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'[EE]: Rectifier Question'
2003\02\08@234548 by Jai Dhar

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Hello all,

For those of you who knew about my little I2C problem that I had, I have
officially given up on it until I receive my new sensors from Microchip - in
which case I will try again :-) Anyway, working on the other parts of my
project, I started on getting my MAX787 5V SMPS regulator working. It's a very
simple connection, so it seems, according to the data sheet on maxim's web
site (http://www.maxim-ic.com/quick_view2.cfm?qv_pk=1271). I am using their
EXACT circuit, and getting a nice solid +5V output. The problem is at the
input from the rectifier. Basically, I have a transformer that goes to a full
wave rectifier, and then the positive part of the rectifier is fed into the
input of the regulator. As soon as I connect the input of the regulator to the
output of the rectifier, the voltage at the input jumps up to ~45V. Without
the regulator connected, the voltage at the output of the rectifier holds at
~25V. Checking the negative side, with no regulator connected, it gives a -
25V - so it's symmetric, which it seems fine. But as soon as I connect the
regulator, when the positive swings up to 45, the negative becomes about -5.
How come the voltage swings like that? Is this normal? Is there anyway I can
control it?

Thank you,

Jai



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2003\02\08@235835 by Daniel Imfeld

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The behavior sounds like that of a clamper, which employs a diode,
capacitor, and second voltage source to shift an AC source by a certain DC
level, in this case about +20 volts.  I suppose it's possible that the cap,
diode and 5V output on the output side of the regulator are combining to
clamp the input signal up 20V.  I'm afraid I can't tell you how to fix this
problem though.  If this is what's going on, it might not be such a problem
since I think you would still have 5VDC for the output.  I could be wrong
though.

Daniel Imfeld

{Original Message removed}

2003\02\09@001146 by Jai Dhar

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Quoting Daniel Imfeld <spam_OUTdimfeldTakeThisOuTspamSOFTHOME.NET>:

> The behavior sounds like that of a clamper, which employs a diode,
> capacitor, and second voltage source to shift an AC source by a certain DC
> level, in this case about +20 volts.  I suppose it's possible that the cap,
> diode and 5V output on the output side of the regulator are combining to
> clamp the input signal up 20V.  I'm afraid I can't tell you how to fix this
> problem though.  If this is what's going on, it might not be such a problem
> since I think you would still have 5VDC for the output.  I could be wrong
> though.
>

You are right - it wouldn't be a problem since I'm getting +5 anyway (although
specs say 40 MAX on the input... so that might be pushing it a little). BUT..
I need to feed a +12 regulator ALSO, which is just a linear - and the max is
only 30V on that. I tried hooking it up (just for fun since I had a lot), and
it suddenly became a +15V regulator instead of a +12 :-) So that's why I need
to maintain it around +25. If this isn't possible, I will have to drop the
voltage a bit with a resistor - which is fine, I just want to know if there is
another way around.



> Daniel Imfeld
>
> {Original Message removed}

2003\02\09@020251 by Russell McMahon

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Can you provide a circuit diagram.
Would very likely allow us to see what is happening.


       RM

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2003\02\09@090414 by G.Smith

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On 8 Feb 2003, at 23:45, Jai Dhar wrote:

>  I started on getting my MAX787 5V SMPS regulator working. It's a very
> simple connection, so it seems, according to the data sheet on maxim's web
> site (http://www.maxim-ic.com/quick_view2.cfm?qv_pk=1271). I am using their
> EXACT circuit, and getting a nice solid +5V output. The problem is at the
> input from the rectifier. Basically, I have a transformer that goes to a full
> wave rectifier, and then the positive part of the rectifier is fed into the
> input of the regulator. As soon as I connect the input of the regulator to the
> output of the rectifier, the voltage at the input jumps up to ~45V. Without
> the regulator connected, the voltage at the output of the rectifier holds at
> ~25V.
My guess is that you don't have a reservoir (bulk) capacitor and when you say
"connect the input of the regulator to the output of the rectifier" you are
including the 220uF cap which is on the regulator input.

George Smith

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2003\02\09@090903 by Dave Dilatush

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Jai Dhar wrote...

>Is this normal?

No.

What you describe indicates something is very, very wrong: you've
got a malfunctioning component, or a wiring error, or an
inappropriate circuit.

If you could post a complete circuit diagram of your power
supply, it would be a lot easier to help you.

Dave D.

