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'[OT]: Project Construction'
2003\02\25@131310 by Jai Dhar

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Hello all,

I have almost completed prototyping my current project - just a simple power
supply (5/12v and a few others) and a 16f628/877 programmer all in one.
Basically a useful tool for prototyping. Anyway, I had already built a power
supply before in the past, and let's just say I came to realize that my
physical construction skills weren't the best :-) I ended up with a pretty
sloppy end-product, that worked... but I ended up frying anyway, heh.

So, considering the amount of time and effort I have put into this, I want to
end up with a better finished product - both physically and electrically. With
this in mind, I have a few questions that I hope can be answered:

1. What is the best way to cut metal for creating jacks such as mains
receptacle jack (the exact same 3-pronged jack you can find on your computer
power supply). It's a funny shape - so what tools would I need to cut this? A
friend of mine suggested a dremel, but I'm not sure how accurate this would
be. I also need holes cut for a 16x2 LCD, and other misc stuff.

2. Short-circuit protection - is there a relatively simple method I can use
for protecting my power supply from being short circuited?? I have reversed
GND and Vcc in the past before, so I thought this might be a good idea to
implement.

3. One of my regulators is a linear type (the +12v), and boy does it tend to
heat up fast! First of all, how can I minimize this heat. Of course, a
heatsink is probably the first method of protection - which I have already
used. I have simply bolted the heatsink onto the case (it's a to-220), but
should I be using some kind of compound between them? Sort of like the stuff
you might find between your CPU and the heatsink. Are there more efficient
ways of cooling the regulator (leaving aside liquid cooling of course). I have
also used a +12V fan, and the darn thing sucks up 200mA as it is, so with that
as my only load, the regulator is already heating up! Mind you, it doesn't
heat up to the point where I think it would be damaging (with the heatsink
bolted on), but if the fan itself is causing some heat.. how much more can it
take? (ironic how a method of cooling ends up generating more heat).

Any other suggestions will be greatly appreciated. Of course, don't feel the
need to answer all of these - I know I have a lot to ask. I figured it would
be better to get it out all at once since I have been noting down these things
as they come to mind.

Thank you all!

Jai



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2003\02\25@132603 by Ned Konz

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On Tuesday 25 February 2003 10:12 am, Jai Dhar wrote:

> 1. What is the best way to cut metal for creating jacks such as
> mains receptacle jack (the exact same 3-pronged jack you can find
> on your computer power supply). It's a funny shape - so what tools
> would I need to cut this? A friend of mine suggested a dremel, but
> I'm not sure how accurate this would be. I also need holes cut for
> a 16x2 LCD, and other misc stuff.

A "nibbler". Even Radio Shack has these.

> 2. Short-circuit protection - is there a relatively simple method I
> can use for protecting my power supply from being short circuited??
> I have reversed GND and Vcc in the past before, so I thought this
> might be a good idea to implement.

You should have a replaceable fuse in your power supply.

You can put a power diode across your VCC supply input on your boards
(cathode to VCC, anode to ground) so that a reversal will put all the
current through the diode instead of your microcontroller (and should
blow the fuse you added).

> 3. One of my regulators is a linear type (the +12v), and boy does
> it tend to heat up fast! First of all, how can I minimize this
> heat.

Feed it with a lower voltage.

Or just use a switching power supply instead.

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2003\02\25@133430 by Jai Dhar

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Quoting Ned Konz <nedspamKILLspamBIKE-NOMAD.COM>:

> On Tuesday 25 February 2003 10:12 am, Jai Dhar wrote:
>
> > 1. What is the best way to cut metal for creating jacks such as
> > mains receptacle jack (the exact same 3-pronged jack you can find
> > on your computer power supply). It's a funny shape - so what tools
> > would I need to cut this? A friend of mine suggested a dremel, but
> > I'm not sure how accurate this would be. I also need holes cut for
> > a 16x2 LCD, and other misc stuff.
>
> A "nibbler". Even Radio Shack has these.
>
> > 2. Short-circuit protection - is there a relatively simple method I
> > can use for protecting my power supply from being short circuited??
> > I have reversed GND and Vcc in the past before, so I thought this
> > might be a good idea to implement.
>
> You should have a replaceable fuse in your power supply.

