get bipolar power from a wall-wart?

Post by plushterry » Tue Feb 09, 2021 7:16 pm

MikeDB wrote:
Tue Feb 09, 2021 4:36 pm
Yes you may as well combine the switcher output and LDO input caps, provided they are close up.

I know the TI datasheet says 4.7uF on the input but I always use 2 or 3 10uFs just to keep crap out of the input cable. Also the TPS65130 isn't spread spectrum so RFI inductors on the input would be a good idea as well.

Not sure on your loads, but check 10uF on the output is enough.

Put some large diameter vias and a large area of exposed copper under the ICs to get rid of any heat.

thanks!

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