Simple Constant Current Device or Circuit

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

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.

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.

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

Greg

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.

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.

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 th e 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 R e less critical so you should be able to fine tune the output more easily u sing 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 le ss 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 curre nt back at higher temperatures, but don't let it go open. Losing the connec tion there will turn the thing on full blast. If you need any thermal integ ration 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.

UPDATE:

I don't have one (LM317T), but

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...

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 hea droom a regular diode might result in the battery never getting a full char ge.

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 abou t that particular deal is then obviously it wouldn't be able to operate off the wallwart and charge at the same time, would it ?

disclosed none....unless I missed that post. "

I doubt you missed it :-) However it is his job to find out of the LM317 ca n handle the current, and if not then maybe use a pass transistor. In eithe r 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-io n battery should be charged. I know some batteries now can be charged faste r than you drain them, but we still do not know the particulars here.

Actually after this last power outage I am considering doing something simi lar 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 co ntinuously 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 te thered 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 witho ut opening the base. In those cases where the wallwart puts out AC you're s crewed.

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.