Re: Getting 36v from a 12v battery

On Sun, 01 Apr 2007 20:36:37 -0400, clare at snyder.on.ca wrote:

On Sun, 01 Apr 2007 17:38:29 -0500, Don Foreman
<dforeman@xxxxxxxxxxxxxxxxxxxx> wrote:


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.



Yup - you can do it that way too. Just a bit more complex to describe
and do.
Like I've said before, I'm no engineer, and I often design / build
things a bit over robust - but they generally work. My notation that
the solar panels are not a "hard" supply, and therefore less current
capacity would be required for both the Zener and the resistor than
the calculations indicated. How much less? I couldn't say, but likely
significantly. My design woudn't burn out anyway!!

A photovoltaic array can't deliver much more current to a dead short
than it can at about 80% of open-circuit voltage. If it is clamped by
a zener (or zener-like circuit) that can handle the panel's power
output then nothing will burn out.

I've had a small solar panel charge an 8 ah battery at significantly
more than the specified float voltage, and it would shorten the
battery life somewhat.

That surprises me, but I don't doubt your word that you saw it happen.
Given that, use of a zener or equivalent would certainly be prudent.
Automotive and 80 or 105 ah deepcycle batteries can easily handle an
amp of trickle charge, but an 8 ah battery might not like much over
100 mA. 5 watts at 13.6 volts (or 15 watts at 40.8 volts) is 0.368
amps which certainly wouldn't harm automotive-sized batteries but may
well be too much for "float" on little 8 ah batteries. 13.6 volts
is a safe float voltage for lead-acid batteries regardless of size.
For 3 batteries in series that'd be 40.8 volts.

1N5259 is a 500mW 39-volt zener for 36 cents. That and the Vbe drop
of an NPN power darlington transistor would make a "power zener" quite
close to 40.8 volts. The transistor might be a TIP120 (5 amps, 60
volts,65 watts) , 74 cents. The base-ground resistor might be 180
ohms 1/8 watt or whatever's handy. Mount the transistor on a chunk of
sheetmetal to serve as a heatsink. It wouldn't take much heatsink to
make this a 15-watt zener equivalent -- no more than an actual
15-watt zener would require. The actual zener here is just a voltage
reference, the transistor does the heavy lifting.

--------
| |
Z |
| |/C
|------|
\ |\E
/ |
\ |
/ |
---------

The regulation of this circuit isn't quite as "sharp" as an actual
power zener, but it is way lotsa plenty good enough for this
application. At 40.6 volts it "leaks" about 11 mA; at 40.8 volts it
shunts about 94 mA, and if shunting the entire 368mA output of the
three series-connected panels it clamps to 41.0 volts (13.67 volts per
battery). If ya wanted to raise the voltage a touch you could stick
a 1N4004 diode (5 cents) in series with the emitter. That'd raise
the voltages for given shunt currents about 0.75 volts or about 0.25
volts per battery.

I've never used a blocking, or isolation diode
on an unregulated solar panel, but using the zener regulator it is a
good idea.

To your point, Clare:

"Shunt Regulator - Shunt regulators are common in photovoltaic (PV)
systems since they are relatively cheap to build and simple to design.
The charging current is controlled by a switch or transistor connected
in parallel with the photovoltaic panel and the storage battery.
Overcharging of the battery is prevented by shorting (shunting) the PV
output through the transistor when the voltage reaches a predetermined
limit. If the battery voltage exceeds the PV supply voltage the shunt
will also protect the PV panel from damage due to reverse voltage by
discharging the battery through the shunt. Series regulators usually
have better control and charge characteristics."

Source: http://www.mpoweruk.com/chargers.htm


As for "clamping to ground" that is ESSENTIALY what it
does. At over the rated current it turns on, and on a "hard" supply,
if there is no resistance in the line, it shorts rather spectacularly.

It clamps to zener voltage (not ground) but it would indeed soon fail
if driven much above zener voltage by a stiff source. My point is
that a solar panel is far from being a stiff source.

Without the resistor on a small solar panel, it would just cause the
panel to heat - which generally is not a good thing. Adding a series
resistance for regulation helps prevent the "magic smoke" from getting
out. As you say, perhaps not essential - but I'd do it "just to be
safe"

The panel wouldn't self-heat any more than when it's delivering rated
power to a load. The panel is generating electric power, not
dissipating it. The zener (or equivalent) would dissipate available
power in excess of the battery's ability to accept it at float
voltage. This would be at most 15 watts with three 5-watt panels,
considerably less with a battery connected and accepting charge.

Perhaps the confusion comes from the power rating. If you draw more
than rated power from a bench supply or transformer, or deliver more
than rated to a transistor or resistor (or zener) it'll overheat
because the power rating is based on its permissible temperature
rise. A solar panel's power rating is what it can deliver to an
optimally-matched load in bright sunlight. The only way to get it to
deliver more is to turn up the sun.
.