Simple Constant Current Device or Circuit

Hi,

I searched the web for a simple constant current circuit so
I can charge a 19V rechargeable "power pack" that contains NMH cells
after they "run down" using a timer. My DC source will be 24V.

The simple circuits I found are for lighting LEDs, anything
requiring more current involves complex circuitry.

Does anyone know of a device like a simple 3 lead voltage
regulator, but for current, or a simple circuit?

Thank You in advance, John

PS, Remove "ine" from my email address if you have an image file.

 

[Pilgrim]

In article <56l6c8hu2vp1p7vrbbvkc5lm9gv4lidt37@4ax.com>,
jaugustine@verizon.net wrote:

Hi,

I searched the web for a simple constant current circuit so
I can charge a 19V rechargeable "power pack" that contains NMH cells
after they "run down" using a timer. My DC source will be 24V.

The simple circuits I found are for lighting LEDs, anything
requiring more current involves complex circuitry.

Does anyone know of a device like a simple 3 lead voltage
regulator, but for current, or a simple circuit?

Thank You in advance, John

PS, Remove "ine" from my email address if you have an image file.

Look in some of the older[20+years] application books from such as
National. The had ckts using voltage regulators as constant current
sources.

CP

 

[William Sommerwerck]

Let's look at some basic electronics theory.

A junction transistor can be modeled as having a constant-current output.
You can use this to produce a constant-current charger. It should be obvious
how, but I'll explain it.

Suppose you want a constant current of 100mA, and the transistor has a
current gain (beta) of 20. Put 5mA into the base and the output will be
100mA, /regardless of how much voltage is dropped across the load/.

Of course, the power supply has to be at least a couple of volts higher than
the battery's maximum voltage. You should be okay with 24V.

The charging current can be varied by varying the bias current.

If you don't know how to bias transistors, now is a good time to learn.

 

[mike]

On 12/8/2012 6:56 AM, jaugustine@verizon.net wrote:

Hi,

I searched the web for a simple constant current circuit so
I can charge a 19V rechargeable "power pack" that contains NMH cells
after they "run down" using a timer. My DC source will be 24V.

The simple circuits I found are for lighting LEDs, anything
requiring more current involves complex circuitry.

Does anyone know of a device like a simple 3 lead voltage
regulator, but for current, or a simple circuit?

Thank You in advance, John

PS, Remove "ine" from my email address if you have an image file.

This is not a simple question.
No doubt, you'll get sage advice from people who sound like they know
what they're doing. They'll
GUESS what you're doing relative to THEIR
experience. But the advice may be counter productive. This is the internet
where anybody can be an expert.
I'll surely get angry responses that I'm an idiot.

You have given insufficient information for anyone to recommend a
solution.
About the best anybody can do is to try to teach you about a wide
variety of issues.

So, EXACTLY what do you have?

19V is not a typical number for a battery.
One possible guess would be that it's
a box containing a battery that puts out 19VDC regulated to power
a laptop computer or similar device. In that case,
the internal battery may be higher or lower voltage than 19.
About the only thing you can count on is that it is NOT exactly 19V.
All depends on the design. And if there's an input jack for
charging, the internal charger parts may or may not depend on
some characteristic of the charger.
How much current do you intend to stuff into the charge port?
What's the capacity of the cells you're charging?

If you mean that you're using a timer to stop the charging,
that works fine if you charge at low current.
If you expect to fast-charge, that's quite another issue.

And, is your 24VDC regulated? Or some wall wart that might be
anywhere from 24-35V unloaded.

One way to get a relatively constant current over a decent
range of voltage difference is to use an incandescent flashlight bulb.
I've used it many times to charge batteries of known characteristics
using a known power supply.
That's the info you requested, but not necessarily the info you need.

 

[gregz]

<jaugustine@verizon.net> wrote:

Hi,

I searched the web for a simple constant current circuit so
I can charge a 19V rechargeable "power pack" that contains NMH cells
after they "run down" using a timer. My DC source will be 24V.

The simple circuits I found are for lighting LEDs, anything
requiring more current involves complex circuitry.

Does anyone know of a device like a simple 3 lead voltage
regulator, but for current, or a simple circuit?

Thank You in advance, John

PS, Remove "ine" from my email address if you have an image file.

With a small differential like that, a resistor would work. Not the best
way to charge that battery.

