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'Need another set of eyes on MOSFET switched capaci'
2000\01\01@131055 by Donald L Burdette

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Byron et al,

I see you're going to try the "simple switcher" design.  Great.  We used
one in a project at work a while ago, and it works great.

However, the thread got me thinking (a dangerous proposition at best, but
this time it worked out well and the smoke is clearing as we speak) -
could a switcher be designed that would use parts available from ANY
Radio Shack or well-stocked junk box?  I'm happy to report the answer is
YES!

I went into my workshop yesterday afternoon, and after a few hours had
cobbed together a design that works pretty well.  For the inductor it
uses the secondary of a 12V step-down transformer.  The control element
is a '555 timer and the power transistor (yes, only one) is an old PNP
thing I had (2N5981).  A 1N4001 diode, a 12V zener, some 2N3904's, a few
resistors and caps finished the parts list.  The output cap I used is
6000uF, but your output ripple voltage requirements may change that.

I get about 11.25 V because my zener is 10.7V instead of 12V.  I get
about 0.7A before the thing comes out of regulation.  The power is
limited mostly by the gain of the power transistor and the rating of the
transformer.  It would probably work better with a FET or newer high-gain
bipolar.  I get about 100 mV peak to peak ripple, most of which is due to
the ESR of the capacitor, which must be about 0.05 ohms.

Mine runs at 2000 Hz, but you could increase that to maybe 5K or even 10K
if you have a good FET.  At 5K my transistor gets hot (that would be ok
if I put a heatsink on it).  Unfortunately, the transformer gets less and
less efficient (hotter) as the frequency goes up.

The advantages and disadvantages I see (compared to "simple" designs)
are:
1.  Parts are easily available.
2.  Layout and construction methods are not critical.
3.  Design can easily be tinkered with to scale up or down, or adjust to
your requirements.
4.  You can get any voltage or any group of voltages you need.  If you
have the right transformer, you can get +5V, -5V, and -12V from one
inverter.
5.  Parts are larger.  My transformer is about 1.5 x 1.5 x 2 inches.
Caps are bigger too.
6.  Parts count is higher, though maybe not compared to 3 separate
"simple" designs.
7.  Parts cost may be higher unless you have a good transformer on hand.
But again, maybe not compared to 3 separate "simple" designs.
8.  Regulation is not as good.

Let me know if you want details.

Don

2000\01\01@133416 by Dave VanHorn

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face
> I went into my workshop yesterday afternoon, and after a few hours had
> cobbed together a design that works pretty well.  For the inductor it
> uses the secondary of a 12V step-down transformer.  The control element
> is a '555 timer and the power transistor (yes, only one) is an old PNP
> thing I had (2N5981).  A 1N4001 diode, a 12V zener, some 2N3904's, a few
> resistors and caps finished the parts list.  The output cap I used is
> 6000uF, but your output ripple voltage requirements may change that.

Try their 100uH inductor. I've used it in boost/buck projects several times.
It's air core, so no saturation problems :)
It's not stellar, but you'll get your switching speed up out of the dirt.
100kHz or better easy.

2000\01\01@153210 by Chris Eddy

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face
Cool.  Hats off to your success.

One other suggestion, although maybe not necessary, add a comparator in front
of the 555 to do the regulation.

By the way, do you think you can hear the 2000 Hz in the audio circuit he is
building?

Donald L Burdette wrote:

> However, the thread got me thinking (a dangerous proposition at best, but
> this time it worked out well and the smoke is clearing as we speak) -
> could a switcher be designed that would use parts available from ANY
> Radio Shack or well-stocked junk box?  I'm happy to report the answer is
> YES!

2000\01\01@195003 by Mark Willis

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face
www.seanet.com/~karllunt/junkbox.htm is Karl Lunt's "junkbox
switcher", I haven't looked at it in a while but remember all it takes
is one P-Channel MOSFET & a few scratch parts;  Need to get the PS
printer online here & re-print that <G>  Keith Payea (both guys are
local here <G>) designed it originally.

