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Old 19th May 2008, 04:54 PM   #1
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Default Maida for Screen Supply

I am building a regulated screen supply using a Maida type regulator. I am trying to use 2 180V zeners to lift the adjust pin of the LM317. The zeners burn out....

The output of the regulator is connected to a 100uf Cap for the screen and 3 RC sections to feed three low current stages.

I want to get 360 for screen, 340, 330, 320 for the remaining stages. The RC sections have 47uf 450V caps.

Is it a mistake to try to do this?
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Old 19th May 2008, 05:50 PM   #2
kevinkr is offline kevinkr  United States
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How much current are you running those zeners at? If you run with a 120 ohm resistor between ref and out you will have a reference string current of 10mA which with 180V zeners means each must dissipate 1.8W - so you should be using well ventilated 5W zeners as a minimum.

One alternative would be to reduce the reference current to 1-2mA which will allow the use of commonly available 1W zener diodes. You can also use more lower voltage zeners in series to reduce individual dissipations if desired.

Make sure that all caps on the output have a discharge path through something other than the LM317 when power is removed from its input. Usually diodes back to the input supply are best for this.

Avoid the temptation to use very large caps on the output as they look like shorts during initial application of power - be sure that the pass transistor you use can handle the full supply voltage (and current) under any circumstance. Limit inrush currents..

Use a varistor across the transformer primary to protect against line borne transients.

I don't much care for the maida, summer thunderstorms or tube failures here killed every one I ever built. (long ago) In fact I don't like low voltage ss regulator chips floating on HV at all. YMMV and others here do seem to like them.

ELI and SY may have some tips on higher voltage chips that work well..
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Old 19th May 2008, 06:13 PM   #3
SY is offline SY  United States
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Kevin and I have had startlingly similar experiences and conclusions, but this is one place we diverge. I've never lost a Maida regulator to anything other than stupidity and carelessness. One important note though: tube guys tend to get a bit careless about heatsinking, and the stand-off pass transistor may need to dissipate quite a bit of heat.

But I'm somewhat confused by your mention of Zeners. A Maida reg uses a resistor string to set the voltage. The version I use for the Red Light District amplifier would be quite suitable for what you want to do. I've built about a dozen of them, they're all in daily service for 1-2 years, and zero failures.
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Old 19th May 2008, 08:01 PM   #4
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Attached see what I had in mind.

Thanks for the input.

Sy, so you recommend changing the Zeners for a divider and adding bypass caps?

Still, are the subsequent stages driven by the output of the regulator a bad choice?

AV
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File Type: jpg maida ps.jpg (19.6 KB, 324 views)
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Old 19th May 2008, 08:28 PM   #5
SY is offline SY  United States
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Is this for the screens of an output stage? If so, you don't have to futz around much with bypassing the reference voltage.

A few minor suggestions. I will usually put a stopper resistor between the pass transistor and the 317; sometimes 317s will oscillate if the input isn't either bypassed or a stopper put in. 100 ohms will do, and I think that's the value that Maida used. I don't know the ratings of the 1N4736, but if it's 12-15V, you're in gravy; if it's the 6.2V I've seen in a few published designs, you're skirting danger. It's usually a good idea to snub the output cap; a 4-6 ohms will work fine. And Kevin's point is well-taken about idle current. I wouldn't go below 5mA, but the over-10 mA you have is more than needed.
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Old 19th May 2008, 09:19 PM   #6
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SY,

Yes, primarily it is for the screens of an output stage, but the other three are for two 12ax7 amp stages and a 12au7 concertina splitter.

I will try both you and kevins suggestions. I think the idle current was causing the zeners to overheat. So I could change the ref resistor to 200 and use 7x51v zeners that should run quite cool.

How about heatsinks? I figure the drop in the pass transistor is max 50V @ around 60ma, that is just three watts.

AV
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Old 19th May 2008, 09:45 PM   #7
Salas is offline Salas  Greece
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Quote:
Originally posted by kevinkr
I don't much care for the maida, summer thunderstorms or tube failures here killed every one I ever built. (long ago) In fact I don't like low voltage ss regulator chips floating on HV at all. YMMV and others here do seem to like them.

ELI and SY may have some tips on higher voltage chips that work well..
No reliability problems with Maida. Many in service. Mosfet version.
The more the current drawn the more the voltage difference between Maida input and output is needed for best regulation and immunity to mains born transients. Good heat sink for the IRF840 must be employed.
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Old 19th May 2008, 10:14 PM   #8
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Quote:
Originally posted by salas

The more the current drawn the more the voltage difference between Maida input and output is needed for best regulation and immunity to mains born transients.
I don't quite understand. Are you talking about current in the reference resistor or through the whole regulator? What do you mean with higher voltage difference? Eg: regulating from 450 to 360 instead of 390 to 360?

AV
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Old 19th May 2008, 10:21 PM   #9
Salas is offline Salas  Greece
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Quote:
Originally posted by avincenty


I don't quite understand. Are you talking about current in the reference resistor or through the whole regulator? What do you mean with higher voltage difference? Eg: regulating from 450 to 360 instead of 390 to 360?

AV

-Current through the whole reg.

-Eg: 450 to 360. Correct.
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Old 19th May 2008, 10:30 PM   #10
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The regulator you have drawn is like the LastPAS which was in Audio Amateur in 1981 and 1982 -- without the fussy bypassing. Putting a 2W in series with the collector of the pass transistor will make the job a little easier for the little critter.

The LM317 is the source for the zener string, and the 100R resistor sets the current at about 12mA. If you are using 180V zeners they have to dissipate over 2W each, so you need 5W zeners. Better to use more zeners than one high wattage variety.

There was a correction (maybe a couple of them) to the article. I can hardly wait for Ed to come out with the 1980 issues on CD-ROM.
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