Re: Getting 36v from a 12v battery

On Sun, 01 Apr 2007 15:10:15 -0400, clare at snyder.on.ca wrote:

On Sun, 01 Apr 2007 15:17:52 GMT, Jenny3kids@xxxxxxx wrote:

On Fri, 30 Mar 2007 20:35:57 -0400, clare at snyder.on.ca wrote:

Hi Clare,

You didn't read the rest of the post - the charge controller only
needs to cost about $10 if you buy all the parts new. A good scavenger
could likely do it for $2.(or free)

That is beyond my electronic capabilities to design much. I am
comfortable at building kits, fault finding etc and I have designed
some simple DC motor controls at times using Gottlieb's books. I did a
search for charge controllers and came up pretty much empty in the 12v
to 36v range..

Would you care to provide a schematic that will work? Maybe a website
or two where I can get started?

I don't want to get between you and Don on the argument of series v
parallel, charge/discharge, way out of my depth. But I do like the
idea of a single 36v controller instead of the 3 HF units. I was
planning on the HF units as the most simple solution I could handle.
But a 10-buck 12-36 charger sounds good and I do already have the 5W
panel.

Thanks
Dead simple. Decide what your charging voltage should be. I'd say 42
volts. Get a heavy duty 42 volt Zener diode (capable of handling the
maximum output of the solar cells). Get a silicon diode , 50 volt PIV
and heavier than the maximum output of the solar cells. Get a power
resistor that will handle the charging current. One and a half amp
panels? That means you need to pass 1.5 amps with negligible voltage
drop. E=IxR, so 1.5*.7 ohms = 1.07 volt drop. when charging. Should be
doable. Now, what happens when the Zener goes into regulation mode,
and clamps the output to ground. assume, for worst case scenario, that
the solar cells are a "hard" supply (they are not) at 42 volts. .7
ohms and 42 volts - i-e/r, or 42/.7= 60 watts. A bit heavy - 60 watt
Zeners and power resistors are pricy, so back to the drawing board and
try a 1.5 ohm resistor.1.5x1.5= 2.25 volts. Should be able to handle
2.25 volt drop at full load, the voltage drop will diminish as the
charging current drops. 42/1.5= 28. A 30 watt 1.5 ohm power resistor,
or something reasonably close, should be fairly easy to find in a
wire-wound ceramic surplus, and a 25 watt (2 amp +/=)zener should also
be do-able. Now, connect the - of the solar panel string to the -
battery terminal of the 36 volt string, and connect the resistor to
the + of the solar panel string. Connect the Zener from the other end
of the resistor to ground so that when the voltage gets over 42 volts
it pulls it to ground. Connect the silicon diode from that same
connection to the + of the battery string.
The silicone diode prevents the battery from discharging into the
solar cells or shorting to ground through the Zener.
The Zener prevents the voltage from excedint 42 volts by clamping the
output to ground, dissipating the excess power in the zener and the
resistor.
In the real world, with the solar panels not being a 'hard" supply
(having significant internal resistance themselves) a 15 watt resistor
and zener will likely be more than adequate.
In the real world, 42 volt Zeners are not a standard - 43 volts
are,The charging voltage will actually be about 42.7, more or less,
and because of the forward drop of the isolation diode,using 2 diodes
in series would give you about 42 volts The old ECG number for the
Zener would be ECG5206a for a stud-mount 10 watt, or ECG5272a for a
50 watt.
A 43 volt 3 amp zener would be a 1n5260 series - 10 for $1.15 from
DigiKey

The isolating diodes would need to be more than 2 amp units to be
safe. Mount on a heat sink to be safe..The isolation diode can be any
ecg5800 series (1N5400 series)


The resistor, in the real world, could be anything from about 0.7 - 2
ohms and 10-25 watts.

Ebay has Item number: 120103830342, 20 3 watt 2.2 ohm resistors for
$5. 2 in parallel gives you 1.1 ohm at 6 watts. 3 in parallel is .7
ohms at 9 watts. - just for an example.
Or Item number: 120103830367, 20 4.7 ohm 3 watts for $5. 4 in
parallel is 1.17 ohms and 12 watts. 5 in parallel is 0.94 ohms and 15
watts.

Looking a bit farther, Item number: 110110012402 is 20 of 1 ohm, 25
watt. Mix and match to your hearts content- you can have half ohm, 1
ohm, 1.5 ohm, 2 ohm, 2.5 ohm, whatever you need with lots of power
capacity to spare. Likely get them for $5. (and you don';t need to go
to e-bay and buy large lots if you have a local surplus depot or even
electronics supply house close by).

Remember, if you find surplus parts that are way over capacity, but
cheap - use them!!!

I've seen 150 watt resistors, 1.5 ohm, for a buck. I've seen 300 amp
600 volt diodes for a buck. The only part you might NEED to buy new is
the Zener.

Wow.

A zener doesn't clamp to ground when it starts conducting. It just
draws current. In conjuction with series resistance, it can limit
voltage. It acts very much like a ...uh ... battery in series with a
diode.

The solar panels themselves can't deliver more than safe trickle
charge current, so a zener not necessary here and serves no useful
purpose.

How to charge a battery with a 5-watt solar panel having sufficient
voltage for the battery in question:
1. Connect solar panel to battery + to + and - to -.
2. Declare victory

Solar panels are themselves diodes, though a series diode probably
would reduce leakage so it isn't a bad idea -- although it does cost a
bit in efficiency.

If you do insist on a zener, you can make any voltage you want with an
LM336 (96 cents) and a couple of 3-cent resistors. Power zener:
LM336, a jellybean PNP transistor (2N4403, 13 cents) and a TIP3055
($1.76) that can dissipate up to 90 watts.






.

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