Reply by P E Schoen April 26, 20122012-04-26
"Tim Williams"  wrote in message news:jnaq0d$47v$1@dont-email.me...

> "P E Schoen" <paul@peschoen.com> wrote in message=20 > news:jna5tf$mja$1@dont-email.me... >> when I return to that one I can try the ferrite cores again. I have >> both #26 and #52 material (which I used). Maybe the #26 >> is better?
> Yuck, eww! #26 and 52 are powdered iron numbers, not ferrites. > Both are "high permeability", which is only 75. You really > want >500 for a transformer, and >10k for a good transformer > (pulse transformers, CTs, small signal isolation, etc.).
> Common powdered iron toroids are yellow/white (#26), which > is very lossy, only good for low ripple smoothing chokes, or > blue/green (#52), which is about half the losses and same > performance otherwise, which isn't really saying much as > it'll still cook off easily with much ripple. These are common on =
older=20
> motherboards, where the smaller cores > can tolerate some ripple at moderate frequencies, but > even then they can get quite hot.
Yes, actually I got the types reversed. What I have in the working unit = is=20 Fair-Rite 9477016002, which is a type 77 ferrite. It was the powdered = iron=20 core that was ringing and unable to work well. Thanks for the heads-up! = This=20 was a project I had started last year, got the samples, and then put = aside=20 for a couple months while I worked on something else. When I returned to = this project I did not check the part numbers, and the packaging did not = specify whether the cores were powdered iron or ferrite. I just downloaded the Fair-Rite catalog, and that seems to be very = helpful.=20 The powdered iron cores were from http://www.micrometals.com/. [snip - but saved for future reference]
> Or you can buy the whole thing in an e.g. RECOM isolated converter, > albeit for >$20/ea.
The closest DC-DC converter I could find that might meet my needs was = fro=20 Traco Power, but although it was rated for 4000 volt breakdown, it was = not=20 rated for 480 VAC mains isolation.
> FYI, are you aware your message encoding is quoted-printable? It > makes it very difficult to quote as I have to enter the ">" manually. > Also, the word wrap is slightly off by my reckoning. IIRC, =
traditional
> is ~76 characters, yours might be set 77-78, which isn't bad, just > looks funny after a few layers of quoting.
Yes, someone had noticed that previously, but I am using Windows Live = Mail=20 for my newsreader, and although it is set to plain text, it uses the = quoted=20 printable, and also it no longer performs the automatic word wrap and=20 quoting characters, so I also must add the ">" manually. I have used = X-News,=20 but it's more convenient to use the same application as my default email = client. Maybe I should get something better, especially if it will also=20 handle email as well as news. I have no idea how difficult it may be to handle the quoted printable. = My=20 posts look fine to me, of course, and they also seem OK when I have used = X-news. I think there are add-ons for most newsreaders that handle it. Thanks, Paul=20
Reply by Tim Williams April 26, 20122012-04-26
"P E Schoen" <paul@peschoen.com> wrote in message 
news:jna5tf$mja$1@dont-email.me...
> when I return to that one I can try the ferrite cores again. I have both > #26 and #52 material (which I used). Maybe the #26 > is better?
Yuck, eww! #26 and 52 are powdered iron numbers, not ferrites. Both are "high permeability", which is only 75. You really want >500 for a transformer, and >10k for a good transformer (pulse transformers, CTs, small signal isolation, etc.). Common powdered iron toroids are yellow/white (#26), which is very lossy, only good for low ripple smoothing chokes, or blue/green (#52), which is about half the losses and same performance otherwise, which isn't really saying much as it'll still cook off easily with much ripple. These are common on older motherboards, where the smaller cores can tolerate some ripple at moderate frequencies, but even then they can get quite hot. Typical numbers for ferrite include Fair-Rite #77 and #78, Ferroxcube 3C80, 3C90 and such, and Magnetics types R and P for power (mu ~ 2200), #43 for high frequency (mu ~ 800, good up to ~MHz), 3F3 and 3F4 for high frequency power (up to about 1MHz), etc.; 3E6 and such, Magnetics type W for high-mu, etc. Obviously, you only get the best effective permeability with minimum gap. Line filter chokes actually go so far as to use not just toroids but also rectangle and "digital figure 8" shapes, single piece no mating faces, for absolute maximum permeability. Such ferrites go up to 15k, even 20k permeability: when it gets that high, it's a fragile thing, easily spoiled by ambient bias fields and current imbalance, but when small signal levels are all that matters (swallowing up a little RFI on the line), it does a great job.