Where does this fuse go? As it is, I have a fuse in series from the hot-line
that comes from the mains in the wall. One end is attached to the hot-line in
mains, the other to the primary in the xformer. Is this the correct location?
{Quote hidden}

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2003\02\25@135229 by Ned Konz

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On Tuesday 25 February 2003 10:33 am, Jai Dhar wrote:
> > > 2. Short-circuit protection - is there a relatively simple
> > > method I can use for protecting my power supply from being
> > > short circuited?? I have reversed GND and Vcc in the past
> > > before, so I thought this might be a good idea to implement.
> >
> > You should have a replaceable fuse in your power supply.
>
> Where does this fuse go? As it is, I have a fuse in series from the
> hot-line that comes from the mains in the wall. One end is attached
> to the hot-line in mains, the other to the primary in the xformer.
> Is this the correct location?

That's a good place to have one to keep the transformer from burning
up when the diodes short out.

However, you should also have one on the output of the power supply.

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2003\02\25@135655 by Walter Banks

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Ned Konz wrote:
>
> On Tuesday 25 February 2003 10:12 am, Jai Dhar wrote:
>
> > 1. What is the best way to cut metal for creating jacks such as
> > mains receptacle jack (the exact same 3-pronged jack you can find
> > on your computer power supply). It's a funny shape - so what tools
> > would I need to cut this? A friend of mine suggested a dremel, but
> > I'm not sure how accurate this would be. I also need holes cut for
> > a 16x2 LCD, and other misc stuff.
>
> A "nibbler". Even Radio Shack has these.
>
Draw an outline on the panel of the hole you want to create. Use
the suggested nibbler until you get close then use a file, flat or round.
It will take a few minutes (after a few holes a minute or two) but no
one will ever take the feeling of pride you get when it looks good.

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2003\02\25@140306 by Sid Weaver

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Ddownload a free program named Front Panel Designer from
http://www.frontpanelexpress.com.

You can layout your panel, then print it out full size.  Big help.



Sid Weaver
W4EKQ
Port Richey, FL

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2003\02\25@140924 by Lawrence Lile

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On Tuesday 25 February 2003 10:12 am, Jai Dhar wrote:

> 1. What is the best way to cut metal for creating jacks such as
> mains receptacle jack (the exact same 3-pronged jack you can find
> on your computer power supply). It's a funny shape - so what tools
> would I need to cut this? A friend of mine suggested a dremel, but
> I'm not sure how accurate this would be. I also need holes cut for
> a 16x2 LCD, and other misc stuff.

Nibblers are cheap, and will do one project but they are not very durable
and won't do thick metal.  I have two with slivers of metal irretrievably
locking the nibbler tooth closed.  There are various punches that are
better, if you want to spend some dough get a hand press and custom
punches.  If you will do a lot of sheet metal this is the way to go.

Dremels are a "machine shop in a can".  They are as accurate as a pencil,
using a heavy duty cutoff wheel you can cut a square shape out of a panel
pretty efficiently.  I like to draw my cutouts on the panel with a razor
knife, because it is very accurate and the dremel doesn't cover it up like
a pencil line.  Dremels make a lot of noise and a big mess and you should
wear a full face shield and earplugs.


-- Lawrence Lile





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02/25/2003 12:23 PM
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       Subject:        Re: [OT]: Project Construction


On Tuesday 25 February 2003 10:12 am, Jai Dhar wrote:

> 1. What is the best way to cut metal for creating jacks such as
> mains receptacle jack (the exact same 3-pronged jack you can find
> on your computer power supply). It's a funny shape - so what tools
> would I need to cut this? A friend of mine suggested a dremel, but
> I'm not sure how accurate this would be. I also need holes cut for
> a 16x2 LCD, and other misc stuff.

A "nibbler". Even Radio Shack has these.

> 2. Short-circuit protection - is there a relatively simple method I
> can use for protecting my power supply from being short circuited??
> I have reversed GND and Vcc in the past before, so I thought this
> might be a good idea to implement.

You should have a replaceable fuse in your power supply.

You can put a power diode across your VCC supply input on your boards
(cathode to VCC, anode to ground) so that a reversal will put all the
current through the diode instead of your microcontroller (and should
blow the fuse you added).

> 3. One of my regulators is a linear type (the +12v), and boy does
> it tend to heat up fast! First of all, how can I minimize this
> heat.

