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'OT: dc to ac inverters'
2000\02\14@000824 by John Pearson

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Looking for  a way to power a leaf blower from a car battery for no more
than 10 seconds.
My leaf blower is 120 volt and 7.5 amps, so that would be 900 watts?
Any inexpensive ways to power my leafblower for 10 seconds at a time? The
cheapest inverters I can find for 1000 watts continous are about $500.00.

Thanks
John

2000\02\14@003330 by Dave VanHorn

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> Looking for  a way to power a leaf blower from a car battery for no more
> than 10 seconds.
> My leaf blower is 120 volt and 7.5 amps, so that would be 900 watts?
> Any inexpensive ways to power my leafblower for 10 seconds at a time? The
> cheapest inverters I can find for 1000 watts continous are about $500.00.


10 12V batteries in series.

You can get 12V at 2AH for about $10 each at Mendelssons, in Ohio.
That gives you 240WH, or 900W for roughly 15 mins

Use relay(s) to put them all in paralell for charging, and series for use.
Use a fuse, and watch your self, 120VDC is lethal.
(120VAC isn't a lot better)

2000\02\14@033911 by Russell McMahon

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>> Looking for  a way to power a leaf blower from a car battery for no more
>> than 10 seconds.
>> My leaf blower is 120 volt and 7.5 amps, so that would be 900 watts?
>> Any inexpensive ways to power my leafblower for 10 seconds at a time? The
>> cheapest inverters I can find for 1000 watts continous are about $500.00.


Questions -

-Why? (inquiring minds etc ...)
- How much "blow" do you need?
- Do you have to power an existing blower necessarily or might a 12 volt
motor be used instead?
- Is the motor "series wound" (as in many vacuum cleaners and other blower
applications) - if so, it will run from DC and possibly from less than the
full 120V
Less V = MORE current with a series wound motor.

Lets see what's needed.
900 watts rated - say 1000watts
Assume 8 volts under "cranking".
10 seconds = 10,000 watt second
= 10,000/3600/8 = about 0.4 Ah
= small fraction of 40Ah auto battery capacity.
At 100% efficiency the DC load needed is about 1000 watts/8v = 125 amps
That's a cranking load - you probably want an auto size battery even though
capacity required is smallish.
At real world efficiencies you'll need more like 200A

For such a short period you could use a bank of FETs with relatively modest
heat sinking.
You can get low Vds hi current FETS good for the better part of 100A at "not
too much".
Say 4 at 50A or 8 at 25A.
Transformer is annoying :-)
Either
- You go low frequency and use a humungous transformer (1 KVA 12v lighting
transformer). large and heavy. Or rewind an "old" isolating transformer or
surplus item. or
- Go high frequency and rectify this to make 50 Hz AC.
At minimum you then need a half bridge switch on the high voltage side but
this is easy compared to the low voltage problems..

So far I'd say even with junk box parts and a bit of skill you are still in
the $US100 range (depending on how good your junk sources are).

The bank of 12v batteries that someone mentioned looks attractive :-)
The current drain is much more acceptable - OK possibly for smaller
batteries (1.2AH?)
These need NOT be relay switched for charging - you can charge them in
series from rectified mains - a particularly dangerous sounding device ;-).



regards,




     Russell McMahon
_____________________________

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(* - or woman, child or internet enabled intelligent entity :-))

2000\02\14@063705 by Dave VanHorn

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> The bank of 12v batteries that someone mentioned looks attractive :-)
> The current drain is much more acceptable - OK possibly for smaller
> batteries (1.2AH?)
> These need NOT be relay switched for charging - you can charge them in
> series from rectified mains - a particularly dangerous sounding device
;-).

The smaller batteries have higher internal resistance, I'd be nervous.
Also on the relays, he'd have to come up with a regulated voltage source at
135-153V A little scary, and not real simple.

A regulated source at 13.5 - 15.3V is a lot easier to come by.

2000\02\14@074805 by Russell McMahon
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>The smaller batteries have higher internal resistance, I'd be nervous.
>Also on the relays, he'd have to come up with a regulated voltage source at
>135-153V A little scary, and not real simple.
>
>A regulated source at 13.5 - 15.3V is a lot easier to come by.


A constant current source would probably suffice if very rapid charging
times are not needed.
Mains peak rectified gives about 150 VDC from 110 VAC
9 batteries gives 108v dc nominal - less under load.
V charge of about 13.6V (traditional) = 9 * 13.6 = 125v say.
This gives about 25v for a current source to work on.
A 220r gives about 100ma at about 2.5 watts dissipation.
Use a 1K and it would float indefinitely at about the 40 hour rate.