> I can't go much higher in frequency with my prototype design because I'm > using a PIC18F2420 which does not have a PWM module. But when I redesign > the board I can use a PIC that has built-in PWM, or I could use a > separate device to drive the transformer. I'm using an L298 module > (which is way overkill, and I'm only using half). So maybe I'll try an > IRS2453D which is a self-oscillating full bridge driver that costs only > about $1.35 and can drive some small MOSFETs. I don't need regulation. > I have a 12V supply and I just want about 8-12 VDC on the secondary for > the gate drive circuitry.
Frequency isn't a big deal; I've done the same thing with a discrete oscillator (what could be simpler than a two transistor multivibrator?), a little current boost (more discrete switches..), and a common mode EMI filter choke ran "sideways". Such inductors have tons of leakage between windings, but the price is right for the amount of isolation you get. Independent of size, I've found most CM chokes are only good for a watt or two: if you run at a higher frequency, you can supply more voltage, but the higher frequency drops more voltage across the leakage inductance, making the supply "squishier"; for some desired output stability (like 10%), maximum load current goes down, as a result, power remains roughly constant. I personally wouldn't use L298, because it's a slow bipolar device with lots of voltage drop (about 2V total under typical load, IIRC). Gate drive chips are the best choice these days; same basic operation (it pulls up to the supply, it pulls down to ground), some even have disable pins (tristating the output), and many are available in multiple units per package (a quad driver with disable is equivalent to a CMOS L298, sans ground return pins). With such inductive loads (as the powdered iron cores), you might need schottky clamp diodes on the outputs to use CMOS chips; all that magnetizing current might otherwise induce latchup. (Gate drivers rated for maybe 1A output are probably good for 0.25A reverse current; design accordingly.) Dual "complementary" packaged transistors are also handy. Sometimes too handy: I once made the mistake of doing this, http://myweb.msoe.edu/williamstm/Images/EE409_PWM3.jpg forgetting that discrete transistors don't have the same softness of monolithic CMOS circuits! Poor tantalum capacitor was probably sweating its balls off, while the transistors pulled 10-20A peak currents out of it for ~50 nanoseconds each cycle. (Solution: nix the cap, soften the supply rail with series resistance and inductance. Supply bypass is actually a BAD thing sometimes, and it's important to see when!) SO-8 MOSFETs and arrays of course come in many sizes, these just happened to sink that much current under switching conditions. Protip: SO-8s are more common than DPAKs or other packages. Bizarre when you'd rather have a compact, heatsinkable SOT89 than a flimsy SO-8, but I guess that's just how the market is. I've gathered a lot of SO-8 dual FETs from hard drives, probably what's "driving" the market, among others. For many purposes, a plain 2N7002 + BSS84 pair wired as a dumb CMOS inverter is equivalent to most smaller monolithic drivers (0.5-1A peak current), with the advantage that they are discrete transistors and can be operated much faster. Anyway, as it's late and I'm rambling again, I'd suggest a flyback converter instead. Something with UC3843 perhaps, like so: http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/12-6_DC-DC_Converter.png (UC3842 shown, because the supply is 18V; 3843 has a lower UVLO threshold, allowing it to work on >8V supplies.) The feedback winding can be local, in which case both output channels have the same mediocre regulation; it can be placed on one output (via TL431 and opto, in the common arrangement), in which case one output is perfectly regulated and the other is mediocre. (Note that, since flyback stores energy, but it demands large ripple, gapped ferrite is preferred over powdered iron in the transformer core.) Or you can buy the whole thing in an e.g. RECOM isolated converter, albeit for >$20/ea. FYI, are you aware your message encoding is quoted-printable? It makes it very difficult to quote as I have to enter the ">" manually. Also, the word wrap is slightly off by my reckoning. IIRC, traditional is ~76 characters, yours might be set 77-78, which isn't bad, just looks funny after a few layers of quoting. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Reply by P E Schoen April 25, 20122012-04-25
"Tim Williams"  wrote in message news:jn8sif$rqp$1@dont-email.me...

