I'm trying to design a cheap, simple power supply. It's for an educational workshop I'll be leading later this year on DIY synthesizers for beginners. The power supply should take a single power source and then step down to produce three different voltage levels: +12V, -12V, and +5V. For the initial power supply, I'm planning to use a wall charger or power bank with a USB-C PD trigger. Most USB triggers can be easily configured to output specific voltage levels ie. 5V/9V/12V/15V/20V, and with EPR in USB PD 3.1/3.2, available voltage increased to 28V/36V/48V, provided the charger can provide the needed power (100W-240W). I'm trying to figure out if my circuit will work with a standard 45W charger providing 20V, or if it'll require a 100W charger for 28V.

In my prototype, I'm using dual TPS5430 step down converts to derive +12V and -12V from the USB trigger. I found a cheap premade board called XL5430 that does this, and it works fine (tested with a multimeter, I was typically reading about +11.9V and -12.1V which is less than ideal but probably good enough!). I'm also using a MP1584EN buck converter to derive the +5V signal. I'm not sure if the signal will be cleaner/better if I wire the MP1584EN up to the USB trigger directly, or if it would be better to wire it to the +12V output of the XL5430. I'm guessing than the latter would risk introducing more noise/ripple, but it might be less prone to overheating.

The real question I have is: will this circuit be more reliable if the USB trigger is configured for 20V, 28V, or something else?
It's important that this circuit is A) easy to build for beginners, B) low-cost, and C) able to provide stable voltage, especially on the +12V and -12V rails.

According to the datasheets, the MP1584EN's input voltage range is 4.5V to 28V, and the TPS5430's input voltage range is 5.5V to 36V, however the description for the XL5430 states that it's input voltage range is 6V to 28V. I was also considering potentially replacing the MP1584EN with a different component such as the LM2596 which has an input voltage range up to 40V.

Thanks for and clarity you can bring to this subject!

 

My two Yen; someone may offer two Rupees or two Shekels instead.

Stick to whatever is most ubiquitous.
But in the end, everything will depend on how much power will the circuit require, with significant headroom for expansion.
I ignore whether 45 and even 100 watts are still within the capabilities of 12 volt USB supplies, but most likely the cheapest and widest availability.
But thinking in future expansion, a higher voltage would perhaps be a better option.

 

Use a center tapped 24 volt transformer . The center tap to the common, and a bridge rectifier off the ends. You get +12 and -12 off the bridge. Filter it well.Then use power-tab regulators with wrap-around power transistors. Use a 7805 power tab off the plus side , also with a wrap around transistor, for the 5 volts.
The transformer watts should be a bit more than the projects anticipated watts.



and forget any thought of any USB stuff.
My response was way off base, I totally did not understand the project.! My apology!!

 

@MisterBill2, I'll need to do more research on center tapped transformers. Thanks for bringing them to my attention.

and forget any thought of any USB stuff.

As for this comment, I should justify why I'm interested in USB PD. This circuit is for a workshop that aims to make analog synthesizers accessible to more people, specifically by lowering their cost. Having a +12V / -12V / +5V power supply is just a prerequisite for the actual DIY workshop to take place, but this circuit won't be a major focus of the workshop. Ideally it should be quick and simple for a beginner to build, and very affordable. I'm talking about ideals here, and I know that things are more complicated and challenging in reality. I'm posting here to find help exploring and understanding what's possible so that I'll be in a better position to make an informed decision about what type of power supply circuit to use in this workshop.

The only reason that I'm interested in a power solution that uses USB PD is because USB PD wall chargers and USB-C cables are so common that it's reasonable to expect most or all workshop attendees will already have a spare USB power supply available to use. In effect, they're available for free, $0, no extra cost since everybody already owns one. This makes them significantly more cost effective than purchasing a new AC-DC wall power supply unit for each workshop participant. For comparison, something like the Adafruit ±12V 500mA Switching Split Power Supply costs $15. So far that's the cheapest and easiest solution I've found -- all it's missing is the +5V output which should be easy enough to get from a voltage regulator or buck converter or transformer. The most significant drawback is that 500mA is on the low side. It's acceptable, but support for more current would be preferable.

 

I checked Digikey for 24V center tapped transformers, and it looks like the cheapest is the F-45X for about $15.

 

If you have any defunct older computers their power supplies have 12V, -12V and +5V voltages.
How much current is required?

 

+12 V, -12 V, +5 V PSU is classic for hobbyists experimenting with both analog and digital electronics.
100 mA ±12 V and 1A @ 5 V will satisfy your needs in most cases.

I built my own PSU using Hammond 166J14 and 166F25 transformers, 7805, 7812, and 7912 voltage regulators.

If you hunt around, you might be lucky to find an older power adapter for printers that has all three supplies in one adapter.

 

Thanks, @MrChips and @sghioto, i appreciate your perspectives. Since this circuit is for a workshop, I'll need to acquire enough materials to build about 15-30 of them. So it seems unlikely that I'll be able to find that many old computer power supplies lying around.

The specific kind of DIY synthesizer that we'll be building is in the eurorack format, where it's more typical to draw most heavily from the +12V rails, then a bit less from the -12V rail, and typically least power drawn from the +5V rail. Something like 1A @ +12V, 700mA @ -12V, and 400mA @ +5V should be plenty. These numbers are somewhat arbitrary, but roughly in that ballpark is good. More wouldn't be a bad thing, and less could be fine too.

