In-circuit recharging?

I'm working on a project that I'd like to run off either batteries or
mains.

It'll be a small clock (2"x3"x3"). The electronics run at 5V and will
pull a max of 30mA.

My past projects I've powered through an LM7805 and either a 9V or a
bank of 8 AA's. This one will be too small for 8 AAs, and I need longer
battery life than I'll get off a 9V.

I'd thinking about using an ST619LB DC-DC regulated charge pump. This
takes an input voltage of between 1.8 and 3.6V and outputs a regulated 5V.
First, charge pumps are more efficient than voltage regulators, and
second, a pair of AAs hold a lot more mA-hours than an 9V.

That much, I'm pretty sure I know how to do.

Alternatively, I could power it from a wall-wart. With adequate caps, I
could use my familiar 7805.

What I would like to do - and what I don't know where to start - is to do
both. Run it off the wall-wart when it's plugged in, and off the AAs when
it's not. AND - have the wall-wart charge the AAs.

Electronic devices that run off mains when plugged in and off of
rechargeable batteries when not aren't at all unusual. But I've not had
any luck chasing down circuit designs on the web.

Any ideas?

--
Rightful liberty is unobstructed action according to our will within
limits drawn around us by the equal rights of others. I do not add
'within the limits of the law,' because law is often but the tyrant's
will, and always so when it violates the rights of the individual.
- Thomas Jefferson

 

Hi Jeff,
This is the first thing that comes to mind. Check it out at:
http://www.fncwired.com/PowerExample/
Let me know if you have any questions. This circuit will give you options on
the wall-warts size.
Brian

 

That looks very much like what I'm looking for. But it's not very well
labeled.

I'm assuming the rectangle on the right is the voltage regulator. And I
recognize the resisters, caps, diode and zener. But what's that stuff in
the middle?

--
Political tags - such as royalist, communist, democrat, populist,
fascist, liberal, conservative, and so forth - are never basic criteria.
The human race divides politically into those who want people to be
controlled and those who have no such desire.
- Robert Heinlein

 

Okay, I have labeled and explained everything, go back to
http://www.fncwired.com/PowerExample/ again.
Brian

 

That helps a lot. I didn't realize this was a circuit you'd drawn, rather
than one you'd found.

I've never used Zeners - I knew a charging circuit would require a current
limiter, but I'd normally handle that with an LM317. Amazing how many
different ways people find of solving the same problem.

Ditto for the idea of putting components inside the wall-wart wire.

I don't see any mechanism for shutting off the charging current when the
battery is fully charged. Is this likely to be a problem?

The project being powered by this will have a microcontroller with spare
I/O pins - I'd been musing about having it monitor the battery voltage and
shut the current off when appropriate.

If I'm reading this circuit correctly, it's assuming that the two
non-ground terminals of a stereo jack are connected when the plug is not
inserted.

I'm working on another project using 2.5mm sub-mini stereo jacks and
plugs, and have three different sub-mini jacks on hand, and this is true
for none of them.

--
When...[government] gets into difficulties it can raise money by seizing
it, in the form of taxes, from those who have earned it. So long as
such persons confine their resistance to academic protests, it will
continue well-heeled, and ready for ever new and worse extravagances.
Even when it finds, on trying to shake them down, that their pockets
are quite empty, it can still borrow on the security of their future
earning power. Legally speaking they are its slaves. It can dip into
their bank account whenever it pleases, and if those bank accounts turn
out to be too scanty for its needs, it can mortgage whatever money they
seem likely to accumulate tomorrow, or next month, or next year...It is
a millstone around their necks that grows heavier every time they try
to throw it off...The Bill of Rights gives a long list of things that
the government may not do to the citizen in his person...There is only
one provision dealing with his property: the government is forbidden to
take it without paying for it. It seems me that there is a hint here.
Why not a new Bill of Rights, definitely limiting the taxing powers of
the government? Why not...[an] Amendment restoring it to its simple
and proper functions, and forbidding it forever to collect or spend a
cent for any purpose lying outside them?
- H. L. Mencken

 

You will most likely want to be trickle charging the batteries, when the
power is plugged in (in which case you won't need to turn the charging
current off). Looking at a 3 volt wall-wart from Digi-Key (part #
MT7133-ND), it puts out 5 volts with a 30 ma. load on it. You could use a 2
volt zener diode (or 3 regular diodes in series to get the 2 volt drop).
Look on the internet to find out what trickle charge rate is needed for your
batteries. Yes, there are usually many different ways to arrive at the same
solution in electronics.