> Yuck. Powdered iron has no redeeming value for transformers anywhere. =
> Its inductivity is too low for good transformer action, and in=20 > applications where low inductance is desirable (e.g., isolated flyback =
> converter), the losses are so high that you get more delta B from =
ferrite=20
> (though the average B can be higher, saturation ~0.8T typically).
> Most likely, the reason you saw less ringing is because the inductance =
was=20
> lower and the core looks like a solid resistor, damping most anything. =
> Optimized drive does the same thing without the power loss.
It is also possible that there was a little gap in the ferrite E cores. = When=20 I took it apart to replace it with the powdered iron I had used for the=20 other transformer, it seemed that there may have been a bit of = insulating=20 tape pinched between the E core mating surfaces. I'm working on another=20 project now, but when I return to that one I can try the ferrite cores=20 again. I have both #26 and #52 material (which I used). Maybe the #26 is = better? I can't go much higher in frequency with my prototype design because I'm = using a PIC18F2420 which does not have a PWM module. But when I redesign = the=20 board I can use a PIC that has built-in PWM, or I could use a separate=20 device to drive the transformer. I'm using an L298 module (which is way=20 overkill, and I'm only using half). So maybe I'll try an IRS2453D which = is a=20 self-oscillating full bridge driver that costs only about $1.35 and can=20 drive some small MOSFETs. I don't need regulation. I have a 12V supply = and I=20 just want about 8-12 VDC on the secondary for the gate drive circuitry. Thanks, Paul=20
Reply by Jon Elson April 25, 20122012-04-25
P E Schoen wrote:


> My experience has been more with toroids and variable transformers > (powerstats), which have thin tape wound laminations and are rated up to 1 > kHz or even 2 kHz. >
http://www.danaherspecialtyproducts.com/uploadedFiles/Siteroot/Superior_Electric/Products_and_Solutions/POWERSTAT&reg;_Variable_Transformers/powerstat%20cat.pdf
> (8MB) > > I made a transformer from a powerstat and it worked up to 15 kHz, although > I didn't try to get much power from it. However, I will soon test it at > about 1500 watts at 1 kHz, and it's original rating was about 500 VA at > 50/60 Hz. I also tried a regular E-I transformer from a 650 VA UPS, and it > worked OK at about 500 Hz. Again, I did not try to get much power from it.
Well, sure, if you keep the peak flux down, the dissipation will also be reduced. That kind of goes against the reason for raising the frequency, but it does help reduce cap size. Jon
Reply by Tim Williams April 25, 20122012-04-25
"P E Schoen" <paul@peschoen.com> wrote in message 
news:jn88ko$j7g$1@dont-email.me...
> My experience has been more with toroids and variable transformers > (powerstats), which have thin tape wound laminations and are rated up to > 1 kHz or even 2 kHz.
We get transformers and inductors from an industrial transformer manufacturer (they regularly use copper pipe windings), they use iron up to about 1kHz, ferrite beyond. You might hope a 100kVA transformer at 600Hz has some amount of compactness to it, but you'd be wrong -- it's still about the size of a cube fridge. And that's running it near saturation (Bmax ~ 1.5T, I think), so the cooling pipes run through the core as well.
> For another project, I wound a small transformer on a split bobbin > (COSMO 4143-1-20) and I tried both a ferrite core (Lodestone E75-52) > and a powdered iron core (Lodestone 9477016002), with a frequency of > 57.6 kHz. The ferrite core did not produce as much power and exhibited > a lot of high frequency ringing, but the powdered iron showed a very > good square wave and worked much better. This is just for about 2 > watts for SCR gate drives, but I need a very reliable high voltage > isolation especially when used on 480V mains.
