School project, dual power source question

[JJ Q Student]

Hi folks,

First time posting here, after lurking for a couple of weeks. I have a
question concerning a project I am working on for school.

I am building a software-based digital clock, using a Texas
Instruments MSP430 uC, a 32768 crystal, an LCD panel, a piezo buzzer,
and a rotary encoder for input, and any necessary support components.
Now the plea for help:

I won't be fooling with AC or transformers at all. I want to power the
circuit from a 9 VDC regulated wall wart, as well as a 9 Volt
rechargable battery. The idea is that the wall wart will provide
primary power, and the battery will be there to keep the clock ticking
if primary voltage is removed. The incoming voltage will then be
regulated to 5 Volts for the LCD, and 3.3 Volts for the micro and
buzzer. I know it's a little redundant to regulate a regulated power
supply, but where would I get a 5 Volt battery other than combining
AA's or such? For that reason I chose 9 Volts, because of availability
and size of a standard 9V battery.

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart? Won't the 9V wall
power serve to keep the battery healthy, as well as recharge it if
used, after primary power returns? Is any switching circuitry
necessary to isolate the supplies from eachother? If you folks feel
that the voltage should be altered from 9V, that's fine- I'm still
designing things.

Thanks for the help- I truly appreciate all replies!
JJ

 

[Tim Wescott]

"JJ Q Student" <not@real.com> wrote in message
news:g5sc50130n80b5ovtdo7llpneuvsqvu1lq@4ax.com...

Hi folks,

First time posting here, after lurking for a couple of weeks. I have a
question concerning a project I am working on for school.

I am building a software-based digital clock, using a Texas
Instruments MSP430 uC, a 32768 crystal, an LCD panel, a piezo buzzer,
and a rotary encoder for input, and any necessary support components.
Now the plea for help:

I won't be fooling with AC or transformers at all. I want to power the
circuit from a 9 VDC regulated wall wart, as well as a 9 Volt
rechargable battery. The idea is that the wall wart will provide
primary power, and the battery will be there to keep the clock ticking
if primary voltage is removed. The incoming voltage will then be
regulated to 5 Volts for the LCD, and 3.3 Volts for the micro and
buzzer. I know it's a little redundant to regulate a regulated power
supply, but where would I get a 5 Volt battery other than combining
AA's or such? For that reason I chose 9 Volts, because of availability
and size of a standard 9V battery.

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart? Won't the 9V wall
power serve to keep the battery healthy, as well as recharge it if
used, after primary power returns? Is any switching circuitry
necessary to isolate the supplies from eachother? If you folks feel
that the voltage should be altered from 9V, that's fine- I'm still
designing things.

Thanks for the help- I truly appreciate all replies!
JJ

Actually most wall warts aren't well regulated, so re-regulating to 5V isn't
a bad thing.

You should put in blocking diodes so the battery won't feed out the
wall-wart socket when the wall-wart isn't connected. You should also plan
or feeding the 9V battery with about 1/10th it's amp-hour rating in current
(so around 5-10mA). NiCd and NiMH batteries don't really like to be charged
forever without a chance to discharge; you might get better battery life by
using a dry cell. If you _really_ want to get ambitious get a "supercap" --
but you want to get what you have now working _first_.

 

[Ken Taylor]

"JJ Q Student" <not@real.com> wrote in message
news:g5sc50130n80b5ovtdo7llpneuvsqvu1lq@4ax.com...

Hi folks,

First time posting here, after lurking for a couple of weeks. I have a
question concerning a project I am working on for school.

I am building a software-based digital clock, using a Texas
Instruments MSP430 uC, a 32768 crystal, an LCD panel, a piezo buzzer,
and a rotary encoder for input, and any necessary support components.
Now the plea for help:

I won't be fooling with AC or transformers at all. I want to power the
circuit from a 9 VDC regulated wall wart, as well as a 9 Volt
rechargable battery. The idea is that the wall wart will provide
primary power, and the battery will be there to keep the clock ticking
if primary voltage is removed. The incoming voltage will then be
regulated to 5 Volts for the LCD, and 3.3 Volts for the micro and
buzzer. I know it's a little redundant to regulate a regulated power
supply, but where would I get a 5 Volt battery other than combining
AA's or such? For that reason I chose 9 Volts, because of availability
and size of a standard 9V battery.

