Electronics > Projects, Designs, and Technical Stuff

Power Supply Design : Questions about Dual Supply and Transformer

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MosherIV:

--- Quote --- Any transformer above 40, 50 volts, are hundreds of dollars.
How the hell do people make and sell non-logic-level electronics for a price people can afford?
Am I searching wrong or something?
--- End quote ---
Most people, professional or hobbiest, do not make their own mains Switch Mode Power Pupply.
Part of the reason is that the high freq transformers are not available off the shelf, they are made to order due to the characteristics being very specific to each psu.

Most audio hobbiests either build simple unregulated linear psu or
Buy a SMPS rated for their project.

It is not impossible to make your own SMPS, getting the parts and getting the design to work right is difficult.

The reason why these off the shelf SMPS are so cheap is that they are made in huge volumes, and with the cheapest parts possible (see others comments about the capacitors). The large volume also means that the custom transformer can be made to order at a cheap price.

No you are not going to find a transfors in that power range cheap!

As others have said, if your current psu design does not have a transformer to isolate from mains - abandon the design - it is highly dangerous (leathal) !

LittleRain:

--- Quote from: TimNJ on August 18, 2020, 05:25:06 am ---By the way, can you please explain what you're trying to do here? Please share a schematic. From your 3D render, it doesn't seem like it's isolated? (I see transformer T1, but that does not appear to be in the main power path. Maybe stepping down for your low voltage circuitry.) Unless you have a system which is extremely well insulated, (AKA "double" insulated), you need a proper transformer.

You could put a big 50/60Hz transformer before the switch-mode supply...but this basically defeats the purpose of using an SMPS in the first place.

Regarding dual supplies. If you have two isolated power sources (that is, transformer isolated), you can wire them in series and get call the center point "0V". The only requirement is that each individual output is not earthed. Sometimes an SMPS's output negative is tied back to earth. If you try to wire two earthed-output power supplies in series, you wind up shorting one of the power supplies out. So, they need to be truly floating. (You can still tie the center-point to chassis earth, if you'd like.)

--- End quote ---

I'm just trying to build a +/-42V AC to DC PSU for an audio amp, here is schematic for my single supply version, since I can't use it for an audio amp, I'm going to turn it into a lab bench PSU(after I figure out how to isolate it for a price that doesn't break the bank).
No it is not isolated, you are correct T1 is for low voltage circuitry.


Hmm, yeah that just leads me back to transformer issue, I can't find one that isn't a couple hundred dollars with the VA rating I need, and then I would need 2 of them.
Thanks though that is good to know.





--- Quote from: bdunham7 on August 18, 2020, 05:32:28 am ---
--- Quote from: LittleRain on August 17, 2020, 09:22:16 pm ---
But my single supply PSU does not have a transformer on the high voltage side, so I'm just wondering,


--- End quote ---

Are you saying that your PSU is mains powered but not isolated?   :scared:

In a nutshell, that's probably not suitable for an audio application even if you did find a way to do it.

--- End quote ---

No its not lol.
Hmm didn't think about that, I did add a ton of capacitance for noise, but I am not very knowledgeable with analog circuitry, I have more of a programming background and mostly work with logic level digital circuits.


--- Quote from: MosherIV on August 18, 2020, 07:30:59 am ---
--- Quote --- Any transformer above 40, 50 volts, are hundreds of dollars.
How the hell do people make and sell non-logic-level electronics for a price people can afford?
Am I searching wrong or something?
--- End quote ---
Most people, professional or hobbiest, do not make their own mains Switch Mode Power Pupply.
Part of the reason is that the high freq transformers are not available off the shelf, they are made to order due to the characteristics being very specific to each psu.

Most audio hobbiests either build simple unregulated linear psu or
Buy a SMPS rated for their project.

It is not impossible to make your own SMPS, getting the parts and getting the design to work right is difficult.

The reason why these off the shelf SMPS are so cheap is that they are made in huge volumes, and with the cheapest parts possible (see others comments about the capacitors). The large volume also means that the custom transformer can be made to order at a cheap price.

No you are not going to find a transfors in that power range cheap!

As others have said, if your current psu design does not have a transformer to isolate from mains - abandon the design - it is highly dangerous (leathal) !

--- End quote ---

Man how do they afford to make a linear PSU, guess they aren't worrying about cost...

Yeah that makes sense, I just don't understand how they can sell it for that much and still make a profit. You'd think you'd be able to build your own for around the same price as their marked-up cost.
I think I might try to make my own isolation transformer, unless I can find a similar one they used in that link from china...
This is starting to be a pretty good learning experience, possibly a little more than I currently have time for.

Edit: Do you by chance know if stainless steel can be used for a core? I have tons of stainless steel sheet metal, and can get as much as I need just down the road for free.
And I can print the plastic bits, not sure what its called, the former?

Is it really that bad to not have it isolated? I see tons of documentation online from TI, and other big IC makers who design buck converters specifically for non-isolated purposes.

Yup they are very expensive haha!
I already ordered them though so I really want to see this project through, I will surely isolate it though.
Luckily I found some high power transistors for a decent price, all be it from China.
All in all I've spent around $100 on the project, but I'll have enough components to make 5 of them(at least when I get more LV transformers, only getting 1 or 2 to start), and of course that doesn't include my isolation transformers.



And thanks to everyone for their input.

Dulus:
Holy s*** this guy's making a buck converter   :wtf: :wtf: :wtf:

Littlerain you're either trolling us or you seriously dont know what you're doing.
Let me point out some highlights:

* transistor Q4 blows, and you're shaking hands with rectified mains.
* MCU doesent wake up properly, Q4 latches up, aaaand you're shaking hands with rectified mains
* you cant drive a POWER FET gate with a 100ohm and 1k pull-up to 10V..... i cant even find out where to start  :palm: :-BROKE
* what will the FET transistors do at the first few cycles of mains. Clearly there will not be any low voltage supply ready yet, however you'll have rectified mains at the FET inputs...
I dont mean to discourage you or anything but, this is serious business and you're going in a very wrong direction.

If you're determined to carry on, please share pictures  >:D
We may even start betting on which components would blow first  :-DD :-DD

Benta:
I'm submitting this for the Darwin Awards.
A non-isolated buck converter for converting 120 VAC to 42 VDC for a power amplifier? And extending it to +/- 42 V?

Man, you can't touch anything on that amp without getting killed... input jacks, loudspeaker leads, you name it.

TimNJ:

--- Quote from: LittleRain on August 18, 2020, 08:18:47 pm ---
Is it really that bad to not have it isolated? I see tons of documentation online from TI, and other big IC makers who design buck converters specifically for non-isolated purposes.


--- End quote ---

Aaah...I see your confusion. Yes, it is very, very bad. There are appliances and devices out there with non-isolated power, but those devices make it essentially impossible for the user to come in contact with anything dangerous. As I mentioned before, this is called "double insulated". You may have seen the term on an AC power tool or kitchen appliance. A power amp is surely not double insulated, unless you take some very particular precaution. Any exposed jacks, metal housing, and that's instantly defeated.

The TI documentation you mention is referring to stepping up/down a voltage from an already isolated source. For example, in a computer, the point-of-load DC/DC converters near the CPU are non-isolated buck converters...but the input voltage is derived from the isolated ATX power supply. Another example is a battery source that needs to be stepped up or down.

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