Electronics > Projects, Designs, and Technical Stuff
Power Supply Design : Questions about Dual Supply and Transformer
TimNJ:
And you will never beat the cost of commercial audio products. When I was about 16, I thought I could make a 3 x 50W amp that was cheaper and better than anything you could buy in a store. Let's just say I stopped keeping track of the cost after my 5th or 6th Mouser order, each of which was about $100. :-DD
Oh well. But, hey it's a lot of fun!
LittleRain:
--- Quote from: Dulus on August 18, 2020, 09:07:39 pm ---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
--- End quote ---
Hey man, its my first time working with AC, I kind of thought it was obvious I don't know what I'm doing, but you don't need to take the piss out of me, what was your first time working with AC like, I'm generally curious?
And this is exactly why I'm here, I need help with designing it, this is a forum exactly for learning electronics, so come on man, there are better ways to get your point across.
Also my initial plan, before I even made the post, was to add a transformer, but again, they are $300 friggen dollars for anything close to what I need, which is 1KVA, its why I asked if anyone could recommend a transformer in my initial post.
And this guy here says adding an isolation transformer will stop GFCI protection?
I'm getting conflicting information everywhere, so its been difficult to learn what is right and what is wrong in the matter of a few days.
But even if I add a transformer, it will still be 120V DC after rectification, so if Q4 blows then whats the difference, other then the fact now I won't have GFCI protection, or is that post wrong(Honestly curious about this also)?
Is it floating power rails that make it less dangerous?
So then if I get a transformer that can convert 120 to 42(which I cant, there are none on the shelf, again anything close to spec is $300), what is the point of a buck converter for cases like this, efficiency?
Maybe you can let me know if I'm wrong about this, I added Q5 there for when the low voltage side/MCU is warming up, believe it or not, but I did think about that, I'm just not sure if I went about it the right way.
If I were to take a guess at why this wont work, it would basically be a floating gate, is that correct or no?
So how would I go about fixing this?
Whats wrong with +10V? The VGSS max for that PFET is +15V, and VGSM is +25V?
Here is the datasheet, I would appreciate if you could tell me what I'm doing wrong with the 10V.
Edit: Oh I just noticed the 100r resistor R9, I just forgot to remove it so calm down lol. The circuit was different before I added the transistors, have you honestly never left a component in by accident after changing it?
Like I said in other replies, I come from a programming background, and only have been working with logic level DC for years, this is literally my first AC/(HV DC) (side)project.
I'll admit I'm rushing this, maybe a little too much, I have boards I want to order for my main projects, and was going to add this to the order.
I actually do appreciate the pointers you gave, but you gave them in the wrong manner, and you could of provided a way to fix this issue as well, instead of making fun. Remember, this is a place for learning.
I will post pictures though, and video if you want.
If you have eagle, and of course if you want to, I'll post the schematic and you can show me what changes you would make.
--- Quote from: Benta on August 18, 2020, 09:27:51 pm ---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.
--- End quote ---
Oh god not me haha.
No the 120 to 42V was before I realized I needed a dual supply, I was going to make a new design using the same components, and yes with a transformer because I couldn't see any other way to get a common ground, and because everyone else told me it would be too dangerous.
I don't even think you could create a dual supply with that design unless you stepped it up to 84V, and created a virtual ground, but don't worry that is not going to happen, the components are not rated for that, and I'm not up to finding/buying new ones.
Just to be clear though, I did know it was dangerous, but I saw it as my only option other than buying a $300 transformer.
But yeah, I've decided on making my own transformer, I have tons of steel sheet metal, and silicone/epoxy, all I need is 12-14 gauge wire and I should be set.
This is actually whats on the back of the board :P
Edit: why aren't the BBCODE attachments working properly?
Anyways the image is on the bottom of the post.
--- Quote from: TimNJ on August 18, 2020, 10:18:51 pm ---
--- 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.
--- End quote ---
Ohh ok, yeah that makes much more sense. I do really need to stop rushing this, maybe order these with my next batch of boards instead.
--- Quote from: TimNJ on August 18, 2020, 10:31:39 pm ---And you will never beat the cost of commercial audio products. When I was about 16, I thought I could make a 3 x 50W amp that was cheaper and better than anything you could buy in a store. Let's just say I stopped keeping track of the cost after my 5th or 6th Mouser order, each of which was about $100. :-DD
Oh well. But, hey it's a lot of fun!
--- End quote ---
Yeah I hear ya, I actually have about 6 1000 watt rack mounted amplifiers already, I really don't need another amp, but yeah I was mainly doing this for the learning experience, and as you said, just for fun.
TimNJ:
I feel like I've seen (and have been part of) very similar discussions on this forum about working safely with mains, but don't know the names of those topics off the top of my head. Maybe others can point you towards them.
In general, two issues, not to be taken lightly:
1. Working on/debugging your mains powered project
2. Using your mains powered project
When working on a mains powered project, you'll want to probe the non-isolated side of your power supply with your scope. To do this, your device under test needs to be powered through a lab isolation transformer. This lets you put the scope's earthed ground clip virtually anywhere you want on the board. Without an isolation transformer, you are limited to putting the ground clip on earth only, which is not useful. So, if you want to design a 1KW power supply, you need a 1KVA isolation transformer, just for bench testing.
Assuming you built an isolated supply, once all your debugging is done, and the design is buttoned-up, you can remove the 50/60Hz isolation transformer, because you no longer need to poke around with the probe.
The above isolation transformer is a big chunky iron-core low frequency transformer. Remember, its main purpose is to help you safely debug.
But, you still need to provide galvanic isolation in the finished product. That's accomplished by a (relatively) small ferrite-core switching transformer. (This is one of the reasons we use switch-mode power supplies in the first place.) It doesn't really make sense to use a big 50/60Hz low frequency transformer in conjunction with a switching supply. The beauty of SMPS is down-sized magnetics.
As noted a few times above, one of the issues with mains is that it is earth-referenced. We humans are also earth referenced, as long as we are standing on the earth, or a floor, etc. So, if some fault condition happens, there's a direct loop between the low-impedance main supply through your body. A transformer breaks that loop, since there is no galvanic connection, thus limiting the current through your body to the small parasitic leakage current through the transformer.
TimNJ:
I encourage you to try building a mains powered supply...just there are many pre-requisites to do it safely. 500W-1KW is not a trivial task. I also don't think you've considered things like power-factor correction and EMI. No one is going to hunt you down with your hobby project, but in general, the commercial supplies meet these international requirements. The ones on AliExpress might at least try to.:-DD
If you want to try building something, I'd recommend <100W to start.
NiHaoMike:
--- Quote from: TimNJ on August 18, 2020, 10:31:39 pm ---And you will never beat the cost of commercial audio products. When I was about 16, I thought I could make a 3 x 50W amp that was cheaper and better than anything you could buy in a store.
--- End quote ---
Amplifier boards and surplus telecom PSUs are really cheap nowadays, definitely well under the cost of a preassembled amplifier of equal specs. I guess it depends on if you call the board a "commercial audio product".
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