What's the efficiency on your design, any idea?

I collect circuits, I'd love to see the "gory details" <G>

 Mark

Donald L Burdette wrote:
{Quote hidden}

--
I re-ship for small US & overseas businesses, world-wide.
(For private individuals at cost; ask.)

2000\01\01@212257 by piclist.com

face picon face
Don't tease. Of course we want the details <GRIN> we are on the PICList!

Wouldn't a small hand wound (with wire from RS) air core inductor be better?
My dad did that sort of thing with a plastic sewing bobbin screwed onto a
bolt chucked in a hand drill for radio circuits when I was a young'n. IIRC,
He would wind the primary until it got to the resistance he wanted and then
wind the secondary until it worked in the circuit, stopping to test every so
often and laying a piece of some special tape in-between the windings to
compensate for scratching off the insulation at the test points.

If one of you electronic types can spec the number of windings, wire size
and resistance it might be worth trying... for the fun of it!

James Newton
spam_OUTjamesnewtonTakeThisOuTspamgeocities.com
1-619-652-0593 phone

{Original Message removed}

2000\01\02@093806 by Donald L Burdette

picon face
The reason I used the transformer secondary for an inductor is that this
design requires a huge inductance - around 100mH, rather than 100uH.
That is so I can keep the frequency low, which is what makes the design
so easy to implement.  You could push the frequency of this design to 50
or maybe 100 kHz, with proportional decrease in the inductance, but the
higher you make the frequency, the more attention you have to pay to the
layout, transistor switching speeds, diode reverse recovery time...

It is quite possible that the 2000 Hz ripple will get into the audio.
You'd have to know what it's used for, or just try it and see.  You could
set the switcher for 14V and use a 7912 to remove the ripple.  You could
increase the frequency.  You could improve the output caps.  But maybe
you don't need to.

As for the gory details, I'll have to apologize.  I forgot who I was
talking to.  Since the schematic is too detailed for ASCII art, I'll have
to try to figure out how to get it into electronic form.  If someone
would be willing to receive a fax, scan it and post it, that would be a
lot easier for me.  I'm still in the pre-scanner dark ages.  If you'd
like to volunteer, e-mail me off-line since I don't get the digested
piclist until after midnight!

Don

2000\01\02@123127 by andy howard

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face
> As for the gory details, I'll have to apologize.  I forgot who I was
> talking to.  Since the schematic is too detailed for ASCII art, I'll
have
> to try to figure out how to get it into electronic form.  If someone
> would be willing to receive a fax, scan it and post it, that would be
a
> lot easier for me.  I'm still in the pre-scanner dark ages.  If you'd
> like to volunteer, e-mail me off-line since I don't get the digested
> piclist until after midnight!

Here's a good tip.  Set up an electronic fax-to mail account for
yourself and then fax it to that account. Instant scanner...

I use http://www.efax.com which works for me and it's free.  You can have a UK
or US fax number forwarded to your email. Other services are available
for a fee, but the free fax receiving is fine for my needs.


Andy Howard
Fax +44 870 131 6959

2000\01\02@170812 by Mark Willis

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face
Andy Howards' idea is great <G>

I can receive faxen & translate 'em (I'll work on getting a SPICE faq
together, I think.)

(360) 802-2558 Fax, hit 'Send' when the stupid old thermal paper fax
machine answers the phone (I'll get the Fax Server up, some time, then
it'll just be Fax modem tones if someone calls <G>)
(360) 802-2557 Voice (Please don't send Faxes to there <G>)

I don't guarantee to be THE fastest person to get to these, if you want
speed, someone else might be a little faster.

 Mark

Donald L Burdette wrote:
{Quote hidden}

--
I re-ship for small US & overseas businesses, world-wide.
(For private individuals at cost; ask.)

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