Re: AC to DC 65+ volts 60 amp Power Supply Plans?


"Tom Bruhns" <k7itm@xxxxxxx> wrote in message
news:1190932958.723753.170040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Sep 27, 11:58 am, "Ulysses" <eatmys...@xxxxxxxxxxx/> wrote:
"Tom Bruhns" <k7...@xxxxxxx> wrote in message

news:1190871995.762989.194470@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx





On Sep 26, 1:00 pm, "Ulysses" <eatmys...@xxxxxxxxxxx/> wrote:
Greetings. I hope I'm in the right place.

I want to build or buy a power supply which will in effect mimic the
output
of solarvoltaic panels. I want to feed the output into an OutBack
MX60
charge controller. The maximum input is 141 VDC and 60 amps. I
can
use
any voltage above about 63 VDC (up to 141) to charge my 48 volt
battery
bank
by programming the MX60. This charge controller will automatically
adjust
the absorb, float, and equalize charge rate voltages and times. I
plan
to
power it with a 120 VAC 60 Hz generator.

What I had in mind is a step-down transformer, a couple of
rectifying
diodes, and a smoothing capacitor. The problem that I expect to
encounter
is ripple. The MX60 is only designed for solar panels so it may
behave
incorrectly (or worse) if there is any ripple or pulsing present.
Any
suggestions? I don't want to do anything that might damage my
rather
expensive charge controller. I'd be happy to buy a power supply if
it's
not
too expensive but so far all of my searches have not turned up
anything.
Are there any web sites that have high current power supply plans?
I
couldn't find any.

Thanks.

That's a lot of solar panels you're wanting to mimic. At 10%
efficiency, I suppose it's about 25 to 30 square meters of surface.
Do you really need that much current? If you really want your source
to look like solar panels, you should have it look like a moderately
high impedance. Into a short circuit, solar panels look about like
constant current sources. Also, if a shadow passes over the panels
(even the right small portion of them) the output drops drastically.
Controllers have to be able to handle that sort of thing.

If I was going to be using that many solar panels, then yes, it would be
quite a few. That just happens to be the maximum amount that the charge
controller will accept. I could go down to, say, 40 amps and still ge
reasonably content. I'm just trying to cut down on generator run-time
and
hopefully gasoline consumption.



A mains-frequency transformer that size won't be a small piece of iron
either. You'd probably be a lot better off with switching supplies,
and you can filter the output to get the ripple to a low value with a
lot smaller components than it will take to do it with a mains-
frequency supply. Now if you have three phase mains available, the
ripple can be much more tolerable, but it's still a lot of capacitance
(or a little less capacitance and some inductance; a little inductance
goes a long ways in reducing ripple especially if you want to get to a
very low ripple percentage.

I just have single-phase AC. Someone suggested a surplus line
transformer
(has a catchy name but I can't think of it right now) but there don't
seem
to be any such things available near where I live. The closest town is
basically a huge trailer park and I can't find any junk yards or places
that
sell surplus electrical stuff. No scrap metal places either. Before I
buy
a huge transformer I want to be reasonable sure I can deal with the
frequency/ripple problem effectively. But then I could always find a
use
for a big 2:1 transformer...



There are surplus houses that deal in power supplies...though 4kW
supplies aren't all that common anyway. Also, I'm not too surprised
you're not finding plans for them (switching type, at least) on the
web. That's a lot of power, potentially kinda dangerous to be playing
with, and because of the currents involved, the physical construction
details and proper selection of components is critical.

Cheers,
Tom

Sounds like the safest way for you to do this, given the lack of
resources, is find a big 2:1 transformer and rectify and filter it.
Unfortunately, a "pole pig" is very unlikely to be 2:1, though you
could use the secondary as a center-tapped winding that you put 120V
across and get 60V from. Does the output have to be ground-
referenced? If so, things get ugly again because you don't have a
grounded center-tap you can work from -- unless you can float the 120V
input. You'll need to float either the input or output, to make a
center-tapped full wave rectifier. A half-wave rectifier would be
just too awful to try to filter.

For the filter, I'd highly recommend using a choke in your filter, if
you want to get to seriously low ripple. A quick simulation using an
0.1 farad filter cap with 1 milliohm effective series resistance and a
40 amp load shows about 1/4 volt ripple, which is probably plenty low
enough for you. But if you can get a 20 millihenry 60 amp choke, you
can use just a couple 4700uF caps (less than 1/10 as much total
capacitance as in the first case) and have about 1/10 volt ripple.
Not that the choke would be either cheap or easy to find, but it can
help a lot in the filtering.

Cheers,
Tom


Wow. Millihenrys. Sounds big. Makes Faraday sound like a midget. I asked
about this on another NG a while back and a couple of people told me not to
use full-wave rectification but to just use two diodes. What you are saying
makes more sense to me. I've built several small (1-2 amp) power supplies
but never anything this big. My problem seems to be that I have no formal
education in electronics and it has always just been a hobby for me.
Because if this I don't know what it is that I don't know.

By "choke" do you mean an AC capacitor connected across the outputs before
it is rectified? And then the 4700uF caps after it's rectified? I think
it's time to call my father-in-law. He just might have something like that.

From what everyone has said it sounds like the ripple can be dealt with
satisfactorily. It also sounds like I'm going to have to break down someday
and get a better oscilloscope. The one I have is a Heathkit from the 60s
and the tubes are so weak I can't even get a good sine wave image any more.


.

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