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart? Won't the 9V wall
power serve to keep the battery healthy, as well as recharge it if
used, after primary power returns? Is any switching circuitry
necessary to isolate the supplies from eachother? If you folks feel
that the voltage should be altered from 9V, that's fine- I'm still
designing things.

Thanks for the help- I truly appreciate all replies!
JJ

Take a look at National Semi's site, specifically;
http://www.national.com/pf/LM/LM2574.html
or more generally
http://www.national.com/parametric/0,1850,1758,00.html if you need more
power or some options.

They have very good application notes which should help. A switching
regulator will be much more efficient than a linear, giving you better
battery life.

Cheers.

Ken

 

[JJ Q Student]

Take a look at National Semi's site, specifically;
http://www.national.com/pf/LM/LM2574.html
or more generally
http://www.national.com/parametric/0,1850,1758,00.html if you need more
power or some options.

They have very good application notes which should help. A switching
regulator will be much more efficient than a linear, giving you better
battery life.

Cheers.

Ken

Are switchers such a hot idea for microcontroller circuits? Don't they

introduce alot of noise, which would upset a clock?

Thanks for the responses so far!
JJ

 

[John Woodgate]

I read in sci.electronics.design that JJ Q Student <not@real.com> wrote
(in <g5sc50130n80b5ovtdo7llpneuvsqvu1lq@4ax.com&gt about 'School project,
dual power source question', on Tue, 16 Mar 2004:

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart?

Not usually.

Won't the 9V wall
power serve to keep the battery healthy, as well as recharge it if
used, after primary power returns?

What sort of rechargeable battery are you planning to use? Get the
manufacturer's data sheet and see how it has to be charged. It varies A
LOT between different types and sizes of battery. But you can't charge a
9 V battery from a 9 V regulated supply.

Is any switching circuitry necessary
to isolate the supplies from eachother?

It's usual to use a steering diode to prevent the two power sources from
'seeing' each other. You can implement a charging circuit as a parallel
path to the steering diode.

Use Courier font:


Wall wart----+---------------+----- to circuit
Charging |
circuit |
Battery -----+---|>|---------+
Diode

Common ground--------------------- to circuit

If you folks feel that the
voltage should be altered from 9V, that's fine- I'm still designing
things.

9 V is quite OK for the wall-wart. What voltage does the circuitry that
needs to be kept going during a power-outage use? If it runs from 3.3.
V, you might consider a 3.3 V lithium battery rather than a 9 V whatever
battery.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[JJ Q Student]

On Tue, 16 Mar 2004 09:00:01 +0000, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that JJ Q Student <not@real.com> wrote
(in <g5sc50130n80b5ovtdo7llpneuvsqvu1lq@4ax.com&gt about 'School project,
dual power source question', on Tue, 16 Mar 2004:

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart?

Not usually.

Won't the 9V wall
power serve to keep the battery healthy, as well as recharge it if
used, after primary power returns?

What sort of rechargeable battery are you planning to use? Get the
manufacturer's data sheet and see how it has to be charged. It varies A
LOT between different types and sizes of battery. But you can't charge a
9 V battery from a 9 V regulated supply.

Is any switching circuitry necessary
to isolate the supplies from eachother?

It's usual to use a steering diode to prevent the two power sources from
'seeing' each other. You can implement a charging circuit as a parallel
path to the steering diode.

Use Courier font:


Wall wart----+---------------+----- to circuit
Charging |
circuit |
Battery -----+---|>|---------+
Diode

Common ground--------------------- to circuit

If you folks feel that the
voltage should be altered from 9V, that's fine- I'm still designing
things.