Feed it with a lower voltage.

Or just use a switching power supply instead.

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2003\02\25@141547 by Jai Dhar

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I think I will be looking into this nibbler - and will be buying relatively
thin metal. I can't afford the high-end press stuff, so that is out of the
question. Especially for one-off's.... but anyway, does anyone have any
suggestions regarding the heatsink?? (considering you guys have answered all
my other questions already!! You are all awesome!).

Quoting Lawrence Lile <EraseMEllilespamSALTONUSA.COM>:

{Quote hidden}

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2003\02\25@141932 by Vern Jones

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Jai Dhar wrote:
>

>
> 1. What is the best way to cut metal for creating jacks such as mains
> receptacle jack
Chassis punches are nice, but a nibbler will work..
>
> 2. Short-circuit protection - is there a relatively simple method I can use
> for protecting my power supply from being short circuited?? I have reversed
> GND and Vcc in the past before, so I thought this might be a good idea to
> implement.
Use inline protection diodes and color code Vcc and GND.
> 3. One of my regulators is a linear type (the +12v), and boy does it tend to
> heat up fast!
Lower the input DC voltage, use a generous heat sink with heat sink
compound or heat conducting thermal washer. Use the 3A TO3 version if
you are pushing things to extremes and really need a linear supply. Use
a switcher....

Use an IEC power receptacle that has a built in line fuse and power
switch, only 1 square hole to cut.

Fuse all of your power supply output voltages between the regulator and
the load.
Use the smallest size that will supply the load plus about 10% and you
shouldn't blow up the supply again..

In addition to the Nibbler, a good set of Files and Drills help as well,
if you have access to a school electronics lab or sheet metal shop, a
shear and brake will let you cut and form the sheet metal.

Vern
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2003\02\25@141934 by Spehro Pefhany

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At 01:53 PM 2/25/2003 -0500, you wrote:


>Draw an outline on the panel of the hole you want to create.

Better yet, draw the holes in a CAD program, print them out in
exact size on a laser printer (check the dimensions carefully),
then spray glue them to the panel with temporary spray adhesive.

<then do this good stuff>

>Use
>the suggested nibbler until you get close then use a file, flat or round.
>It will take a few minutes (after a few holes a minute or two) but no
>one will ever take the feeling of pride you get when it looks good.


Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2003\02\25@142935 by SavanaPics

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If you have an air compressor, try <A HREF="http://www.harborfreight.com">http://www.harborfreight.com</A>.  They have a very
reasonable phneumatic nibler.  They also have some ver reasonable prices on
some of the their brakes and sheers.  We just got a local one here in
Savannah Georgia and I have been buying up some tools

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2003\02\25@142937 by Jai Dhar

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Quoting Vern Jones <.....soundresspam_OUTspamFOOTHILL.NET>:

{Quote hidden}

This is the last bit of info I need clarified. What are some common heat sink
compounds that I can find in stores? Are there different grades that I should
be looking at? What characteristics are typically associated with these
compounds. And as for the regulator, I don't need a linear one, but it's all I
know how to work with. Fortunately, the MAX787 is a nice +5V SMPS regulator
that was easy for me to implement, so I used that for my +5 side... I couldn't
find anything for +12V tho. Are there any out there?
{Quote hidden}

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2003\02\25@145225 by Spehro Pefhany

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At 02:28 PM 2/25/2003 -0500, you wrote:
>If you have an air compressor, try <A
>HREF="http://www.harborfreight.com">http://www.harborfreight.com</A>.  They have a very
>reasonable phneumatic nibler.  They also have some ver reasonable prices on
>some of the their brakes and sheers.  We just got a local one here in
>Savannah Georgia and I have been buying up some tools

Have you actually tried it? It leaves mess everywhere and cuts a fairly
inconveniently shaped kerf.

Best regards,

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2003\02\25@150431 by SavanaPics

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I tried the nibler, I have had  good succes with it.  I just mentioned the
brake as it was mentioned in the list.

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2003\02\25@151850 by jim barchuk

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Hi Spehro!