A little playing around to refine the figures but something like this could
be made to work.

Depends very much what its for and why and where & how it's going to be
used.

Charging a 12v battery is easier but then needs either lots of switches or a
rather grunty almost unused inverter.

The desired load of 7.5 amps is about the 9 minute nominal rate for a 1.2AH
cell so you may get several minutes actual .
Recall that the spec was "no more than 10 seconds".

Do-able, I think. albeit not elegant.



RM

2000\02\14@080300 by paulb

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Dave VanHorn made the suggestion.


Charge             :                 :
+ o-+-----------------+-----------------+-----------------+
   |              :  |              :  |                 |       Output
   +-|>|--+-----+ :  +-|>|--+-----+ :  +-|>|--+-----+    +-|>|--+-----o
       ___|___  | :      ___|___  | :      ___|___  |        ___|___  +
      |   +   | | :     |   +   | | :     |   +   | |       |   +   |
      |       | | :     |       | | :     |       | |       |       |
      |  12V  | | :     |  12V  | | :     |  12V  | |       |  12V  |
      |       | | :     |       | | :     |       | |       |       |
      |___-___| | :     |___-___| | :     |___-___| |       |___-___|
          |     +--+        |     +--+        |     +--+        |
          |       :|        +----+  :|        +----+   |        |
          |       :|        |    |  :|        |    |   |        |
          |       :|        o    |  :|        o    |   |        o
          |       :|     (D) \(C)|  :|     (D) \(C)|   |     (D) \(C)
          |       :+-------o  o  |  :+-------o  o  |   +-------o  o
Common     |       :           |  |  :           |  |              |
- o--------+-------------------+  +--------------+  +--------------+
                  :<Repeating unit >:

 Notes:  Relays are wired for minimum voltage across contacts.  Power
diodes must be rated for full output voltage and current.
(D) = Discharge, (C) = Charge.
--
 Cheers,
       Paul B.

2000\02\14@081335 by Dave VanHorn

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> A constant current source would probably suffice if very rapid charging
> times are not needed.

NOT for lead-acids. Any significant CC will cook them.
They want constant voltage charge.

Nicad AA cells are another option for this, but the cost will be
interesting, and I would really wonder about cell reversal problems with
this many in the stack.
They will like the CC charging method though.

2000\02\14@094723 by Gordon Varney

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Service-Mate P700 made by Translectric Inc.  (8 Amps at the outlet)
1-573-243-5186 they won't sell direct, but they will tell you where to buy.
This device is designed to jump start 12-12 volts 12-24 volts and will
provide 85 Vdc to an outlet. (Yes DC) Any brush motor including drills,
saws, vacuum cleaner, and your leaf blower will run perfectly on 85 Vdc. You
can also jump start a road grader or bulldozer with your car, from this
thing. I have owned one for many years, I built a dock on my lake with it.
Run 500 Watt quartz hologen lights at night and cut up the wood with a
circular saw and drilled the holes during the day. Its a portable box with
cables that connect to the battery in your car. Keep your car running if you
plan to use it for a long period of time but for short term uses, it will
run off of the battery.

Gordon Varney

> Looking for  a way to power a leaf blower from a car battery for no more
> than 10 seconds.
> My leaf blower is 120 volt and 7.5 amps, so that would be 900 watts?
> Any inexpensive ways to power my leafblower for 10 seconds at a time? The
> cheapest inverters I can find for 1000 watts continous are about $500.00.
>
> Thanks
> John

2000\02\14@195941 by Brent Brown

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Here's another idea. You could get this voltage from an alternator
on your vehicle. Add a second one, pull out the regulator and
rewind it for the voltage you want. Or with a bit more ingenuity do
some other tricky thing like modifiy your existing altenator to have
a regular and high voltage mode. Switch modes for the 10 seconds
you mentioned and direct the high voltage to your blower. The
typical maximum power output of an alternator is similar to that of
your blower.

FWIW figures I remember suggest the turbo requires something
like 7hp on a 1600cc engine with around 8psi boost (these figures
are very rusty so don't quote me). One horse power is about 746W.

For 10 seconds of boost it might be more efficient to use 3W of
power to open a solenoid valve on a nitrous oxide canister in your
intake manifold...

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz

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