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2003\02\09@113334 by Jai Dhar

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Ok, I have composed a schematic in Paint :-) (if anyone can recommend any
FREE, commonly-used schematic creators, please do). I will just re-iterate
that I am measuring the voltage at the positive end of the rectifier (when it
gets converted to DC). On a better note.. measuring the AC inputs to the
rectifier (right from the transformer) don't change when I connect the
regulator. If that means anything. And I will copy the link to maxim's site
again below for reference (since I didn't stick the regulator circuit in
there).

http://www.maxim-ic.com/quick_view2.cfm?qv_pk=1271

Thank you again for all your help,

Jai

Quoting Dave Dilatush <EraseMEdilatushspam_OUTspamTakeThisOuTCOMCAST.NET>:

{Quote hidden}

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2003\02\09@115914 by Olin Lathrop

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> Ok, I have composed a schematic in Paint :-)

Um, OK, but I don't see it.

> (if anyone can recommend any
> FREE, commonly-used schematic creators, please do).

I hear there are free and low cost versions of Eagle with various
limitations.  I think all of them can do at least one page of schematic
capture.

However Eagle, like any complicated software package, takes a little while
to learn and won't help you immediately.  You could sketch the schematic
with pencil and paper, scan it, and attach the GIF file or put it on a web
site if it's too big.  That sounds a lot easier than trying to use Paint
to draw a schematic.

> And I will copy the link to maxim's site
> again below for reference (since I didn't stick the regulator circuit in
> there).

I don't know about others, but I'm not going to go chasing several links
just to answer a PIClist question.  You will get a better response if you
put everything we need to know in one place that is easy to get to.
Remember, you are asking people to do you a favor, and everyone has
something else to do other than answering PIClist questions.  You want to
make the hassle factor as low as possible.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\02\09@115921 by Gordon Niessen

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AutotraxEDA is free.  http://www.autotraxeda.com   It is still in
development stages, but works.

At 10:33 AM 2/9/2003, you wrote:
{Quote hidden}

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2003\02\09@121155 by Jai Dhar

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part 1 2673 bytes content-type:text/plain; charset=ISO-8859-1 (unknown type 8bit not decoded)

My apologies to all - I forgot the picture (my webmail works in an odd
fashion). I did not expect anyone to chase links by any means, so my apologies
again. Here it is.


Quoting Gordon Niessen <gordonEraseMEspam.....GNIESSEN.COM>:

{Quote hidden}

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part 2 24771 bytes content-type:image/pjpeg; name="circuit.jpg" (decode)


part 3 2 bytes
-

2003\02\09@121408 by Dave Dilatush

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Jai Dhar wrote...

>Ok, I have composed a schematic in Paint :-)

Could you post it, please?

DD

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2003\02\09@130050 by Dave Dilatush

picon face
You wrote...

>My apologies to all - I forgot the picture (my webmail works in an odd
>fashion). I did not expect anyone to chase links by any means, so my apologies
>again. Here it is.

I don't see any reservoir capacitor in your circuit.  Are you
relying entirely on the 220uF capacitor shown in the Maxim
diagram?  If so, I doubt that's enough to hold up the input
voltage to your regulator adequately under load.

Whether you are or not, that wouldn't explain why the rectifier
output voltage is going **UP** when you connect the regulator;
that just doesn't make any sense.

You ARE measuring this voltage with respect to circuit ground,
right?  And circuit ground (i.e., the transformer center tap) IS
connected to regulator circuit ground, right?

I really can't make sense of this.  Are you sure your voltmeter
is working properly?

DD

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2003\02\09@130707 by Oliver Broad

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I think I see what's going on, maybe...

There's no smoothing capacitor in your diagram. I bet there's a reasonable
smoothing cap on the input of the regulator circuit though.

When you measure without the regulator you measure the average of the
rectified waveform, which is still dipping down to zero between each half
cycle so you measure a lot less than peak, possibly less than RMS even?

When you connect the regulator there's a capacitor present so it
peak-detects and reads >50% higher. This is the relevant reading. If this is
too high you have the wrong transformer.


{Original Message removed}

2003\02\09@132538 by Dave Dilatush

picon face
Oliver Broad wrote...

>I think I see what's going on, maybe...
>
>There's no smoothing capacitor in your diagram. I bet there's a reasonable
>smoothing cap on the input of the regulator circuit though.
>
>When you measure without the regulator you measure the average of the
>rectified waveform, which is still dipping down to zero between each half
>cycle so you measure a lot less than peak, possibly less than RMS even?
>
>When you connect the regulator there's a capacitor present so it
>peak-detects and reads >50% higher. This is the relevant reading. If this is
>too high you have the wrong transformer.

Duh.  Why didn't I think of that...

That's exactly what's going on, I'm sure of it.