On Tue, 25 Feb 2003, Spehro Pefhany wrote:

> At 02:28 PM 2/25/2003 -0500, you wrote:
> >If you have an air compressor, try <A
> >HREF="http://www.harborfreight.com">http://www.harborfreight.com</A>.  They have a very
> >reasonable phneumatic nibler.  They also have some ver reasonable prices on

> Have you actually tried it? It leaves mess everywhere and cuts a fairly
> inconveniently shaped kerf.

I've never even seen the pneum nibbler mentioned but have used the manual
a lot. I hate them. Really only as a last resort if a hacksaw blade won't
fit. (Just the blade, no frame.)

By definition they nibble, the don't 'cut.' Ever eaten an apple 'neatly?'
LOL! They're a *bitch* to make a truly straight line or smooth curve.  If
the item to be mounted has no bezel I nibble up to the last 1/16"-1/32"
and file the remainder. Annoying ant time consuming. If no bezel I use the
nibbler roughly and file just a little.

Have a :) day!

jb

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2003\02\25@153943 by Spehro Pefhany

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At 03:23 PM 2/25/2003 -0500, you wrote:
>Hi Spehro!

Hi
 leaves mess everywhere and cuts a fairly
> > inconveniently shaped kerf.
>I've never even seen the pneum nibbler mentioned but have used the manual
>a lot. I hate them. Really only as a last resort if a hacksaw blade won't
>fit. (Just the blade, no frame.)

They are okay if you have thin cheesy aluminum, like in those ubiquitous
ugly plastic/aluminum utility boxes. Otherwise they wreck your hand
quickly. They are sort of a knock-off of a nice US made tool that I forget
the manufacturer of (Roper?) - more like $60 than $10 for the real thing.

The penumatic nibbler cuts a semi-circle and your comments about straight
lines are right on. The cutter is a round bit with a slot in it that slides
in and out. You slide the metal in and the slot chomps a bit off.

BTW, Harbor Freight is sometimes called "Harbor Fright"- their quality
is a bit dubious, look carefully before you buy. Expect stuff that was
packed in crates  China and not looked at until it shows up at your
dock. The pneumatic tools are generally better quality- made in Taiwan.
The brand names "Central xxx", "Chicago Electric" etc. don't mean anything,
they are slapped on the product by factories du jour in Anhui Province,
Taipei or whatever. There's no store close to me, though I manage to
visit the Pasadena one fairly often.

>By definition they nibble, the don't 'cut.' Ever eaten an apple 'neatly?'
>LOL! They're a *bitch* to make a truly straight line or smooth curve.

The nice way is to have a 350lb compound hand press with an appropriate
tool. They are not cheap though, about $2K US. I've tried an electric
reciprocating nibbler with a table like a scroll saw that works VERY
nicely, but unfortunately they are also expensive (and I'd have to
get it from overseas with all the implications regarding spares).

>the item to be mounted has no bezel I nibble up to the last 1/16"-1/32"
>and file the remainder. Annoying ant time consuming. If no bezel I use the
>nibbler roughly and file just a little.

You might be able to use a jeweler's saw, but there is a lot of skill
involved in not snapping the little brittle blades.

>Have a :) day!

Fortunately you only have to make ONE prototype, the rest can come from
a supplier ready to use. ;-)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2003\02\25@161144 by Dave Dilatush

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Jai Dhar wrote...

>1. What is the best way to cut metal for creating jacks such as mains
>receptacle jack (the exact same 3-pronged jack you can find on your computer
>power supply). It's a funny shape - so what tools would I need to cut this? A
>friend of mine suggested a dremel, but I'm not sure how accurate this would
>be. I also need holes cut for a 16x2 LCD, and other misc stuff.

I use a nibbler, followed by file and sandpaper to smooth the
finished cutout.  It's slow, and takes a bit of care to get
good-looking results, but it works.

>2. Short-circuit protection - is there a relatively simple method I can use
>for protecting my power supply from being short circuited?? I have reversed
>GND and Vcc in the past before, so I thought this might be a good idea to
>implement.

I include current limiting in my supplies, as in the adjustable
outputs of this general-purpose 3-output supply:

http://mywebpages.comcast.net/dilatush/labsupp.html

In this example, the outputs are limited to around 60 mA.

For +5.0 volt regulators, I usually use National's LP2951CN
regulator.  It current limits at about 150 mA and includes
overtemperature protection.  I've used hundreds of them, and
haven't blown one up yet.

If you use current limiting, you don't have to worry about
blowing things up.

