On Sun, 09 Dec 2012 12:30:42 -0500, in sci.electronics.repair you wrote:

On Sun, 9 Dec 2012 01:03:49 -0000, in sci.electronics.repair you wrote:

SNIP

The very simple answer is the LM317T. Just Google "LM317T as a constant
current source." Just needs one resistor to set the required current. I
have been using these adjustable voltage regulators as constant current
sources for probably 30 years without issue. Other more powerful three
terminal adjustable voltage regulators can be used in exactly the same way
if you need more current.

Arfa

Hi Arfa,

Yes, you are correct, it is simple! I LOVE IT!!!

I will look though my collection of semis for it (no ECG/NTE sub for it).
If unfound, I will order it or it's equivalent.

Thank You Very Much, John

UPDATE:

I don't have one (LM317T), but www.parts-express.com has them for less
than $3.00 each !!! I ordered two.

Again Thanks, John

On 12/10/2012 9:29 AM, wrote:
On Sun, 09 Dec 2012 12:30:42 -0500, in sci.electronics.repair you wrote:

On Sun, 9 Dec 2012 01:03:49 -0000, in sci.electronics.repair you wrote:

SNIP

The very simple answer is the LM317T. Just Google "LM317T as a constant
current source." Just needs one resistor to set the required current. I
have been using these adjustable voltage regulators as constant current
sources for probably 30 years without issue. Other more powerful three
terminal adjustable voltage regulators can be used in exactly the same way
if you need more current.

Arfa

Hi Arfa,

Yes, you are correct, it is simple! I LOVE IT!!!

I will look though my collection of semis for it (no ECG/NTE sub for it).
If unfound, I will order it or it's equivalent.

Thank You Very Much, John

UPDATE:

I don't have one (LM317T), but www.parts-express.com has them for less
than $3.00 each !!! I ordered two.

Again Thanks, John


As you're reading the datasheet to verify that the voltage of your fully
charged battery plus the 1.2V across the resistor plus the minimum in/out
overhead of the chip is less than 24V and calculating the heat sinking
requirements for a fully discharged battery, you'll find many sample
circuits.
Remember that those reference circuits are to demonstrate ONE particular
characteristic of the device, not a complete design.
In particular, pay close attention to the section on protection diodes.
Depending on the characteristics of your 24V source, when you plug on a
battery before turning on the supply, you just might short the chip.
Then, when you come back later, your battery will be cooked...or worse.
Or whatever undisclosed stuff may or may not be between the charger and the
battery may take care of that...or not...

The simplest solution for protecting the chip is a series diode,
but that shaves another 0.7V off your
already marginal headroom. You might be stuck with the reverse diode
from input to output. Just make sure it can handle the peak energy
when you charge the 24V output cap from the battery.

One of my internet pet peeves is that smart, experienced, well-meaning
people rattle off solutions for unspecified problems. They don't ask
for or care the details of your particular application. And you've
disclosed none....unless I missed that post.

I've used the LM317 as a current source. But, I've characterized the
source and load to verify that it works under all conditions.
And the result has never been as simple as a lm317 and a resistor.

I consider myself to be smart and experienced...but I've blown up my
share of batteries based on assumptions that turned out to be false.
I once blew up a $2000 laptop by putting the specified voltage on the
charger input.
Was a bonehead computer design that nobody could have anticipated.
Whodathunk that the charge current regulation was in the wall-wart.
Luckily I was able to scrape the burned spot off the board and
replace the FET.

And there are a bunch of other issues related to battery charging
that haven't been discussed.

It's rarely as simple as it looks.
Good luck...

"You might be stuck with the reverse diode
from input to output. Just make sure it can handle the peak energy
when you charge the 24V output cap from the battery"

I wouldn't even attempt to run the circuit without it. a series diode isn't the greatest idea unless you want to use a Schottky. With only 5 volts headroom a regular diode might result in the battery never getting a full charge.

"I once blew up a $2000 laptop by putting the specified voltage on the
charger input.
Was a bonehead computer design that nobody could have anticipated.
Whodathunk that the charge current regulation was in the wall-wart. "

Oh, my life has been repleat with "whodathunkits". What really gets me about that particular deal is then obviously it wouldn't be able to operate off the wallwart and charge at the same time, would it ?

"And you've
disclosed none....unless I missed that post. "

I doubt you missed it :-) However it is his job to find out of the LM317 can handle the current, and if not then maybe use a pass transistor. In either case, the heatsinking is also his problem. If he wants an amp and sticks a battery that's very discharged on it, a clipon is not going to cut it. We don't even know what type of battery.

Which brings us to those "other issues". I am not even sure yet how a Li-ion battery should be charged. I know some batteries now can be charged faster than you drain them, but we still do not know the particulars here.

Actually after this last power outage I am considering doing something similar with a cordless phone. In fact I'm surprised manufaturers haven't done it. (that I know of) The wallwart is usually 12 volts, so I intend to stack up 12 volts worth of nicads and use a diode and resistor to charge them continuously when there is power. The diode will be reverse biased when there is power and the battery will charge until it floats. Then when the power goes out, the battery wil feed the power to the base to run the phone, and maybe even charge the handset.

Was strange, when the power was out here the landline still worked but the cellphones couldn't get a signal. So if we wanted to make a call we were tethered to the wall. Amazing how unfamiliar that was, after all these years, when it was the norm a while back.

Anyway, this phone idea with the batteries might be useful. I might even be able to sell it. As long as the wallwart puts out DC, it can be done without opening the base. In those cases where the wallwart puts out AC you're screwed.

Next I want a buck convertor to run my laptop off 12 volts, as it takes 19 volts. Should be fairly easy. I am lazy but I will get around to it.