Jai, if you are intending to get +5V and +12V out of this power
supply, there is no need for you to be using a transformer which
puts out so much voltage.  Instead of 50 volts center-tapped, I
think 28 volts center-tapped would suffice.  That would give you
around 18-20 VDC at the input to the regulators.

DD

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2003\02\09@150921 by Jai Dhar

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Quoting Dave Dilatush <KILLspamdilatushspamBeGonespamCOMCAST.NET>:

> Oliver Broad wrote...
>
> >I think I see what's going on, maybe...
> >
> >There's no smoothing capacitor in your diagram. I bet there's a reasonable
> >smoothing cap on the input of the regulator circuit though.
> >

I have only the smoothing cap that the maxim site recommends (220uf)
{Quote hidden}

You are absolutely right - I don't need a xformer that powerful. The problem
was the surplus store only carried that type at the time. this is used of
course, new xformers are sooo expensive that I didn't want to fork out that
much just for this. Obviously, the better solution would be to wait until they
get more in (Which they will) and use a more appropriate xformer. But anyway,
your explanation makes perfect sense to me, thank you sooo much!! I actually
understand why now.

I appreciate all your help!
> DD
>
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2003\02\09@152341 by Dave Dilatush

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Jai Dhar wrote...

>I appreciate all your help!

You're very welcome.  Best of luck with your project.

DD

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2003\02\09@191241 by Jai Dhar

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For those of you who tend to be careless at times like me - do NOT play guitar
and touch the hot wire from mains at the same time. I just enjoyed quite the
spunky shock since my hands happened to be on the strings while I switched the
power on to my transformer (hence touching the hot wire by accident). Didn't
even think twice about it... just letting those of you know who already
don't :-)

Jai

Quoting Dave Dilatush <.....dilatushspam_OUTspamCOMCAST.NET>:

{Quote hidden}

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2003\02\09@195713 by Tom Messenger

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At 07:11 PM 2/9/03 -0500, you wrote:
>For those of you who tend to be careless at times like me - do NOT play
guitar
>and touch the hot wire from mains at the same time. I just enjoyed quite the
>spunky shock since my hands happened to be on the strings while I switched
the
>power on to my transformer (hence touching the hot wire by accident). Didn't
>even think twice about it... just letting those of you know who already
>don't :-)
>
>Jai
>

Jai, you are the first person ever to get me to type: ROTFLMAO!

Not really... but it did just about make beer shoot out my nose.  Hope you
are ok.  I have read about musicians actually being killed by this stunt
falling into a swimming pool.

It totally amazes me that the things are not transformer isolated.  Maybe
they are and it was just some leakage current?

Well, be more careful in the future, Jai.  We need all the 'pic-listers'
we've got.  It would be a shame for this to get around on the net: "Hey,
over there on the pic-list, some guy fried himself playing guitar while
booting some code on his pic gadget.  Stick with Atmel AVR's or SX's and
stay away from those PIC weirdos...".

In jest, Tom M.

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2003\02\09@213139 by Dave Tweed

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Oliver Broad <RemoveMEoabdelphispamspamBeGoneCOAXIALPOWER.COM> wrote:
> When you measure without the regulator you measure the average of the
> rectified waveform, which is still dipping down to zero between each half
> cycle so you measure a lot less than peak, possibly less than RMS even?

Yes. the average value of a half-sinewave is 2/pi, or 63.66% of the peak
value. The RMS value is sqrt(2)/2, or 70.7% of the peak value.

40V (peak value, with regulator attached) * 63.66% = 25.46V (average value)

-- Dave Tweed

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2003\02\10@073027 by Roman Black

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Dave Dilatush wrote:

> >Is this normal?
>
> No.
>
> What you describe indicates something is very, very wrong: you've
> got a malfunctioning component, or a wiring error, or an
> inappropriate circuit.


Maybe he has some type of buck SMPS circuit
without a properly functioning flyback diode...
:o)
-ROman

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2003\02\10@074901 by Roman Black

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Jai Dhar wrote:

> > >too high you have the wrong transformer.
> > Instead of 50 volts center-tapped, I
> > think 28 volts center-tapped would suffice.
>
> You are absolutely right - I don't need a xformer that powerful. The problem
> was the surplus store only carried that type at the time.