{Quote hidden}

The best thing to do is feed your regulator chip all the voltage
it absolutely needs to keep the output in regulation, and no
more.  Any excess input voltage will only make the regulator
hotter.

HTH...

Dave D.

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2003\02\25@161806 by Jai Dhar

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Quoting Dave Dilatush <spamBeGonedilatushEraseMEspamCOMCAST.NET>:

{Quote hidden}

This is a good idea - EXCEPT - I plan on using my supply to the greatest
potential, if needed. IE: I want to be able to use all 3A+ that my SMPS
regulator can provide, and all ~1.5A that my 12V regulator can dish out. Are
there any current limiters that you could recommend for this current range?
{Quote hidden}

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2003\02\25@162809 by jim barchuk

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Hello Jai!

> question. Especially for one-off's.... but anyway, does anyone have any
> suggestions regarding the heatsink??

(Also regarding your compound question.) There are really too many
variables to fool around with much 'calculation.' Starting at the die,
there's the operating temp range, transistor case temp, mounting method,
thermal conduction, heatsink material and mounting, outer case material,
fan CFM, ambient temps, routing of air through the box. Yuck.

No, the fan isn't adding any -appreciable- heat. :) Meaning unless you're
designing a sealed/closed environment with *totally* measured and
controlled thermals it's not enough to make any difference.

You've got plenty of venting going -out- of the case to allow for all the
air going -in-, right? Meaning square inches of vent greater then the fan.
Any pressure in the case caused by insufficient venting will increase
temps; the more vent the better.

Best and safest is to just overkill it. You mentioned the TO-220 mounted
to the case. You can add more metal outside the case, anything with fins
will help. Still too hot? Add more metal. :) Best always is fins mounted
vertically so warmer air can rise 'up' through the fins with less
resistance.

The variations in thermal compounds won't make any difference, unless, of
course, you're measuring everything mentioned above. :) 'Characteristics?'
Well, 'messy' comes to mind. :) For the future I suggest mail ordering
some Bergquist sheets and cut to fit, or when you order semis order the
Bergquist at the same time.

Just be sure to squeegie the compound as thin as possible.

Have a :) day!

jb

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2003\02\25@163015 by Russell McMahon

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> >3. One of my regulators is a linear type (the +12v), and boy does it tend
to
> >heat up fast! First of all, how can I minimize this heat.

... fan .. 200 mA

> The best thing to do is feed your regulator chip all the voltage
> it absolutely needs to keep the output in regulation, and no
> more.  Any excess input voltage will only make the regulator
> hotter.

As several people have said - don't have excessive input voltage to the
regulator.
You seem to have taken lots of effort over your system so far - using an
appropriate transformer should be worthwhile.
If your fan load of 200 mA is heating the heatsink noticeably with the fan
blowing it then it suggests quite a lot of dissipation. Depends on heatsink
size but this suggests 5 or 10 watts +.
So at 200 mA that means Vin = 12 + 5/.2 = well over 30 volts.
Is this correct?
If so it is far far too high.

What is the DC input voltage to the regulator.
Around 15 volts would be more than ample if the supply is "stiff" enough (ie
transformer is adequately sized for the load).

If you MUST use a higher voltage then you can use a series resistor between
DC supply and regulator to drop some voltage. Size it like this.

Rseries is less than (Vin - Vreg - Vdropout)/Imax

Imax is the maximum current in amps that the regulator will supply.
Vdropout is the minimum voltage (Vin - Vout) the regulator needs to operate.
See spec sheet but assume 2 volts for now.
For an example assume Vin = 24 volts under full load and a max load current
of 0.8 amps

In this case this gives R < (24 - 12 - 2) / 0.8 = 12,5 ohms
Use a 10 or 12 ohm resistor.

This needs to be rate at greater than P = Imax^2 x R
= 0.8^2 x 12 = 7.7 watt
Use a 10 watt, 12 ohm  resistor

This will still dissipate the same amount of heat but now a lot of it is
dissipated in the resistor which is designed to take this power level (if
you use the correct size). Placing the resistor in the fan air flow will
help it.



