# DC-DC "buck" convert - how to invert the voltage

#### voidstar

Joined May 7, 2022
15
I have a set of 3x "buck' converts doing 19Vdc to +12 / +8.5 / +5 Vdc.

I use a separate "power brick" to do the 120 Vac to 19Vdc / 4A conversion.

How do I safely "invert" the 12V and 5V power? Can I tether off the existing "buck" that I'm using for 12V and 5V? Or do I need to use two additional "buck" converters?

Attached is what I have so far.

I saw notes here:
Making a Voltage Inverter from a Buck (Step-Down) DC-DC Converter (maximintegrated.com)
But, still confused. Does the GND input on the buck now become an output?? Just want to integrate in with the same common ground.

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#### ThePanMan

Joined Mar 13, 2020
367
Welcome to AAC.

Do you know what an inverter is? If so - forgive my boldness to explain, but an inverter takes DC voltage and changes it into AC volts. Is that what you want?

While I'm not versed in inverters, I don't see how that diagram from the linked page is an inverter of any kind. The only thing they did was change the grounded capacitor into an output. I just don't see how that can be an inverter. But like I said, I'm not versed in those beasts.

#### voidstar

Joined May 7, 2022
15
No, I don't need DC back to AC -- I have those for converting 12V large-battery back into AC (300W on up to 2000W inverters), for projects unrelated to this.

I'm talking within the same circuit, I need +12V and -12V, and also +5V and -5V. It's an old PC PSU that uses the negative power for certain functions.

And when I say "same circuit", I mean using the same 120Vac source. In the diagram I attached, I'm not sure how to connect up the single wires for the -12V and -5V pins.

#### Papabravo

Joined Feb 24, 2006
18,427
You cannot easily get negative voltage directly from an off the shelf buck converter. You can get them with an inverting buck-boost converter which is another animal altogether.

The app note says that you can do it provided you have control over the internal circuitry. This may not be possible with an off the shelf product. There is a risk of serious destruction of the equipment and injury to yourself when fooling around with this stuff. Whatever you do, please be careful.

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#### ronsimpson

Joined Oct 7, 2019
2,067
I have a set of 3x "buck' converts doing 19Vdc to +12 / +8.5 / +5 Vdc.

#### voidstar

Joined May 7, 2022
15
The "buck" devices I'm using (can try others, there are many variations, these are just what I started with):
Amazon.com: Boost Buck Converter, DROK DC 5.5-30V to 0.5-30V 5V 12V 24V Adjustable Power Supply Regulator Module, 4A 35W High Power Voltage Step Up Down Converter Board with Case LCD Display : Electronics
DROK ZK-DP60

The "power brick" I mentioned is just an old DELL Laptop AC adapter that does 19V/4A out.

I'm just trying to substitute in replacement for the stock IBM PSU from 1978 (see attached photo). It has two large-soda-can-sized caps on the bottom right, they charge up to +167V and -167V (just testing the leads on the backside -- not sure if they're really inverted polarization from each other, or just coincidence in how the components are arranged on the board, but I tested the two leads in the same orientation). There are two brown inductor type components right above those caps, and to the right of those is the two leads for the single main 120Vac input (and the first thing the power does is go through those brown ceramic inductor looking components). Since I'm get 167Vdc, I think it's AC-to-DC conversion is working However, while there is no visible damage to the board, some diode must be malfunctioned, because I'm not getting the down-converted outputs on the right side of the board. [ and I'm not solder-savvy enough to start replacing components, or testing them individually off the board -- but I'm willing to ship and pay for someone to check it out; it's about the size of a shoebox ]

So I figured the job here is just to convert 120Vac over into +12, +8.5, +5, -5, -12 Vdc. So just build something to do that. The buck converters are small enough, they'd fit on a board that could be a drop in replacement to the original PSU. So that's the goal.

Attached are also the confirmed Vdc outputs of the original board.

And attached is also what I have so far as a prototype replacement, with the original wiring harness tethered in.

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#### MisterBill2

Joined Jan 23, 2018
11,888
What the TS wants is a BIPOLAR power supply, one that delivers both positive and negative outputs relative to a central "common" connection which might also be identified as "ground"
OK, Here are a few options. BUT the question is about the isolation between the input and the output. If they are isolated then it can be rather simple. If the input common and the output common are not isolated then it gets complex in a hurry, and there may be some smoke as well.

So the primary question is what are the required output voltages and currents? This really matters quite a bit.