Greg

 

[cjt]

On 12/08/2012 08:56 AM, jaugustine@verizon.net wrote:

Hi,

I searched the web for a simple constant current circuit so
I can charge a 19V rechargeable "power pack" that contains NMH cells
after they "run down" using a timer. My DC source will be 24V.

The simple circuits I found are for lighting LEDs, anything
requiring more current involves complex circuitry.

Does anyone know of a device like a simple 3 lead voltage
regulator, but for current, or a simple circuit?

Thank You in advance, John

PS, Remove "ine" from my email address if you have an image file.


Put it in the feedback loop of an op-amp that also has a constant

current input generated by a fixed voltage through a fixed resistance;
it'll waste some power, and you'll need a beefy op-amp, but it's simple.

 

[Arfa Daily]

<jaugustine@verizon.net> wrote in message
news:56l6c8hu2vp1p7vrbbvkc5lm9gv4lidt37@4ax.com...

Hi,

I searched the web for a simple constant current circuit so
I can charge a 19V rechargeable "power pack" that contains NMH cells
after they "run down" using a timer. My DC source will be 24V.

The simple circuits I found are for lighting LEDs, anything
requiring more current involves complex circuitry.

Does anyone know of a device like a simple 3 lead voltage
regulator, but for current, or a simple circuit?

Thank You in advance, John

PS, Remove "ine" from my email address if you have an image file.

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

 

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

 

Even simpler than that.

https://dl.dropbox.com/u/29948706/csce.jpg

Realize though that all the current you want will be going through the pot. Actually best to trial and error it in the beginning and then decide on the resistor value (Re). That tmeans you don't have to screw with calculating those millivolts. Only one diode is actually needed, but using two makes Re less critical so you should be able to fine tune the output more easily using standard resistor values.

The resistor from the base to ground, a wild guess works. Anything from 15K to maybe 47K should work just fine.

That circuit should only need about 2 volts of head voltage, if you need less just use one diode instead of two and adjust the emitter esitor (Re).

You might also put one of the diodes near the battery pack to cut the current back at higher temperatures, but don't let it go open. Losing the connection there will turn the thing on full blast. If you need any thermal integration there are much better ways. If you can give a bit more head voltage you can put a diode in series with Re and it should do the trick, and if it opens up then the current won't max out.

 

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

 

[mike]

On 12/10/2012 9:29 AM, jaugustine@verizon.net 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.

 

I consider myself to be smart and experienced..
And the result has never been as simple as a lm317 and a resistor.
http://www.hqew.net/product-data/LM317T

 

[Arfa Daily]

<joey899244@yahoo.cn> wrote in message
news:c587ba92-f257-41b1-a894-c64a394000ae@googlegroups.com...

I consider myself to be smart and experienced..
And the result has never been as simple as a lm317 and a resistor.
http://www.hqew.net/product-data/LM317T

Why the link to the data sheet I wonder ? That only gives details of its
original designed purpose as a simple-to-use 3 terminal adjustable voltage
regulator.

As far as the solution "never being as simple as an LM317 and a resistor",
it depends entirely on your requirements. The OP merely requested details of
a 'simple' constant current source to charge a battery pack, and the LM317T,
suitably configured and connected, will do just that - provide a constant
current at any value you like with the addition of one resistor. No more, no
less. I don't think anyone, least of all me, suggested that this would be
the best or kindest way to charge a pack of NiMH batteries, but with the
charging current correctly dimensioned, it would be unlikely in it's 'raw'
form to cause any likelihood of explosion or serious long term damage.

For sure, the circuit could be refined. A simple timer could be added for
instance, to just turn the whole affair off after a time that would ensure
that the pack is fully charged. Or a multistage timer could be incorporated
to change the charge current during the charge cycle. Or you could start
monitoring the terminal voltage and so on and so on. But then, of course,
you are getting away from the OP's original requirement for a 'simple
constant current source'.

Whilst I wouldn't disagree with anyone that a degree of care should be taken
over the charging of NiCd and NiMH cells and packs, I also think that
sometimes, you can get a bit paranoid over it all. Many 'simple' chargers
provided by equipment manufacturers to charge their items, are nothing more
than a basic power supply with a current limiting resistor on the end.
Cordless phones used to be a good example of this, and the battery packs
used to last a long a time on them. The cordless drill that I use every day
in my workshop and charge once every week or ten days is another example.

Arfa