 

Meanwell PSU GP25A13D-R1B

https://www.digikey.com/en/products/detail/mean-well-usa-inc/GP25A13D-R1B/7703337

 

This PSU appears on amazon.ca site:

https://www.amazon.ca/Arcity-Switching-Adjustable-Cocktail-Horizontal/dp/B0CM6F8VKM

 

From aliexpress.com:

https://www.aliexpress.com/item/1005007103372482.html

 

and forget any thought of any USB stuff.

Why? @aaaaaaaaaaaaaaaaa (the thread starter) explained his use case for an inexpensive accessible solution. USB C PD has become ubiquitous, and as he states, many people will already have a suitable power brick available. His desires (at least in this regard!) are well thought out and reasonable.

Without doing much research, a 65 watt USB C PD supply can be had from Amazon for $15 if one isn't already available. (see pictures below.) Let's compare it to some of the suggestions made.

● This power brick is complete and ready to go. The only additions needed are a PD trigger and a bipolar buck converter, which can be housed in a small enclosure.

● This power brick is tiny – 65mm × 65mm × 30mm (2.55" × 2.55" × 1.18") and weighs almost nothing.

○ The traditional approach of center-tapped transformer, two bridge rectifiers, filter caps and linear regulators is a significant project it itself. (Yes, I have built such a supply using a scavenged filament transformer. 50-some years ago). Transformers are expensive these days. A substantial enclosure will be needed to house the heavy transformers. To get the desired current, heatsinks may be required.

○ Open-frame power supplies are large and heavy, require an enclosure, line power wiring, etc.

Just to prove this is a sound idea, here's a picture of a USB C PD supply I've built to power my CPAP. It replaces a power brick that's twice the size and weighs several pounds. When traveling my laptop USB C PD supply does double duty, charging my laptop and running my CPAP.

This supply has a PD trigger set for 20 volts and a buck converter to 12 volts. Why a buck converter? 12 volt output is no longer required in the PD specification. Many supplies still include 12 volt output, but not all. This ensures operation with any supply of sufficient output power.

 

It may take some searching to find a bipolar buck converter – a bipolar boost converter was in a recent thread here.

A word about PD modules: I believe a 65+ watt module will be needed to get 20 volts output. 15 volts should be avaliable from a 45 watt module.

But there is some fine print here (literally). Multi-port modules that claim to be 45 watts or 65 watts may not include the expected output voltage – a module may advertise 65 watts, but that 65 watts is the total available from all the powers. It may not supply 65 watts to a single port which is needed for a 20 volt output. Ads don't always make this clear, so read the fine print on the supply

The enclosure for my CPAP supply shown above may be useful for this project. It's built from laser-cut acrylic and Lego Technic frames. The technique is shown here

.

 

This supply may be suitable.

https://www.ebay.com/itm/373121439498

 

OK, I did not realize what this project actually was! Sorry about getting it wrong! I see quite a few good suggestions since my response. Now I see, (I think) that the plan is for each person to be able to assemble a triple output power supply. There are surplus dealers that may actually have the triple output supplies in stock.

 

@Jon Chandler thank you for those thoughtful replies! At the workshop I'll make sure to inform participants to read the fine print on their wall charger to verify that they're actually capable of providing the stated voltage. This would surely lead to confusion if not addressed.

The lego technic frame is really cool too, and not something I was previously aware of, but it seems like it could be perfect for this project!

This supply may be suitable.

https://www.ebay.com/itm/373121439498

This board looks like it could be perfect. I'll first run more testing to see if the XL5430 prototype that I already have can be made to function, but this looks like a great backup plan. It's a little more costly than the XL5430 but certainly within the budget - and I already found cheaper versions on Aliexpress.

 

One part of my original question remains unanswered: Is there any advantage to configuring the USB trigger for 28V output over 20V output? If I'm connecting a bipolar DC-DC buck-boost convert to the USB trigger to derive +12V and -12V signals, and another step down converter or regulator or transformer to get +5V, then how much voltage does the USB trigger need to draw? Is 20V or 28V or something else the right choice for this situation? It's especially important that the +12V and -12V rails are kept mostly stable with minimal noise/ripple or other interruptions/distortions.

 

You need to check the input voltage specification for that split supply module! That is carefully blotted out in the large print headline. It IS a DC to DC converter, so you would only need one DC supply from a resale shop.

 

Is there any advantage to configuring the USB trigger for 28V output over 20V output?

I suspect 28V output supplies are less common than 20V supplies. And 15 volt supplies will be even more common. 15 volts should be enough overhead for a 12 volt output.

I'd suggest something like a mini-360 buck converter for the 5 volt supply, powered directly from the source. The mini-360 converters are cheap, a buck or less.

Regarding the XL5430 buck converters (assuming in the form of a module):
The negative input/output is common. You can't connect them to the same power source and arrange them in a bipolar arrangement. The common connect of the output will short positive and negative together.

To stack two converters for a bipolar output requires modules with the output isolated from input.

 

It IS a DC to DC converter, so you would only need one DC supply from a resale shop.

I think @MisterBill2 may have missed the point of the PD module..... but the option of using a 19.5 volt laptop power supply salvaged from a dead laptop or purchased from a thrift store is a cost-almost-nothing is a good addition.

Add a terminal strip and a beefy diode... um... make that 2 diodes. One in-line with the positivec output of PD trigger, one in-line with the positive terminal of the terminal block, feeding into the DC-DC converter. Ideally, the PD supply would be used. If a PD supply isn't available, a laptop supply could be used (connected via a terminal block since there's no standard connector). The diodes prevent back-feeding the PD trigger and reversed polarity connection of the laptop supply.