"PC Computer" power supplies used to provide +5 volts at many amps available, +12 volts at a few amps available for the disc drives and fans, and -12 volts at less than one amp, for the RS232 serial communications portion, and possibly for a few other functions. These voltages were all referenced to a common, often called "ground".

#### ronsimpson

Joined Oct 7, 2019
2,067
Now I understand. This is a boost/buck supply. There is no isolation. All "0V" are connected together. That makes things hard. How much current and power do you want for -2V and -5V?
-------------edited--------------
The word inverter is used for several different types of supplies.
----DC to AC (example 12Vdc to 120V60hz ac.)
----DC to a negative DC (example +12V to -12V)

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#### voidstar

Joined May 7, 2022
15
In the -12V and -5V, no idea what the current draw is. The original IBM wire harness is using approximately 18 gauge wiring throughout. The -12V relates to some tape-deck operations (recording, erasing) and the -5V relates to certain ROM chips. I'd just guess somewhere between 1A and 4A.

The +12V is powering an internal 5" CRT.

#### MisterBill2

Joined Jan 23, 2018
11,888
Is the intention to power a computer?? Or some other load, not the original application??I am suggesting that the IBM computer supplies are approaching life's end and should not be used for anything important. And if you do not know the current draw from the new use device then there is no way to reasonably suggest the power supply design. At that point stop and discover what is actually required for the application.
Powering a power supply from a power supply powered by still another power supply is usually a poor plan.And in this instance it seems like a very poor plan, especially as we are given no clue as to what the load is. If the load is an audio system then know in advance that computer power supplies have a fair amount of electrical noise on the output, and that noise will be on all of the audio in the system. And in addition, many of them require some big load on the 5 volt output to stay on. With no load they switch off in a hurry.
Those 19 volt 4 amp supplies are probably for laptop computers. At some point, recycling old computer power supplies becomes a losing proposition.

Joined Jan 15, 2015
6,585
I have an old ATX200 series PSU laying here. the -5.0 volt and -12.0 volt outputs were 0.5 amp and 0.8 amp respectively. So not much current. What I believe you want is just a DC to DC converter with a +/- output. Years ago I used units made by Cosel USA and they put out a wide range of DC to DC converters. Scroll down this page. They also have other models. Years ago I had to buy Made in USA but I would imagine much lower priced stuff is available off the boat. COSEL USA is merely a starter poing for DC to DC converters including bi polar output versions. You may find bi polar DC to DC converters on Amazon.

On early home computers before external audio powered amplifiers the + and - voltage were used on the output audio driving small common speakers. Not much current at all.

Ron

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#### voidstar

Joined May 7, 2022
15
The info at the original link I posted works fine, it inverts/reverses the voltages on these DC-DC buck adapters as described.
Making a Voltage Inverter from a Buck (Step-Down) DC-DC Converter (maximintegrated.com)
Yes, per those notes, the GND of what is normally the input is just instead connected to the (+) of the DC out (but just hook it all up normally first to configure the OUT before hand).

I made two more of those bucks, and am getting the -5 and -12Vdc as needed out of those. However, those two negative volt units run at a higher temp (the temp is apparently proportional to how negative you go):

- all the positive voltage bucks are running about 32degC
- the -5V one is 37degC
- while the -12V one is at 43degC.

Have had them running for about 6 hours now, temps have stabilized - no smoke, wires aren't even warm. These temps are as reported from their own internal sensor on the bucks.

The application is to run an old vintage computer system, but with new PSU components. Still benchtesting, but pretty confident it is going to work. The fans all use their own circuitry, no concern about those. The rest is IC's and low current stuff (except the internal CRT that runs off 12V - studying more about that).

The 19V/4A power brick is just a placeholder for a device to do AC to DC conversion. Essentially any AC/DC power brick between about 15V-30V would do for this.

#### voidstar

Joined May 7, 2022
15
Also, those MeanWell devices do look like might be a suitable option - and they're not that super expensive. Like the RQ-50D maybe. Will ping them to see if they have a closer match, might try it on the next build.

#### voidstar

Joined May 7, 2022
15
Just to follow up, here is the MeanWell RQ-128B based PSU replacement, compared to the original PSU. It supplies the -5V and -12V already, so I just had to add a "boost" or step-up converter (using the +5V line, to boost up to 8.5V).

The "buck-boost" approach worked as well - used an old laptop AC to DC adapter (19V, 4A), but the 5V line ended up needing about 4.8A (so I used a boost-only device on that, which handles up to 6A). The 4A model worked for about 60 seconds before it shutoff after reaching 99degC (we had no idea what the current draw would be, had to guess). But for "inverting" the 12V and 5V to -5V and -12V, that worked out fine (using the notes mentioned:
Making a Voltage Inverter from a Buck (Step-Down) DC-DC Converter (maximintegrated.com)
) on these buck-boost converters.