Find the point where the 2 wires exit the transformer
and are soldered onto the centre tap pin. Then cut one wire,
and re-solder it as 28v in PARALLEL so you get twice the
current. Probably takes 10 minutes and saves you buying a
new transformer.
-Roman

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2003\02\10@080314 by Jinx

face picon face
> Find the point where the 2 wires exit the transformer
> and are soldered onto the centre tap pin. Then cut one wire,
> and re-solder it as 28v in PARALLEL so you get twice the
> current. Probably takes 10 minutes and saves you buying a
> new transformer.
> -Roman

Not 100% on this, good idea, but wouldn't the windings need
to be in phase ? If so, how would you do that safely (temporary
series resistor ?) or would you just measure the voltage - if in
phase it would be 28V, if out of phase, 0V ?

( Been blathering out of one end all day, thought I'd give the
other a go ;-)  )

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2003\02\10@080732 by Roman Black

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For best effect try touching your lips to a grounded
microphone when you are touching metal guitar strings
that are connected to 700v DC via your tube amp. :o)
-Roman


Jai Dhar wrote:
>
> For those of you who tend to be careless at times like me - do NOT play guitar
> and touch the hot wire from mains at the same time. I just enjoyed quite the
> spunky shock since my hands happened to be on the strings while I switched the
> power on to my transformer (hence touching the hot wire by accident). Didn't
> even think twice about it... just letting those of you know who already
> don't :-)

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2003\02\10@082217 by Roman Black

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Jinx wrote:
>
> > Find the point where the 2 wires exit the transformer
> > and are soldered onto the centre tap pin. Then cut one wire,
> > and re-solder it as 28v in PARALLEL so you get twice the
> > current. Probably takes 10 minutes and saves you buying a
> > new transformer.
> > -Roman
>
> Not 100% on this, good idea, but wouldn't the windings need
> to be in phase ? If so, how would you do that safely (temporary
> series resistor ?) or would you just measure the voltage - if in
> phase it would be 28V, if out of phase, 0V ?


Ha ha! You're serious? When paralleling 2 transformers
or 2 transformer windings, just connect 2 wires then stick
an AC voltmeter across the remaining 2 wires. If it reads
double AC volts you guessed it wrong, if it reads close
to 0v you got it right. Or just put your tongue across
the 2 remaining leads... Jinxian roulette. ;o)
-Roman

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2003\02\10@083046 by Jinx

face picon face
> Ha ha! You're serious?

I didn't want to send that last mail and look a plonker but some
bigger boys made me

> When paralleling 2 transformers or 2 transformer windings,
> just connect 2 wires then stick an AC voltmeter across the
> remaining 2 wires. If it reads double AC volts you guessed it
> wrong, if it reads close to 0v you got it right

Obvious of course. I've even done it too ;-)

O  O
   |
___
  U

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2003\02\10@083502 by Jai Dhar

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Considering I am still not comfortable with the fact that I OBVIOUSLY don't
know much about electrical safety (hence my jolt), I am not even going to
attempt this. Thankfully, I found an old xformer lying around that HAPPENED to
dish out 30V instead of ~50... and it works beautifully with the whole setup!
The only thing I can't figure out is that it seems to have 2 centertaps: One
for the primary, and one for the secondary. I just left the primary middle
wire unconnected (actually just chopped it off incase I touched THAt by
accident), and connected the secondary centertap.

Quoting Roman Black <spamBeGonefastvidEraseMEspamEZY.NET.AU>:

{Quote hidden}

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2003\02\10@172918 by Dave Dilatush

picon face
Roman wrote...

>Find the point where the 2 wires exit the transformer
>and are soldered onto the centre tap pin. Then cut one wire,
>and re-solder it as 28v in PARALLEL so you get twice the
>current. Probably takes 10 minutes and saves you buying a
>new transformer.

...and will accomplish absolutely nothing.  Remember from the
diagram he posted, he's using the full-wave center-tapped
configuration.  If he cannibalizes his transformer and runs the
paralleled windings through a full bridge, he'll get **EXACTLY**
the same DC voltage he's getting right now.

In case you missed the relevant post, Oliver figured out the
source of the original confusion:

Oliver Broad wrote:

{Quote hidden}

Voila.  Problem solved.

DD

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2003\02\10@174210 by Spehro Pefhany

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At 10:26 PM 2/10/2003 +0000, you wrote:
>Roman wrote...
>
> >Find the point where the 2 wires exit the transformer
> >and are soldered onto the centre tap pin. Then cut one wire,
> >and re-solder it as 28v in PARALLEL so you get twice the
> >current. Probably takes 10 minutes and saves you buying a
> >new transformer.
>
>...and will accomplish absolutely nothing.  Remember from the
>diagram he posted, he's using the full-wave center-tapped
>configuration.  If he cannibalizes his transformer and runs the
>paralleled windings through a full bridge, he'll get **EXACTLY**
>the same DC voltage he's getting right now.

A little bit less, because of the extra diode drop.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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