Another way to phrase the same thing, which has helped me to explain things in the past: the BJT isn't a current-controlled device. It's a voltage-controlled device, with a low enough input impedance that it appears approximately current-controlled.
> the same job could be done with an isolation transformer
It really cannot -- the isolation transformer doesn't have control of its output, so it can't start or stop cleanly, and it can't ramp voltage cleanly. (An autotransformer kind of can, but it's still not really good enough.) The AC source can stop on a dime, with no inductance of its own, so it is the correct way to do this test.
Source: I have had to do this and refused to use the autotransformer anymore because it was just too much of a pain in the butt. (We rented the AC source.)
Also I probably should have actually addressed your Class B comment:
No, they're not Class B. It's all digital PWM stuff inside. But the duty cycle gets tiny near zero cross, there's very little power in the waveform there, and there's overhead to have a switching device on at all (this is much more noticeable for IGBTs).
So it ends up being a massive simplification to just not care about that section. And it's a simplification that works pretty great, so people do it!
We had to get this truly right in the inverter I mentioned in sibling comment (as it wasn't a grid-feed or backup inverter, it was doing Something Else™ *) and just that piece was actually way harder than the entire rest of the waveform output design.
* hopefully NDA-OK spoiler: let's just say I know way, way more than I'd like to about what's inside that Chroma 61507 mentioned in the article.
> Evidently, AC waveform generation is extremely power-hungry
Yes, it's quite ugly. Open up one of these things and you'll find a big block of four transistors (if not more due to doubling up) on a big heatsink. That's the inverter drive bridge, and it's probably the single largest source of heat in the whole thing. It's not hard to find.
> to avoid wearing out one side of the rectifier inside the PSU
If you are seriously worried about this then the whole thing is trash. Either the design is marginal or it is not. You cannot possibly switch a relay fast enough to make a difference here (and have the relay survive).
I'm also suspicious of the idea of that part wearing out, but if it doesn't matter at all then there's no reason to call things trash.
Your other comment says: If you are truly close to the design failure point of the rectifier, it's not safe to run at all. (You are almost certainly not.)
Well there's no reason to assume it's close to the failure point.
Think of it this way: Draw the line in the sand for where you'll approve the design, but just barely. If someone is running a diode close to that line, then it's not trash but trying to improve longevity isn't crazy either.
The point is that the dance with the relay doesn't move the needle on design acceptance. If it is acceptable with the relay, it will be acceptable without the relay, because the component stresses will be the same.
So if it is unacceptable, it is unacceptable, and needs to be fixed. I said "trash" because it's going to become trash, and with luck just the power supply. Hope there's a fuse inline! Input rectifier failures tend to take down other stuff without one.
I'm worried about long-term thermal wear from uneven heating, so I switch the current direction once a day. Simply because that's the maximum time I can set on a time delay relay that controls it.
Don't bother. That's not how these things fail, and one day is so so far beyond the thermal time constants involved that it's not doing anything useful. You would have to switch on timescales of seconds to minutes to do anything meaningful, and that would kill your relay in short order.
If you are truly close to the design failure point of the rectifier, it's not safe to run at all. (You are almost certainly not.)
If you are worried about the fact that you're only using one element of a multi-element package, again, it's a nonissue. We do this all the time. It's often cheaper to add a second bridge rectifier to get a single diode than it is to add another BOM line item for the "proper" part. As long as that diode isn't operating near absolute maximum ratings (it probably isn't), it doesn't matter that there are or aren't three more in the box.
note: its the additional component and its knock on effects that are the cost- not the half cent diode.
aka- you add a diode, now you have to add procurement, warehousing, extra time on the pick and place, possibly a more expensive/larger+slower one as well(so even more time), then you have wastage, labor for keeping the machine fed...
Phase-shifted full bridge is the way to go. (It might have another name in this area of power electronics, these things do have lots of names....)
