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Voltage dropper

Hi Folks
I'm trying to run a tiny 6 volt radio for as long as possible off a 12
volt car battery. I came across a 7806 but was disappointed to find
that it is nominally only 50% efficient. The heat dissipation (voltage
drop times load current) being much the same as the load (out-voltage
times load current)

I read some primer on dc-dc converters that claimed that synchronous
rectification could get to 95% efficiency. Is there some way to
achieve a reasonable efficiency here? I even thought about centre
tapping the 12 volt battery and using the two 6V halves in parallel.
I suspect I might need a diode in this circuit and that would not bode
well for the hopefully intermittent charging from a small solar panel
in Winter. I will also be running a 12V, 1 Watt LED from this battery
at night when the Sun definitely don't shine.

I'm worried that if there is a weeklong rain period with no charging,
I may flatten the battery too far.

Perhaps two or three car batteries in parallel might be a better bet,
but the ones I have are all different brands, sizes and ages, and
would definitely self-destruct by trying to charge the weaker ones
without diodes, but how do you rig up diodes to allow charging and
draining?

Thanks for any ideas or suggestions, jack
 
H

Homer J Simpson

Jan 1, 1970
0
I'm trying to run a tiny 6 volt radio for as long as possible off a 12
volt car battery.

Two choices. Find a 12 volt radio or use a switching supply to step the
voltage down. The latter may generate noise if not well designed.
 
D

default

Jan 1, 1970
0
Perhaps two or three car batteries in parallel might be a better bet,
but the ones I have are all different brands, sizes and ages, and
would definitely self-destruct by trying to charge the weaker ones
without diodes, but how do you rig up diodes to allow charging and
draining?

Thanks for any ideas or suggestions, jack

Some "bucking" switching supply for the high efficiency voltage drop.
National Semiconductor has a line that is easy to implement and work
well called "simple switchers." They are on line.

You need a pair of diodes for each battery you want to isolate.

From the charger plus lead wire connect the anode of the diode (arrow
on a schematic). Connect the cathode or banded end of the diode to
the battery. Do this for each battery the cathodes will go to a
battery plus terminal and all the anodes will connect together and to
the charger.

The same idea works to isolate them for discharge. One diode anode
connects from the Plus lead on each battery and goes off to the load
and all the cathodes or banded ends go together to the common load.

All batteries must have the same nominal voltage - don't put a 6 V in
a bank of 12 V ones.

The batteries should all have pretty near the same voltage during
charge and discharge. The downside is that one shorted cell in one
battery will prevent the entire bank from charging, by dragging down
the charge voltage for the rest. A shorted cell won't matter to the
load side. The low/shorted battery just won't support the load
current and won't contribute any power.

Each diode has to be rated for the maximum current it will carry and
the maximum current could be the entire output of the solar panel or
charging source or the entire current of the load - since there's no
telling how hard each battery is going to be working in your
application or the battery condition.

Fuses may be a good idea, particularly on the load side - solar panels
don't usually output much current so the charge diodes shouldn't be at
risk.- but you still need diodes - they also prevent the panel from
draining the battery when it is dark.
 
J

Jamie

Jan 1, 1970
0
Hi Folks
I'm trying to run a tiny 6 volt radio for as long as possible off a 12
volt car battery. I came across a 7806 but was disappointed to find
that it is nominally only 50% efficient. The heat dissipation (voltage
drop times load current) being much the same as the load (out-voltage
times load current)

I read some primer on dc-dc converters that claimed that synchronous
rectification could get to 95% efficiency. Is there some way to
achieve a reasonable efficiency here? I even thought about centre
tapping the 12 volt battery and using the two 6V halves in parallel.
I suspect I might need a diode in this circuit and that would not bode
well for the hopefully intermittent charging from a small solar panel
in Winter. I will also be running a 12V, 1 Watt LED from this battery
at night when the Sun definitely don't shine.

I'm worried that if there is a weeklong rain period with no charging,
I may flatten the battery too far.

Perhaps two or three car batteries in parallel might be a better bet,
but the ones I have are all different brands, sizes and ages, and
would definitely self-destruct by trying to charge the weaker ones
without diodes, but how do you rig up diodes to allow charging and
draining?

Thanks for any ideas or suggestions, jack
you can use one of those DC-DC converters how ever, i think you might
find it more interesting if you were to make one of your own.
using a Cmos version of the 555 timer which i think is the 7555 ?
,
these use power, and then you can use this as a PWM (pulse width
modulator) at high freq driving transistor into saturation mode into
an small inductive coil that has a cap on the end to filter all of this.
Then build your self a little regular to alter the charging cycle
of the 555 timer..
I know this all sounds like a lot how ever, you can actually get good
efficiency doing this.
I've do this before when looking around for a high efficient regulator
just for the same reason you are..
 