Yuck. Powdered iron has no redeeming value for transformers anywhere. Its inductivity is too low for good transformer action, and in applications where low inductance is desirable (e.g., isolated flyback converter), the losses are so high that you get more delta B from ferrite (though the average B can be higher, saturation ~0.8T typically). Most likely, the reason you saw less ringing is because the inductance was lower and the core looks like a solid resistor, damping most anything. Optimized drive does the same thing without the power loss. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Reply by P E Schoen April 25, 20122012-04-25
"Jon Elson"  wrote in message=20
news:o9WdnX4Gse7Z4QrSnZ2dnUVZ_q2dnZ2d@giganews.com...

> raypsi wrote:
>> I just bought a 50/60 hz transformer rated 575VAC pri to 230VAC >> sec rated at 2.5KVA on ebay. I remember reading some time >> ago that transformer companies rated their transformers to 1000 hz.
> Nope, certainly not. The laminations in any power transformer > will perform quite badly at much over a couple hundred Hz. > The thickness of the laminations needs to be reduced approximately > linearly with increasing frequency o keep iron losses down. > You can compensate to some extent by reducing Bmax, but that will > work against what you are trying to do. What you would really > want is a custom ferrite core transformer. You should be able > to buy surplus E-core or C-core pieces and possibly parallel a > few of them to get sufficent cross section. At 1 KHz, a 2.5 KVA > transformer will not be very large at all. But, then why stop > at 1 KHz, you can go to tens of KHz and make it even smaller.
My experience has been more with toroids and variable transformers=20 (powerstats), which have thin tape wound laminations and are rated up to = 1=20 kHz or even 2 kHz. http://www.danaherspecialtyproducts.com/uploadedFiles/Siteroot/Superior_E= lectric/Products_and_Solutions/POWERSTAT=AE_Variable_Transformers/powerst= at%20cat.pdf=20 (8MB) I made a transformer from a powerstat and it worked up to 15 kHz, = although I=20 didn't try to get much power from it. However, I will soon test it at = about=20 1500 watts at 1 kHz, and it's original rating was about 500 VA at 50/60 = Hz.=20 I also tried a regular E-I transformer from a 650 VA UPS, and it worked = OK=20 at about 500 Hz. Again, I did not try to get much power from it. For another project, I wound a small transformer on a split bobbin = (COSMO=20 4143-1-20) and I tried both a ferrite core (Lodestone E75-52) and a = powdered=20 iron core (Lodestone 9477016002), with a frequency of 57.6 kHz. The = ferrite=20 core did not produce as much power and exhibited a lot of high frequency = ringing, but the powdered iron showed a very good square wave and worked = much better. This is just for about 2 watts for SCR gate drives, but I = need=20 a very reliable high voltage isolation especially when used on 480V = mains.=20 So even though I could have used higher frequency and a smaller package, = the=20 voltage isolation and safe spacing made that risky, and I did not have=20 severe size limitations. Same with the OP's application, but in his case he needs the windings to = withstand 5kV continuous. So a larger transformer may be better so as to = handle wire with thicker insulation and/or layers separated with thick=20 insulating tape. So I still think a tape wound toroid design may be=20 worthwhile. And for the amount of power and high voltage involved, it = may be=20 safer to use a transformer designed for the purpose. I found some 4160V = to=20 120/240V 1kVA to 5 kVA transformers on eBay for under $100, and they = might=20 be ideal. Even a "potential transformer", normally used for metering=20 purposes, might be able to provide the needed power if run at a higher=20 frequency. At least the insulation ratings are designed for it. Paul=20
Reply by Jon Elson April 25, 20122012-04-25
raypsi wrote:

> I just bought a 50/60 hz transformer rated 575VAC pri to 230VAC sec > rated at 2.5KVA on ebay. I remember reading some time ago that > transformer companies rated their transformers to 1000 hz. >
Nope, certainly not. The laminations in any power transformer will perform quite badly at much over a couple hundred Hz. The thickness of the laminations needs to be reduced approximately linearly with increasing frequency o keep iron losses down. You can compensate to some extent by reducing Bmax, but that will work against what you are trying to do. What you would really want is a custom ferrite core transformer. You should be able to buy surplus E-core or C-core pieces and possibly parallel a few of them to get sufficent cross section. At 1 KHz, a 2.5 KVA transformer will not be very large at all. But, then why stop at 1 KHz, you can go to tens of KHz and make it even smaller. Jon
Reply by Phil Allison April 22, 20122012-04-22
<upsidedown@downunder.com
> > In the tube audio power amplifier output transformer days, I did not > detect much differences between 50/60 Hz mains transformers and 30Hz - > 15 kHz audio output transformers.