9 V is quite OK for the wall-wart. What voltage does the circuitry that
needs to be kept going during a power-outage use? If it runs from 3.3.
V, you might consider a 3.3 V lithium battery rather than a 9 V whatever
battery.

Thanks for the response! I'm a little confused on the ASCII schematic
above, though. How involved is a charging circuit? I'm trying to use a
standard, every day store-bought rechargable 9 Volt battery. I don't
want to be bogged down with the whole charging circuit, so maybe I'll
just use an alkaline 9V. Also, I've seen some dual power source
steering IC's from TI. Perhaps one of those is best.

If it appears that I'm looking for the easy way out, it's kinda true.
I've sorta bit off more than I can chew- I'm only a second year EET
student at a local community college. Most students aren't designing
their own circuits, but using app notes for their projects. The course
is "Design & Layout", and the focus is to teach us what goes into
producing a product, and laying out a PCB. I wanted to do more,
though, so I'm doing this whole micro based clock. Things aren't
looking good at the moment, so I'm trying to abide by "KISS".

Again, thanks for the help!
JJ

 

[John Woodgate]

I read in sci.electronics.design that JJ Q Student <not@real.com> wrote
(in <8tae50lgajhh16s4cfeodkpt31uasmg6tb@4ax.com&gt about 'School project,
dual power source question', on Tue, 16 Mar 2004:

How involved is a charging circuit? I'm trying to use a
standard, every day store-bought rechargable 9 Volt battery.

If it's a NiCd battery, then the charging circuit need be no more than a
resistor. But remember that the on-charge voltage of such a battery is
about 10.5 V, so you can't charge it from a 9 V regulated wall-wart.

The resistor should be chosen to limit the current into a fully-
discharged battery (7 V for a 7-cell '9 V' battery) to 11 mA. So, if you
had a 12 V supply, you would calculate 12 - 7 = 5 V. 5 V drop with 11 mA
is a resistance of 5000/11 = 455 ohms - use 430 ohms or 470 ohms as
these are standard values. 1% tolerance resistors are so cheap that
there is no point in considering anything else. Use V^2/R to calculate
the power dissipated and choose a resistor rated for more than that
power.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Ken Taylor]

"JJ Q Student" <not@real.com> wrote in message
news:nced50hk53014ctru10rmcld2tj1enm908@4ax.com...

Take a look at National Semi's site, specifically;
http://www.national.com/pf/LM/LM2574.html
or more generally
http://www.national.com/parametric/0,1850,1758,00.html if you need more
power or some options.

They have very good application notes which should help. A switching
regulator will be much more efficient than a linear, giving you better
battery life.

Cheers.

Ken

Are switchers such a hot idea for microcontroller circuits? Don't they
introduce alot of noise, which would upset a clock?

Thanks for the responses so far!
JJ

Hmm, well it doesn't upset the clock in PC's, so your call. Follow the App.
Notes and you shouldn't have any problems.

Cheers.

Ken

 

[soundman]

"JJ Q Student" <not@real.com> wrote in message
news:8tae50lgajhh16s4cfeodkpt31uasmg6tb@4ax.com...

On Tue, 16 Mar 2004 09:00:01 +0000, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that JJ Q Student <not@real.com> wrote
(in <g5sc50130n80b5ovtdo7llpneuvsqvu1lq@4ax.com&gt about 'School project,
dual power source question', on Tue, 16 Mar 2004:

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart?

Not usually.

Won't the 9V wall
power serve to keep the battery healthy, as well as recharge it if
used, after primary power returns?

What sort of rechargeable battery are you planning to use? Get the
manufacturer's data sheet and see how it has to be charged. It varies A
LOT between different types and sizes of battery. But you can't charge a
9 V battery from a 9 V regulated supply.

Is any switching circuitry necessary
to isolate the supplies from eachother?

It's usual to use a steering diode to prevent the two power sources from
'seeing' each other. You can implement a charging circuit as a parallel
path to the steering diode.

