# Simple Constant Current Device or Circuit

Discussion in 'Electronic Repair' started by [email protected], Dec 8, 2012.

1. ### Guest

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?

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

2. ### William SommerwerckGuest

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.

3. ### mikeGuest

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

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.

4. ### gregzGuest

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

Greg

5. ### cjtGuest

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.