Some "bucking" switching supply for the high efficiency voltage drop.
National Semiconductor has a line that is easy to implement and work
well called "simple switchers." They are on line.

Thanks so much for your help. I've found these and they look great.
I'm wondering if there is a commercial product that is "plug-n-play"
as I'm a total electronics novice. The cigarette lighter various
voltage adaptors are likely to be inefficient as energy is not short
on a car.
You need a pair of diodes for each battery you want to isolate.

From the charger plus lead wire connect the anode of the diode (arrow
on a schematic). Connect the cathode or banded end of the diode to
the battery. Do this for each battery the cathodes will go to a
battery plus terminal and all the anodes will connect together and to
the charger.

The same idea works to isolate them for discharge. One diode anode
connects from the Plus lead on each battery and goes off to the load
and all the cathodes or banded ends go together to the common load.

All batteries must have the same nominal voltage - don't put a 6 V in
a bank of 12 V ones.

This was understood, but I've got several car batteries that differ
significantly in their charged voltage. There is one little battery
that is a real performer. It is quite old, and I've abused it by
would-you-believe reversing a car up a hill with the starter motor
only. Because of it's chemistry, I asume, it consistently charges to a
higher voltage than all the others including a brand new one.
The batteries should all have pretty near the same voltage during
charge and discharge. The downside is that one shorted cell in one
battery will prevent the entire bank from charging, by dragging down
the charge voltage for the rest. A shorted cell won't matter to the
load side. The low/shorted battery just won't support the load
current and won't contribute any power.

Each diode has to be rated for the maximum current it will carry and
the maximum current could be the entire output of the solar panel or
charging source or the entire current of the load - since there's no
telling how hard each battery is going to be working in your
application or the battery condition.

Fuses may be a good idea, particularly on the load side - solar panels
don't usually output much current so the charge diodes shouldn't be at
risk.- but you still need diodes - they also prevent the panel from
draining the battery when it is dark.

I'm leaning towards a large single 12V UPS battery.
All these diodes are going to sap some energy I assume.

My limiting factor is my solar panel which is rated at 20Watts
(nominal). I have a proper controller for this which includes a diode
for preventing reverse current flow in the dark. It also switches on a
load when the panel charge voltage drops below 3.5V and switches off
in the morning when the PV voltage reaches 5V.

I want the radio on 24/7, so the smallest drain possible will be
needed if the battery is not to go flat if the Sun don't shine for
several days.

Three days of 1W drain (83mA) will be 0.083 x 72 is 6Ah which at 20%
discharge would require a 30Ah battery, and that is just for the
radio. I may have a couple of W of lighting that will be switched on
during darkness. This may require perhaps a 60Ah battery.

Perhaps if I just buy the biggest wet cell marine battery that I can
find, and hopefully only discharge it less than 20%? Just thinking
aloud here. Hoping for a few comments or advice from you experts.
Thanks again for your help, jack
 
Two choices. Find a 12 volt radio or use a switching supply to step the
voltage down. The latter may generate noise if not well designed.

Thanks, Homey
Good advice.
Seems like the simplest way would be that 12V radio.

Anyone got any suggestions?

What would be the likely power requirements for the smallest of
non-hi-fi radios? Any ideas? jack
 
H

Homer J Simpson

Jan 1, 1970
0
Thanks, Homey
Good advice.
Seems like the simplest way would be that 12V radio.

Anyone got any suggestions?

What would be the likely power requirements for the smallest of
non-hi-fi radios? Any ideas? jack

A typical small radio delivers about 50 mW of power to the speaker. Double
that as a guide to power consumed.
 
E

ehsjr

Jan 1, 1970
0
Hi Folks
I'm trying to run a tiny 6 volt radio for as long as possible off a 12
volt car battery. I came across a 7806 but was disappointed to find
that it is nominally only 50% efficient. The heat dissipation (voltage
drop times load current) being much the same as the load (out-voltage
times load current)

I read some primer on dc-dc converters that claimed that synchronous
rectification could get to 95% efficiency. Is there some way to
achieve a reasonable efficiency here? I even thought about centre
tapping the 12 volt battery and using the two 6V halves in parallel.
I suspect I might need a diode in this circuit and that would not bode
well for the hopefully intermittent charging from a small solar panel
in Winter. I will also be running a 12V, 1 Watt LED from this battery
at night when the Sun definitely don't shine.

I'm worried that if there is a weeklong rain period with no charging,
I may flatten the battery too far.

Perhaps two or three car batteries in parallel might be a better bet,
but the ones I have are all different brands, sizes and ages, and
would definitely self-destruct by trying to charge the weaker ones
without diodes, but how do you rig up diodes to allow charging and
draining?