That is correct.

I copied a jack and plug from the Mouser Electronics site, to the drawing.
Go back there, to see it. If they don't make a jack and plug exactly the way
you remember it, then you improvise (as in the updated drawing).
Mechanically it is different, but electrically it is the same.
Brian

 

I'm not concerned about the drawing, or the mechanics.

I'm saying that I tried my continuity tested on the three 2.5mm sub-mini
jacks I have sitting on my desk, and none of them have the terminals
connected to each other when the plug is not in place.

 

Then none of them are the right kind for this application. You can get what
you need from Mouser http://www.mouser.com/ ,they don't have a minimum
order.
Brian

 

Can you tell me what part number you were using?

--
When...[government] gets into difficulties it can raise money by seizing
it, in the form of taxes, from those who have earned it. So long as
such persons confine their resistance to academic protests, it will
continue well-heeled, and ready for ever new and worse extravagances.
Even when it finds, on trying to shake them down, that their pockets
are quite empty, it can still borrow on the security of their future
earning power. Legally speaking they are its slaves. It can dip into
their bank account whenever it pleases, and if those bank accounts turn
out to be too scanty for its needs, it can mortgage whatever money they
seem likely to accumulate tomorrow, or next month, or next year...It is
a millstone around their necks that grows heavier every time they try
to throw it off...The Bill of Rights gives a long list of things that
the government may not do to the citizen in his person...There is only
one provision dealing with his property: the government is forbidden to
take it without paying for it. It seems me that there is a hint here.
Why not a new Bill of Rights, definitely limiting the taxing powers of
the government? Why not...[an] Amendment restoring it to its simple
and proper functions, and forbidding it forever to collect or spend a
cent for any purpose lying outside them?
- H. L. Mencken

 

The phone jack part number is 502-113F (on page 847) and the phone plug part
number is 502-60 (on page 852). The nearest I can tell, this is a match
betwen the jack and plug, but I would call and ask to make sure.
Brian

 

Thanks.

--
Honour, justice, and humanity, forbid us tamely to surrender that freedom
which we received from our gallant ancestors, and which our innocent
posterity have a right to receive from us. We cannot endure the infamy
and guilt of resigning succeeding generations to that wretchedness which
inevitably awaits them, if we basely entail hereditary bondage upon them.
- Thomas Jefferson, A Declaration setting forth the
Causes and Necessity of taking up Arms, 1775.

 

Glad to help. That's what this newsgroup is all about.

 

Here's one way - no need to do any switching
from AC power to battery power - it happens
automatically.

+----->|--------+
| D1 |
6V + | ----- |
Wallwart -+-|LM317|--+ |
----- | |
| | |
+-[R]-+ [100R]
| | -------
| | |ST619LB| Clock
+---->|----+-------|Charge |--->Circuit
+ | D2 | |Pump |
[Batt] [Zd]3.1V -------
- | | |
Wallwart ------+----------+-----------+------->


Use 2 AA NiCd. Set R for a charge rate of ~C/50.
For example, say the NiCds are 1200 mAh C/50 = 24.
Use 51 ohms for R - you'll get about 24.5 mA charge
current, and your batteries can stay on the charger
forever at that rate. Use a 6V wall wart and Shottkey
diode for D2.

The zener will keep the input at the pump to no more
than 3.1 volts. If there is no AC power, the batteries
will keep the clock running, and the zener has no effect.

Ed

 

Use four 1.25V NiCd batteries gets you 5 volts and drops to 4.8 volts
after a short discharge. but stays at 4.8 volts until they go south.
Most NiCd's you can get would be from cordless phones, as NiMh have
taking over the market.