Use Courier font:


Wall wart----+---------------+----- to circuit
Charging |
circuit |
Battery -----+---|>|---------+
Diode

Common ground--------------------- to circuit

If you folks feel that the
voltage should be altered from 9V, that's fine- I'm still designing
things.

9 V is quite OK for the wall-wart. What voltage does the circuitry that
needs to be kept going during a power-outage use? If it runs from 3.3.
V, you might consider a 3.3 V lithium battery rather than a 9 V whatever
battery.

Thanks for the response! I'm a little confused on the ASCII schematic
above, though. How involved is a charging circuit? I'm trying to use a
standard, every day store-bought rechargable 9 Volt battery. I don't
want to be bogged down with the whole charging circuit, so maybe I'll
just use an alkaline 9V. Also, I've seen some dual power source
steering IC's from TI. Perhaps one of those is best.

If it appears that I'm looking for the easy way out, it's kinda true.
I've sorta bit off more than I can chew- I'm only a second year EET
student at a local community college. Most students aren't designing
their own circuits, but using app notes for their projects. The course
is "Design & Layout", and the focus is to teach us what goes into
producing a product, and laying out a PCB. I wanted to do more,
though, so I'm doing this whole micro based clock. Things aren't
looking good at the moment, so I'm trying to abide by "KISS".

Again, thanks for the help!
JJ

If simplicity is the key, why not use an alkaline battery and a 9VDC wall
wart. Put a diode in to each of the supply lines so that only the highest
voltage supply will pass current. A 9VDC unregulated PSU will deliver in
excess of 12V when lightly loaded, so under normal conditions, the mains PSU
will do the work. When the mains fails, the battery will take over. Your
circuit sounds like it will draw a miniscule amount of current, so the
alkaline battery should last for a period in the order of weeks of
continuous use. If the battery is used as a standby, it is likely that the
battery will die of old age rather than enregy usage.

Take the combined power feed into a 7805 voltage regulator and the output
will be your 5 volt supply for the logic circuit. Another reply mentions a
switching power supply for greater efficiency. Although I would agree, in
this case, the current consumption of perhaps 10mA is too low to benefit
from a switching supply. The 7805 regulators are quick, simple and cheap to
use.

Good luck

Peter

 

[N. Thornton]

"soundman" <ph@nowhere.com> wrote in message news:<405b04d2$0$201$812600b3@news.nntpaccess.com>...

I read in sci.electronics.design that JJ Q Student <not@real.com> wrote

Are there any special concerns for implementing two power sources, or
can I just parallel the battery and the wall wart?

you need a diode to prevent curent rushing into the battery
uncontrolled and blowing it up.

Wall wart----+---------------+----- to circuit
Resistor |
| |
Battery -----+---|>|---------+
Diode

Use a resistor if youre using a rechargeable battery. For ordinary non
rechargeables leave out the R, dont try and charge it.

just use an alkaline 9V. Also, I've seen some dual power source
steering IC's from TI. Perhaps one of those is best.

no no

If simplicity is the key, why not use an alkaline battery and a 9VDC wall
wart. Put a diode in to each of the supply lines so that only the highest
voltage supply will pass current. A 9VDC unregulated PSU will deliver in
excess of 12V when lightly loaded, so under normal conditions, the mains PSU
will do the work. When the mains fails, the battery will take over. Your
circuit sounds like it will draw a miniscule amount of current, so the
alkaline battery should last for a period in the order of weeks of
continuous use. If the battery is used as a standby, it is likely that the
battery will die of old age rather than enregy usage.

Take the combined power feed into a 7805 voltage regulator and the output
will be your 5 volt supply for the logic circuit. Another reply mentions a
switching power supply for greater efficiency. Although I would agree, in
this case, the current consumption of perhaps 10mA is too low to benefit
from a switching supply. The 7805 regulators are quick, simple and cheap to
use.

This is the reply to follow.


Regards, NT