** Irrelevant to the issue here - since audio output transforms are rated for or the SAME input power across the band. NOT power that increases with frequency ~!!
> The 50/1000 Hz frequency ratio is just slightly more than 4 octaves, > so I would not expect too much problems.
** The problem is when you drive the primary with 20 times more voltage than rated. The laminated core gets damn hot. The primary insulation, layer to layer and even turn to turn, breaks down. Capice ?? .... Phil
Reply by April 22, 20122012-04-22
On Sun, 22 Apr 2012 08:58:29 -0700 (PDT), "dcaster@krl.org"
<dcaster@krl.org> wrote:

>On Apr 21, 11:27&#2013266080;am, raypsi <rborow...@gmail.com> wrote: >> I just bought a 50/60 hz transformer rated 575VAC pri to 230VAC sec >> rated at 2.5KVA on ebay. I remember reading some time ago that >> transformer companies rated their transformers to 1000 hz. >> > >You do not remember correctly. Why would a transformer company rate >their transformers to 1000 hz? There are a lot of trade offs in >transformer design. It makes no sense to design a transformer to work >at both 60 hz and 1000 hz. To work well it 1000 hz, you need to use >thinner laminations. Designing for 60 only will let you make a >transformer that costs less and can be sold for less money. > >This is not to say that one can not use a 60 hz transformer at higher >frequencies. It will just not work as well as one designed for the >higher frequencies.
In the tube audio power amplifier output transformer days, I did not detect much differences between 50/60 Hz mains transformers and 30Hz - 15 kHz audio output transformers. Of course, you had to use sufficient inductance for the low end performance and possibly do some layout rearrangements for minimal stray capacitances for the high end performance. A 7-8 octave bandwidth was not be something extreme. The 50/1000 Hz frequency ratio is just slightly more than 4 octaves, so I would not expect too much problems.
Reply by Jamie April 22, 20122012-04-22
Phil Allison wrote:

> "P E Schoen" has gone INSANE !!! > ----------------------------------------- > > Toroid transformers will easily handle 1000 Hz, and possibly as high as 15 > kHz, as I have demonstrated. When you go up in frequency, you can also go up > in voltage (and wattage) by a proportional amount. > > > ** That UTTER NONSENSE was PROVED wrong here months ago !! > > While SOME increase in power throughput is possible, it is nothing like > proportional at 20 times. > > > So you could use a > 150-300 VA toroid transformer with a 24 VAC secondary and 240 VAC primary. > Create a 300 VDC bus from the 220 VAC line, and then drive MOSFETs or IGBTs > at 1 kHz into the secondary. > > ** COMPLETE INSANITY. > > The insulation would FAIL in seconds !!!!! > > > The primary > should be a 2200 VAC square wave ... > > ** Sppplaatttttt !!!!! > > > The 1 kHz reduces the filtering requirements. > > ** There is virtually NO filtering needed with a rectified square wave. > > > You might have problems with an ordinary line voltage rated toroid or other > type transformer when running at 3000V. > > > ** No fooling ................ > > > The insulation might be rated at 4kV if it's a very good transformer, > > > ** FFS - WANKER !!!! > > Even if that figure is quoted - it is for 1 minute and from primary > to secondary ONLY !!!! > ------------------------------------------- > > > but that is just a surge breakdown rating > and not designed to be continuous. You may need to make the transformer > using high voltage wire. > > ** REALLY !!! > > Hundreds of turns of test instrument lead ???? > > > A better idea might be a microwave oven transformer. Maybe two in parallel > if they are identical. > > > ** Call the guys in white suits with the straitjackets - NOW !! > > ( rest of this puke's drivel snipped - cos it was making me too nauseas ) > > > > ..... Phil > > > >
And good afternoon to you, too! Jamie