Bob Cordell Interview: Power Supplies - Page 52 - diyAudio
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Old 17th May 2007, 02:36 PM   #511
anli is offline anli  Russian Federation
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Default Re: Re: Re: torroidal power transformers

Quote:
Originally posted by Bob Cordell
Very good observation. Are these "special" toroidal transformers readily available anywhere? Do toroidal transformer makers normally spec the amount of induction? I am certainly not much of an expert on transformers.

Finally, can I get a relative measure of the amount of induction by knowing the volts-per-turn of the toroid (i.e., wind ten turns on it, measure the voltage, and divide by ten)?

Thanks,
Bob
Bob,

I'm not trans-guru also. I (and many other Russian DIYers) order toroidal transformers in a firm which accept single-amount orders and all customer's demands (Vn, I, P, ternary steep and so on), and know how to achieve acceptable inductance (the firm's fellows call such orders as "audio" ). So, I'm a customer here only. The firm is Moscow-located, I think it isn't handy for you. At any case any decent firm MUST can to calculate such trans.
Of course, there are DIY-fellows which can calculate such trans, but I prefer to design schematics rather transformers
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Old 17th May 2007, 02:59 PM   #512
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I had an Tektronix SG505 oscillator (which is a really excellent test oscillator) -- the SG505 has its own power supply (unlike the SG502 which relies upon backplane DC from the TM5xxx) -- I found that by putting a piece of mu-metal shielding (which I had insulated) around the transformer I could get the THD down below 0.001% -- on a newer model SG505 which replaced the old one the THD was 0.0008% -- I don't know if there was an intentional fix by the folks in Oregon or if it was just random variation and I happened to get a better unit.

At any rate, a little mu-metal cures some transformer woes --- just remember that it doesn't like sharp bends.

Jim Williams at Linear Tech had an application note (which I can't find) in which the radiation patterns of various transformers were illustrated.
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Old 18th May 2007, 01:52 AM   #513
AJT is offline AJT  Philippines
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Default Re: Re: torroidal power transformers

Quote:
Originally posted by anli

Bob,

I have had the same effects when "standard" toroidal trans was in use. After replacing of last one with "special" trans the problem has gone. "Special" means reduced induction (down to 1.1-1.2Tl instead of 1.4-1.5Tl), which was achieved by rising turns amount (about 40-50% above "standard"). A drawback is, you need a wire with bigger diameter, or be ready a trans will be warm and "soft" - I mean secondary resistance. Very simple.
implies lower core losses, magnetising current is likewise smaller, the bigger wire is needed to keep the copper losses down, you can do the same thing with EI's...
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Old 18th May 2007, 07:38 AM   #514
AndrewT is offline AndrewT  Scotland
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Hi,
Can I assume that the extra turns on the primary give a different solution than adopting a higher VA rating.

It seems that the thicker wire and more turns solution is going back to the low regulation transformers that were more common 20 or 30 years ago.

Maybe all we need to do is select transformers on the basis of regulation (plus electrostatic screen).
Any thoughts?
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Old 18th May 2007, 06:18 PM   #515
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Quote:
Originally posted by jackinnj
I had an Tektronix SG505 oscillator (which is a really excellent test oscillator) -- the SG505 has its own power supply (unlike the SG502 which relies upon backplane DC from the TM5xxx) -- I found that by putting a piece of mu-metal shielding (which I had insulated) around the transformer I could get the THD down below 0.001% -- on a newer model SG505 which replaced the old one the THD was 0.0008% -- I don't know if there was an intentional fix by the folks in Oregon or if it was just random variation and I happened to get a better unit.

At any rate, a little mu-metal cures some transformer woes --- just remember that it doesn't like sharp bends.

Jim Williams at Linear Tech had an application note (which I can't find) in which the radiation patterns of various transformers were illustrated.

The SG505 was indeed a great oscillator, designed by my friend Bruce Hofer at Tek before he co-founded AP.

Bob
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Old 21st May 2007, 04:31 PM   #516
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OK, suppose we are stuck with a given transformer, which may often be the case. How can we reduce this tendency to radiate a spray of 120 Hz harmonics? Is there some kind of better filtering we can do at or near the ac side of the rectifiers? Will soft-recovery diodes help this? I've tried snubbing and shunt capacitance with limited improvement. Its just surprizing that the toroid was so much more prone to this than a conventional transformer.

Bob
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Old 21st May 2007, 04:49 PM   #517
anli is offline anli  Russian Federation
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Quote:
Originally posted by Bob Cordell
OK, suppose we are stuck with a given transformer, which may often be the case. How can we reduce this tendency to radiate a spray of 120 Hz harmonics? Is there some kind of better filtering we can do at or near the ac side of the rectifiers? Will soft-recovery diodes help this? I've tried snubbing and shunt capacitance with limited improvement. Its just surprizing that the toroid was so much more prone to this than a conventional transformer.

Bob
Bob,

EM-radiation depends little on current consumption. The only way to reduce this negative effect for given transformer is to cover last one with ferrous "bowl". 1mm thickness reduces radiation ~8-10db. Just try in real-time (steal suitable iron bowl from your kitchen) and see a magic
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Old 21st May 2007, 04:57 PM   #518
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by Bob Cordell
OK, suppose we are stuck with a given transformer, which may often be the case. How can we reduce this tendency to radiate a spray of 120 Hz harmonics? Is there some kind of better filtering we can do at or near the ac side of the rectifiers? Will soft-recovery diodes help this? I've tried snubbing and shunt capacitance with limited improvement. Its just surprizing that the toroid was so much more prone to this than a conventional transformer.

Bob

Just thinking aloud Bob.

From what you post it seems you are not certain whether the spray is conducted all the way from the rectifier, or whether it is radiated from the transformer and picked up.

Perhaps removing the transformer and connecting with a long thick cable may help, moving it around, to check if something changes.

If it is conducted - which seems more likely - perhaps the culprit is the lower leackage inductance inherent in a better confined magnetic circuit as is the case for a toroid in comparison with a I-E or E-E lamination. If that were the case, then a small input choke will also make for a measurable difference, just to make sure.

Just a cent, and keep us posted on your results.

Rodolfo
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Old 21st May 2007, 05:15 PM   #519
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I filter my rectifiers, fast or slow, and haven't had a problem with radiated or conducted HF stuff. As for LF magnetically induced noise, I've had little success with mu-metal for power applications. IMO, things need to be fully encased, and bending and joining the stuff reduces the performance unless you can re-anneal it. It's been ok for small signal shielding, but so has regular steel.

Here's a crazy trick that has worked for me. Shield your magnetic fields with aluminum! Foil won't work. 1/4" thick plate won't work. 3/8" plate starts to work, but isn't great. But, 1/2" plate has so much eddy current loss that it can be more effective than steel or mu-metal. It isn't even that expensive, but you need a way to cut it (band saw). I was working on some very sensitive amplifiers (tunneling microscope signal conditioning using OPA627s) and kept picking up transformer fields. A thick aluminum plate between the amps and tranny did the trick, when little else was effective. Being a commercial product, they said "no way" due to cost. The ultimate solution was to mount the transformers diagonally, redirecting the fields away from the sensitive amps. Take your pick.
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Old 21st May 2007, 05:59 PM   #520
PMA is offline PMA  Europe
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Because of its very very high relative permeability, the mumetal easily saturates, and that is the problem.

Skin depth of aluminium is some 11.5 mm for 50Hz, so we have to make it thick . Actually, it does not saturate.

I am designing measuring instruments for high power labs. My last case/housing is milled from 8mm Al plates (outer case), and there is further Al or Fe inner case, depending on application.
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