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Author Topic: Yo................ Vincenzo.......................  (Read 133896 times)
AC5UP
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« on: November 01, 2011, 02:45:33 PM »

Remember a few weeks ago when I beat you about the ears and shoulders regarding rectifier legend & lore in relation to the IGGY?

Here's a quiz for you: Go here --- www.chipamp.com/docs/lm3886-manual.pdf and take a close look at Page 4, the power supply schizmatic.

Tell me if there's anything wrong with the way it's drawn.

( tick, tock )

If you think the instructions look good, clean your glasses and go here: http://diyaudioprojects.com/Chip/LM3886_CA/LM3886_CA.htm

Whoa! Just so you know, sometimes it pays to read kit instruction manuals and schizmatics with a careful eye.  Grin
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N4NYY
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« Reply #1 on: November 01, 2011, 03:24:45 PM »

He tapped one of the Bridges incorrectly.

Other than that, the transformer appears to be a 1:1 instead of step down. But that just means he gets  higher DC out.
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AC5UP
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« Reply #2 on: November 01, 2011, 03:37:38 PM »

Bingo!

Remember the rule that the arrow always points to positive... In the case of a differential power supply the two sides are mirrored relative to ground. One bridge grounds to negative, the other to positive, the filter caps are in series with the center point grounded.

Another way to build that power supply would be to use a center tapped transformer (or tie the two secondaries together in phase) and ground the center tap. Use one bridge across the outside of the secondary winding with the + connection to B+ and the - connection to B-.

Saves the cost of a second bridge and is the traditional design for a half wave rectifier.

BTW: The LM3886 chip and variants from National Semi are interesting items.
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N4NYY
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« Reply #3 on: November 01, 2011, 03:47:43 PM »

Hey, BTW. That D-104 that I restored that I scored with a Courier 23 CB? I am going to do a bad thing to it. I am going to paint the base a non-standard color.
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AC5UP
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« Reply #4 on: November 01, 2011, 05:48:15 PM »

Good Buddy Brown?

Some people say that's too 70's, but I say earth tones never go out of style!
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N4NYY
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« Reply #5 on: November 01, 2011, 06:40:54 PM »

I was thinking of a Lime green metalic like a 1960's color

http://www.google.com/products/catalog?hl=en&rlz=1B3GGLL_enUS421US421&q=metallic+light+green+spray&gs_upl=10053l11801l1l12099l6l6l0l0l0l0l235l990l0.5.1l6l0&bav=on.2,or.r_gc.r_pw.,cf.osb&biw=1366&bih=525&um=1&ie=UTF-8&tbm=shop&cid=7399108343948907555&sa=X&ei=Z5-wToC2D8nY0QHnmIydAQ&ved=0CFkQ8wIwBg
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N4NYY
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« Reply #6 on: November 01, 2011, 07:25:35 PM »

BTW, I did not miswire the bridge. I connected the ground to the negative of the bridge instead of chassis ground.

Now, if you want to talk about metal tubes, well, that is a whole other argument!   Roll Eyes Roll Eyes Roll Eyes Roll Eyes Roll Eyes Roll Eyes
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W8JI
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« Reply #7 on: November 03, 2011, 05:45:24 AM »

He tapped one of the Bridges incorrectly.

Other than that, the transformer appears to be a 1:1 instead of step down. But that just means he gets  higher DC out.

He has a label issue with output terminals and a filter capacitor drawn backwards in the lower supply section, and transformers, while they should be drawn with reduced turns on low impedance or voltage side, can be drawn as a 1:1 if specified in writing as to ratio or value.

That's probably because he didn't have someone proof read his work.
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K9YLI
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« Reply #8 on: November 03, 2011, 07:55:09 AM »

  UP..  i  disagree with the statement the  arrow  points to positive.

  diodes..      K   with a   |   plate accross the  legs..   plate like a  tube diode being the  plus side..     ----K|-------

  electron flow   against the arrow..   electron flow   minus to  plus..

  just so some  rookie reading  you disertation doesnt  take it as gospel and makes a mistake   6 months from now..