       Russell McMahon

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2003\02\25@163017 by Russell McMahon

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> This is a good idea - EXCEPT - I plan on using my supply to the greatest
> potential, if needed. IE: I want to be able to use all 3A+ that my SMPS
> regulator can provide, and all ~1.5A that my 12V regulator can dish out.
Are
> there any current limiters that you could recommend for this current
range?


       LM350

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2003\02\25@165140 by Dave Dilatush

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Jai Dhar wrote...

>This is a good idea - EXCEPT - I plan on using my supply to the greatest
>potential, if needed. IE: I want to be able to use all 3A+ that my SMPS
>regulator can provide, and all ~1.5A that my 12V regulator can dish out. Are
>there any current limiters that you could recommend for this current range?

Not really; it's rare for any of my gadgets to require more than
50 mA, so I don't traffic in high-current regulators much.

If I'm not mistaken, there are switchmode regulators available
from Linear Technology, Inc. and from Maxim that implement
current limiting.  Perhaps you could use one of those.

Dave D.

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2003\02\25@180734 by Robert Ussery

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From: "Russell McMahon" <spamBeGoneapptechKILLspamspam@spam@PARADISE.NET.NZ>
| If you MUST use a higher voltage then you can use a series resistor
between
| DC supply and regulator to drop some voltage. Size it like this.

Well, I'm a newbie to PICs and electronics, but it seems to me that a
rectifier diode placed between the supply and the regulator would be a good
idea. This would provide a voltage drop, and provide the added benefit of
preventing reverse-connection damage (I'm speaking from experience here!
)  ).
Am I thinking correctly?

- Robert

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2003\02\25@183801 by Vern Jones

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I have one of the harbor freight air nibblers, I use it when things are
too large for my shear. I use a healthy straight edge clamped to the
piece I am cutting. I get nice clean edges. It is definitely messy,
metal chips all over the place.

Vern

Spehro Pefhany wrote:
{Quote hidden}

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2003\02\25@195409 by Russell McMahon

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> Well, I'm a newbie to PICs and electronics, but it seems to me that a
> rectifier diode placed between the supply and the regulator would be a
good
> idea. This would provide a voltage drop, and provide the added benefit of
> preventing reverse-connection damage (I'm speaking from experience here!
> )  ).
> Am I thinking correctly?

Yes, more or less.
A series diode does no great harm if voltage drop is needed or acceptable
(as is probably the case here).

Many modern regulators can withstand reverse input voltage and don't
strictly need such a diode but it does no harm as long as its affects are
accounted for.


       RM

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2003\02\25@204749 by Herbert Graf

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> From: "Russell McMahon" <apptechspam@spam@PARADISE.NET.NZ>
> | If you MUST use a higher voltage then you can use a series resistor
> between
> | DC supply and regulator to drop some voltage. Size it like this.
>
> Well, I'm a newbie to PICs and electronics, but it seems to me that a
> rectifier diode placed between the supply and the regulator would
> be a good
> idea. This would provide a voltage drop, and provide the added benefit of
> preventing reverse-connection damage (I'm speaking from experience here!
> )  ).
> Am I thinking correctly?

       Yes, that would work too, however, aside from the added benefit of reverse
protection (which many regulators have as a feature) you are still
dissipating the same power you would with a resistor. The benefit of a
resistor in this case is "number of components", to drop enough volts you
may need several diodes, with a resistor you could conceivably do it with
only one. TTYL

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'[OT]: Project Construction'
2003\03\01@103610 by Jai Dhar
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Quoting Russell McMahon <RemoveMEapptechKILLspamspamTakeThisOuTPARADISE.NET.NZ>:

{Quote hidden}

Sorry to bring this up again, but I was just about to head out to buy my
parts... and I started doing some calculations for the resistor that would
drop some voltage. First of all, the Vin to my regulators is 25V. Now,
straight off, I know this is too high .. I need a better transformer. The only
problem is if I can find one or not. Right now, I'm going on the assumption
that I can't. Hopefully I will be home in a few hours with one more suitable
(that puts out 15V lets say). So anyway, doing the calculations... if I want
to use these regulators to their full potential (3A and 5A), I will need
resistors that are rated at like 25W!!!! Is these a fesable option? Is there
any other way to drop the voltage??