We did a "big" inverter design a while back (500 VA was big for us; perhaps not for you). The guy who did the concept architecture suggested a PSFB design. He then quit to take a a great offer from a startup. Not really being a power electronics team, we hired a specialist consultant. The first consultant did... honestly, I don't know what he did. But it was weird. (This was a problem.) It wasn't a PSFB anymore. It also didn't work. The design then went through five more lead engineers and two more consultants, plus one more if you count me on the side watching and occasionally pitching in (I was the sister subsystem lead). It ended up being a full digitally programmable bridge and we had to figure out how to switch it. Guess how it ended up working?
Phase-shifted full bridge. Just like the first guy (and I!) said it should have been all along!
Please just buy a pair of mains voltage diff probes. They're not expensive (around $500 each new, much less used) and they will eliminate the crazy connection scheme and give you true input -> output fidelity.
Seeing a datacenter proposal in the news come in at 1.3GW was very sobering for me. I spent a lot of time in grad school on the campus of a large nuclear plant, and it turns out one nuclear core is good for about 0.9GW of electricity (or 2.9GW of heat).
A single site consuming more than the entire electrical output of a nuclear core, considering the sheer size and scale of that reactor and its supporting infrastructure and workforce, is just boggling to my mind. It's literally billions of dollars just to feed that one site, if they're being accurate in their proposal.
My best travel advice (for urban areas) is simple: get lost. If you don't know exactly where you are or what's around the corner, you've got to take it as it comes, and that makes it all the more interesting. No FOMO or opportunity cost struggles, just what's in front of you right now.
This is both easier and harder with smartphones and GPS. Harder because, well, you know exactly where you are and have to actively ignore the phone. Easier because when you're ready to be done, you know exactly where you are!
As someone who's traveled quite a bit, I love doing this but I should also warn everyone to make sure they're in a place where it's safe to do so. While most tourist destinations are mostly fine, you don't want to end up in a random shady neighborhood in certain cities - even touristy ones (Barcelona, Paris, etc). It doesn't matter that 20 years ago you did it and it was fine, things have changed.
I can say I've had good and bad - I've wondered through cities with no direction and found -- nothing, two that come to mind are Paris and Barcelona. I'm sure there is interesting stuff to be found here and there but mostly, outside of the main attractions, I found the rest not much more interesting than American suburbia. Yes, I'm glad I saw it to basically see that "life is life". There's the interesting coast, or the restaurant row that's already on the tourist map, then there's the living areas where every block or two there's a convenience store, another non-descript cafe, a hair salon, etc... Maybe once in a while something sticks out but mostly not.
To be clear, I found both cities amazing. But, the "this city is amazing" parts are the parts listed as must visit. The "get lost parts" less so, with a few exceptions.
I've had more luck doing things further down the list. On 3rd or 4th visit, I'm not doing the top 10 most popular things. I'm down on 40th or 50th or 100th.
I personally like navigating to random city parks. I'm not truly lost, but municipal parks vary widely, so there's a good chance of seeing something interesting or pleasant at the destination, and who knows what I'll see on the way!
My wife and I took a walk along one of the Paris canals on a nice early spring day. You are very right that there wasn't much exciting to see once we got out of the touristy areas but locals were out enjoying the day and it was nice to see how the more average Parisian lives rather than only seeing the attractions and neighborhoods occupied by the 7 zeroes and up class.
I liked walking around Paris and did found minor interesting things there: statues, parks, that sort of thing. Paris looked good outside of main tourist attraction to me.
I did that once about twenty years ago. I was in Seoul for a few days for work, and I had the last day free before my plane out in the evening. Without checking a map or guidebook, I got on the subway, rode a few stops, went up to street level, and wandered around; I repeated this four or five times. Other than one nondescript office district, every area I emerged in was interesting: a wholesale textile market, an upscale residential neighborhood, a lively commercial district. Though I don’t know the names of the places I visited, I still remember them all these years later.
This is one of the only sensible comments in this entire thread. Just go wander, talk to and observe people, etc... I've traveled extensively and all of my best experiences are from this sort of thing.
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