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AC5UP
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« Reply #9 on: November 03, 2011, 08:34:30 AM »

Download this: http://www.hammondmfg.com/pdf/5c007.pdf

In the case of full wave, half wave, or bridge rectifier circuits the diode(s) always points to the positive side of the DC output. If you find a power supply schematic on the web where this isn't true, please post the URL so I can take a look. Remember that my comments to N4NYY both here and in a previous thread refer only to AC power supply rectifiers.
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K9YLI
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« Reply #10 on: November 04, 2011, 06:56:47 AM »

  UP     ok  using that  thinking, ou are correct..  I drew up a  full wave bridge to check.    Seems an  odd way  to look at something.

 I always go by the action of individual components.
To add  the  arrow points to the ' plus side of the output'
which is the   minus side of the diode  inrelation to the voltage at the transformer winding.

semantics..

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AC5UP
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« Reply #11 on: November 04, 2011, 08:38:11 AM »

No Prob........... There are times when I'll come up with a mental shortcut that makes sense to no one but myself.

In the case of diode rectifiers the "arrow" of the schematic symbol points to the cathode band on the physical part, which should be nearest the positive side of the rectified output. In the past things like a discrete four diode bridge on a terminal strip or lined up neatly on a PC board could get me turned around (and sometimes still do), but as long as the cathodes are nearest the positive terminal they're probably installed correctly.

Even more counterintuitive (to me) is that when testing diodes the negative test lead connects to the cathode. A DMM in diode check should read the voltage drop across the junction this way. Reverse the leads and a healthy diode should have no continuity. Take a look at this illustration I found on the web:

http://www.kmitl.ac.th/~kswichit/npn/diode.gif

Absolutely ass backwards. If you have a spare diode within reach, try it and you'll see...............  Tongue

(BTW: Just for the hell of it, take the URL for the GIF image above and edit it back to http://www.kmitl.ac.th/ then press enter. This will take you to the source web page. Whoa! Whooda' Thot?)
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KD6KWZ
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« Reply #12 on: November 11, 2011, 02:33:32 AM »

Quote
Absolutely a$$ backwards. If you have a spare diode within reach, try it and you'll see............... 

Uh oh, someone who doesn't know better will be making lots of diode returns at the parts store.  Grin
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AC5UP
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« Reply #13 on: November 11, 2011, 04:21:22 AM »

Yo.............. Vinnie! Here's another quiz for ya'!

I'm tearing down junque in the garage and strip out a power transformer from a 13" video monitor. Primary and one secondary winding with no center tap that measures 108 vac (no load) on the bench. Can't think of what I'd use it for, but I'd hate to toss it because it's a nice piece that's fully shielded and beefy enough to be a 60 - 70 watt item. Wouldn't be afraid to run a 100 watt load as intermittent duty.

Then I'm thinking......................... If I bolt the transformer to the bottom of a two gang electrical box it can stand on its head with no problem (shielded, no risk of electrical shock) and inside the electrical box I could install a power cord, duplex outlet, wall switch and a neon pilot light. Then I'd have a quick & dirty isolation transformer with an independent power switch!

How cool would that be?

On one hand, it's almost free as I have the junquebox parts to build it. On the other, what good is a 100 volt isolation transformer?

So here's the question: Assume the typical AA5 rated at 30 watts of AC draw. Will the radio play at 100 volts? Can faults in an AA5 be accurately diagnosed at 100 volts? If you align the radio at 100 volts will the alignment be totally bogus at 120 volts?

Whatcha' Think?

BTW: Even if I don't use this transformer to isolate AA5's (and at the moment I have none in the house) I could use it to iso the soldering iron, even though there's no screamin' need for that. See? I can find a purpose for the transformer.  Grin
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N4NYY
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« Reply #14 on: November 11, 2011, 04:59:59 AM »

I thought those radios were designed for 100V? Some had a 100-117 volt switch.
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