Still, I prefer the look and setup of the RQ-128B MeanWell PSU better, and each approach cost about the same.

#### MisterBill2

Joined Jan 23, 2018
11,888
There must be some considerable value to that old computer to make it worth the effort to power it.
I did work for one company that had a product that used a "DEC" PDP11-04 computer package, with the software non-documented and the programmer quite hostile towards providing any help at all. So the units they sold in the '386 PC era required some major analysis of the old compiled Fortran to produce code that did the same functions. The net savings in associated I/O hardware made it cost effective, though.
My point being that at some point updating can be worth the effort, especially for a standard product.

#### Ramussons

Joined May 3, 2013
1,264
I have a set of 3x "buck' converts doing 19Vdc to +12 / +8.5 / +5 Vdc.

I use a separate "power brick" to do the 120 Vac to 19Vdc / 4A conversion.

How do I safely "invert" the 12V and 5V power? Can I tether off the existing "buck" that I'm using for 12V and 5V? Or do I need to use two additional "buck" converters?

Attached is what I have so far.

I saw notes here:
Making a Voltage Inverter from a Buck (Step-Down) DC-DC Converter (maximintegrated.com)
But, still confused. Does the GND input on the buck now become an output?? Just want to integrate in with the same common ground.
What you are asking is whether you can use, say, the +12 V output as a -12 V source.
Technically you can simply interchange the + and Gnd terminals to get a -ve voltage. BUT there are limitations.
* You can use that 3x convertor as a SINGLE -12 V / -8.5 V / -5 V unit. You cannot get all the Negative voltages at the same time.
* If the 3x convertor is NOT the isolation type (generally, they are not), you cannot use that 19 volt Power Brick to source a +ve voltage. If you try, you will end up shorting the supply(ies).

#### voidstar

Joined May 7, 2022
15
Initially I thought I could just interchange the +/- wires - I've done that to run DC powered toy-trains in reverse along the track. But that doesn't work in this application.

Here is an example of one of the systems that use this kind of power arrangement: (early/mid 70s system, so just for curiosity not actual practical usage - although they do have 7-digit floating point, and of course BASIC+APL support built in {with a nice set of matrix and trig operations}; complete desktop PCs several years before Microsoft or Apple even existed)
RARE VINTAGE- IBM 5100 PORTABLE COMPUTER 16-BT PALM PROCESSOR, SCAMP, 5" CRT | eBay

I have a couple of them, and one of them the PSU stopped working. So instead of removing components and trying to isolate what failed on the OEM PSU (since I can't solder to save my life), I prototyped a replacement using the following combination of old laptop power brick and several boost-buck converters. That works, ran the system for many hours, no heating issues (using IR camera) and all functions of the system worked fine. (the initial 4A buck on the 5v line didn't work, had to upgrade to a slightly bulkier version) [by 3x or 5x, I meant 3-copies or 5-copies of a converter device; not 3x or 5x the power]

What the system actually needs these negative voltages for is still a mystery to me; they are used for the tape and disk drive support in some fashion.

Having confirmed all that and the Amps needed on each line, I then found an appropriate MeansWell PSU to those specs, and needed just a single step-up converter to get the 8.5v. Both solutions work equally well.

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#### MisterBill2

Joined Jan 23, 2018
11,888
he scheme can work IF, and ONLY IF, the inputsnand outputs of each module are isolated from each other. The drawing in post #17 will have a problem because the bottom two modules do not have the input supply negative terminal connected to anything. So either those modules are not isolated or they are not powered, or there is a drawing error.

#### voidstar

Joined May 7, 2022
15
Not a drawing error, that's what was built and is still working. They are step up/step down buck converters, so I guess the term is they are isolated i/o. I do agree the wiring looks very odd, hence my original post asking if the notes at maximintegrated.com about it were correct - so I just tried it. Here are those original notes:

https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3844.html

And here is an example of one of the converters:
https://www.amazon.com/gp/product/B07VNDGFT6

For the 5V line, I used this other converter (which looks very similar, but it is a "step down" only, smaller adjustment knobs, and handles up to 6A).
https://www.amazon.com/gp/product/B078Q1624B

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#### MisterBill2

Joined Jan 23, 2018
11,888
OK, What I see is that it can be connected as an isolated arrangement, And as a three terminal device. It makes sense but it is not quite standard. I would not expect it to be on every module from most suppliers. Very handy though.