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.

Simple Constant Current Device or Circuit
 

In article ,
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

Simple Constant Current Device or Circuit
 

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.

Simple Constant Current Device or Circuit
 

On 12/8/2012 6:56 AM, 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.

Simple Constant Current Device or Circuit
 

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

Simple Constant Current Device or Circuit
 

On 12/08/2012 08:56 AM, 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.

Simple Constant Current Device or Circuit
 

wrote in message
...
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

Simple Constant Current Device or Circuit
 

wrote in message
...

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