{Quote hidden}

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2003\03\01@150317 by Russell McMahon

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> Sorry to bring this up again, but I was just about to head out to buy my
> parts... and I started doing some calculations for the resistor that would
> drop some voltage. First of all, the Vin to my regulators is 25V. Now,
> straight off, I know this is too high .. I need a better transformer. The
only
> problem is if I can find one or not. Right now, I'm going on the
assumption
> that I can't. Hopefully I will be home in a few hours with one more
suitable
> (that puts out 15V lets say). So anyway, doing the calculations... if I
want
> to use these regulators to their full potential (3A and 5A), I will need
> resistors that are rated at like 25W!!!! Is these a fesable option? Is
there
> any other way to drop the voltage??

As you say, 25b is far too high. If you are in Canada as your email address
suggests there should be more appropriately rated transformers available. If
not there are many trade or hobby type internet suppliers. Jameco, Digikey,
Goldmine, ....

IF you use a linear regulator then that power must be dissipated somewhere.
If you are dropping form 25v to say 15v  then that's 10v to drop. Power = V
x I = 10 x 3A &r 10 x 5A = 30 watt and 50 watt !!!!!!!!!!! While this
requires large resistors it would require equally impressive regulators and
heatsinks without the resistors. What it really means is that, at this power
level this is almost certainly the wrong approach!!!!

Look in the archives or Google on switching regulators and buck converters.
Look hard for a lower voltage transformer. Decide if you really need this
much power at this stage too :-). Note that specifying the transformer
voltage depends on load and smoothing capacitance. There are lots of
references on web to power supply design. Google, as always, is your friend.
As a start, Voutdcpeak = Vac x 1.4.   or Vac = Vdc/1,4 (This is due to the
peak AC voltage being root(2) = 1.414 more than the RMS (average) value. So
15/1.4 = 10.7 volts. There will be some droop and ripple so use say 12 VAC -
a very common transformer value. 12 vac x 1.4 = 16.8Vdc. This will be enough
to supply 15v as well with a suitable low dropout regulator. Smoothing
capacitor will need to be large enough to prevent excess ripple. You must
use a full wave rectifier. There will also be diode drop to account for
which I have not done above. Again, see Mr Google.

A switching regulator is highly desirable in this case unless there are good
reasons not to use one BUT I suspect that starting off with less power for
now may be a good start. If we knew more about your applications we may be
able to better comment.


       Russell McMahon

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2003\03\01@223849 by Jai Dhar

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I guess you are right, there is no substitute for the right part. I do my
shopping at a pretty big surplus store, it's just sometimes they don't have
the exact specs I need. A store down the road usually has everything,
including these transformers, but I have to fork out big $$$ for one since
it's all new stuff (eg. $40 for the xformer I wanted as opposed to $8 at the
surplus store). Anyway, I think I picked up a better one today. While
soldering up the wires, I thought it would be interesting to check the
resistance across the primary and secondary coils. I recall there was some
relationship if you measure the resistance across various terminals in a
xformer... but I couldn't remember. Anyway.. what I found was wierd. Measuring
the resistance across the two wires to the primary coil yielded NO resistance
(ie, close circuit), and the same with the secondary. Is this how it should
be? I did the same with my previous xformer, which was working... and I got
the same results. From what I remember in our intro to Electronics class last
year, xformers are simply just coils side by side (duh)... but with this in
mind, how come there isn't a short since the hot and neutral lines are
connected together???


Quoting Russell McMahon <spam_OUTapptechspam_OUTspamspam_OUTPARADISE.NET.NZ>:

{Quote hidden}

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2003\03\01@231245 by Robert Rolf

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It's called 'reactance'. A DC you have a near short, but because of the
inductance of the windings at 60Hz you have a much higher 'impedance'
(not resistance since you are dealing with an inductor).
Any introductory electronics text will explain this in detail, right
after 'Ohm's law'.
R

Jai Dhar wrote:
{Quote hidden}

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2003\03\01@234557 by Jai Dhar

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Thanks Robert! We actually did reactance last year also, but I failed to
relate it to this application. Unfortunately, we don't get much of the
practical stuff yet :-( All makes sense now tho.. thank you!

Jai

Quoting Robert Rolf <KILLspamrobert.rolfspamspamBeGoneUALBERTA.CA>:

{Quote hidden}

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2003\03\02@143243 by Dwayne Reid

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At 10:36 PM 3/1/03 -0500, Jai Dhar wrote:
{Quote hidden}

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