Thanks for any ideas or suggestions, jack

A radio will draw more current the louder you play it,
so looking for the lowest drain radio is a non-starter.
It needs to be lowest drain at the volume you have in
mind.

A battery lightly loaded will have more total capacity
than a battery heavily loaded. Your 30 Ah battery into
an 83 mA load will last longer than your figures. That
30 Ah rating is figured at a higher load.

A 12 volt radio will produce as much volume as a 6
volt radio at ~1/2 the current, all other conditions
being equal.

Last, but not least, you should have an automatic
shut off circuit to prevent your battery from being
over discharged.

Ed
 
D

default

Jan 1, 1970
0
I'm leaning towards a large single 12V UPS battery.
All these diodes are going to sap some energy I assume.
Sealed Lead Acid batteries are hard to beat for long service in low
current and float charge applications.
My limiting factor is my solar panel which is rated at 20Watts
(nominal). I have a proper controller for this which includes a diode
for preventing reverse current flow in the dark. It also switches on a
load when the panel charge voltage drops below 3.5V and switches off
in the morning when the PV voltage reaches 5V.

I want the radio on 24/7, so the smallest drain possible will be
needed if the battery is not to go flat if the Sun don't shine for
several days.
Small portable radios use much less than a watt of power. If you're
not too picky on fidelity - you can conserve more energy by using less
bass. It takes more energy to play music with a lot of bass compared
to voice .

9 volt transistor radio batteries were designed to power radios in the
100 milliwatt range with an average power of less than 100.

Some of the newer "class D" audio systems will be even more energy
efficient - they work similar to a switching power supply but the
output voltage is modulated with audio. Most of the radio usage to
date has been for cell phones and a few high end stereo systems.
Three days of 1W drain (83mA) will be 0.083 x 72 is 6Ah which at 20%
discharge would require a 30Ah battery, and that is just for the
radio. I may have a couple of W of lighting that will be switched on
during darkness. This may require perhaps a 60Ah battery.

Perhaps if I just buy the biggest wet cell marine battery that I can
find, and hopefully only discharge it less than 20%? Just thinking
aloud here. Hoping for a few comments or advice from you experts.
Thanks again for your help, jack

I'd use a sealed lead acid or even a pair of them - they are hard to
beat. Marine battery or telephone system battery (wet cell) are
great for keeping the cost low, but may need more maintenance.

There's a lot of resources on batteries and off-grid living on the
'net already. http://www.otherpower.com/ has some good information on
generation, conservation and storage. Scroll to the bottom of the
page for the categories.

During hurricanes I operate an aquarium pump, TV and compact
fluorescent from a pair of SLA batteries that I've had since 98 and
they were sold surplus because they were manufactured in 1990 and
hadn't been sold - intended to be used with trolling motors in boats.
Once a year I recharge the pair, and again as a hurricane approaches.
The TV runs off 12 volts the pump and light from a small inverter I
cobbled together from radio shack parts.
 
B

Bill Bowden

Jan 1, 1970
0
Hi Folks
I'm trying to run a tiny 6 volt radio for as long as possible off a 12
volt car battery. I came across a 7806 but was disappointed to find
that it is nominally only 50% efficient. The heat dissipation (voltage
drop times load current) being much the same as the load (out-voltage
times load current)

I read some primer on dc-dc converters that claimed that synchronous
rectification could get to 95% efficiency. Is there some way to
achieve a reasonable efficiency here? I even thought about centre
tapping the 12 volt battery and using the two 6V halves in parallel.
I suspect I might need a diode in this circuit and that would not bode
well for the hopefully intermittent charging from a small solar panel
in Winter. I will also be running a 12V, 1 Watt LED from this battery
at night when the Sun definitely don't shine.

I'm worried that if there is a weeklong rain period with no charging,
I may flatten the battery too far.

Perhaps two or three car batteries in parallel might be a better bet,
but the ones I have are all different brands, sizes and ages, and
would definitely self-destruct by trying to charge the weaker ones
without diodes, but how do you rig up diodes to allow charging and
draining?

Thanks for any ideas or suggestions, jack

You could try an old 12 volt car radio. I have a 1980s Ford AM radio
that only draws 80mA with the dial light disconnected. It's very
sensitive with good fidelity from a 4 inch speaker, and the push
buttons are handy for changing stations in the dark. But you will need
an external antenna and ground.

Don't use any switching power converters if you want to listen to AM
radio. They generate too much noise on weak stations.

-Bill
 
J

jasen

Jan 1, 1970
0
Thanks, Homey
Good advice.
Seems like the simplest way would be that 12V radio.

Anyone got any suggestions?

car dadios aren't real efficient,

A 9v battery radio would be easier to find (used) an probably more
efficient even if powered from a 7809,

my 3V pocket radio and run it from a buck converter could be another option.

Bye.
Jasen
 
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