# LM317 voltage regulator R2 is getting very hot? why?

Paul__B: A SLA battery charger. See his other thread.

Aha, thanks for that. I do strive to know what I don't know, and I can say that I am pretty much ignorant of battery charging algorithms. In general maybe there is a constant current phase for most of the charge but then this tapers off and/or switches to a trickle mode to keep things topped off, but details matter as does battery chemistry. Interesting stuff actually, I should educate myself a bit sometime. So I might look for an IC built for the purpose, although I always understand the attraction of rolling your own.

That said, I'll probably sit the rest of this one out, other than reiterating that before actually attempting to charge any batteries, the OP should Google up some tutorials and ensure they have a very solid understanding of Ohm's Law and also power relationships. Burning up a resistor may smell bad but failure of a lead acid battery could be quite a bit less enjoyable.

Ok so hear me out now. I do believe it is possible to create a constant voltage constant current source. The first LM317 limits volts to 14.1v and the second one limits current to let's say 100mA. When the "device" requires 50v to draw 100mA then the constant voltage will only allow it to draw as much current as 14.1v would allow according to ohms law. So as the "device" changes it's properties like resistance, the 2 LM317's will NEVER allow it to draw more than 100mA or be subjected to more than 14.1v. And I do believe a lead acid battery could be charged this way. It might take long but will still work. Does that not make sense?

calvingloster:
Ok so hear me out now. I do believe it is possible to create a constant voltage constant current source. The first LM317 limits volts to 14.1v and the second one limits current to let’s say 100mA. When the “device” requires 50v to draw 100mA then the constant voltage will only allow it to draw as much current as 14.1v would allow according to ohms law. So as the “device” changes it’s properties like resistance, the 2 LM317’s will NEVER allow it to draw more than 100mA or be subjected to more than 14.1v. And I do believe a lead acid battery could be charged this way. It might take long but will still work. Does that not make sense?

Not much, I’m afraid. Sounds more like a current-limiting situation. Do the maths. V=IR. If V and I are constants, so too must R be constant. So we can only have voltage and current constant with one specific resistance, i.e. the load resistance cannot change.

But don’t let me discourage you. I am perfectly willing to be convinced. Design the circuit, build it, test it, then bring us the schematic and the V/I curves so that we can understand it and duplicate the results.

I am perfectly willing to be convinced.

Well I am not. This is a classic beginners mistake thinking you can have a constant current and constant voltage at the same time.

What the OP is describing is a current limiting supply, the sort of thing you get in bench power supplies. The voltage is constant up to a certain current, then as that current is reached any further reduction of the load to try and increase the current results in a reduction in output voltage. You can do all that with one regulator and a bit of feedback from a current sensor.

calvingloster: I have set up a LM317 to output 13.7v. From the output to adjust I have used a 5ohm resistor and then my R2 which is from the R1 to ground I have used a 50ohm resistor. When I put a 16v power supply on the 50ohm resistor gets smoking hot!!! Like I can even smell it! Why is this?

(the LM-317 data sheet may be a help too......) :)

calvingloster: Does that not make sense?

Yes, but from the device's point of view it won't be a constant voltage supply. The voltage the device sees will vary (up to the maximum allowed).

PS: You can get pre-built, adjustable devices for doing this:

You can even get them with three adjusters so they turn on a LED and shut down when they reach a certain voltage:

This can be done with two LM317, but the current limiter must come first. Essentially what you are doing is building a voltage regulator, then powering it with a current regulator. The current regulator drops the voltage to the voltage regulator if the current reaches the setpoint, and so the voltage regulator's output drops because it just isn't getting enough voltage.

It can be done, but each LM317 requires a relatively large headroom. You need about 5V headroom for each one, so for 14.1V maximum output you'll need nearly 25V. Rather inefficient.

For the current regulator, all the output current flows through the resistor between Output and Adjust, so it must be a higher wattage. At 100mA output, 1.25Vx100mA = 125mW. You always at least double the rating for resistors, so a 1/2W resistor will work. But for the voltage regulator,none of the output current flows through the voltage divider that sets the output voltage, so they can be low wattage resistors.

The LM317 each require a heat sink.

However, as has been pointed out:

1. There are better ways to do this
2. The price for failure can be high (dead or exploding batteries)
3. The OP really needs to read up on the basics: Ohm's law, Kirchoff's current and voltage laws, the voltage/resistance/current/power formulas, etc.

I find this very frustrating and hard to understand. Can I ask that someone answers the questions I am about to ask in a yes no fashion.

1) the Bulk charge of a lead acid battery is done by a constant current charge. For simplicity sake let's say we want to charge our battery with 100mA. Now in order for us to force 100mA into the battery we require a voltage of 20v. So now our power supply only puts out 16v. So we will not be able to push 100mA into the battery, BUT!!!! BUT BUT BUT!!! We will still be push some amount which is LESS THAN 100mA!!! So essentially our BULK charge will take longer than usual BUT it will still work and will NOT damage the battery. Yes or no?

2)Now an LM317 CAN!!!!! CAN CAN CAN!!!!!! Provide the top explained power supply. As a constant current source? YES or NO?

3) for the float charge we charge the battery with a constant voltage source? At 14.1v. Now the LM317 can also provide us with such a power supply!!!!!! Yes or no?

The LM317 can supply us with a constant voltage and a constant current source. I hear u guys are saying it cannot but I'm sure it can because all the first LM317 is doing is limiting voltage to the second one. So the second LM317 never see's any voltage higher than say for example 14.1v. Then the second LM317 regulates current to say 100mA. So the device can never see voltage above 17volts and can never see current above 10mA.

Even if I use one LM317 set up as a voltage regulator regulated to 14.1volts I will be able to charge the battery until it's full!!!!! This will obviously take much longer but it does not damage the battery at all and if I leave it over night then awesome!!!!!

And I know that efficiency isn't going to be great but these are the only components I have!!! I can't just buy stuff like you guys recommend. I have what I have and I want to use it to make what I need. Even if it isn't the most efficient thing on earth.

No.

Typo I meant the device can never see current above 100mA

I don't know if you guys are maybe getting confused. A constant current source means that the device can never draw current above the set limit but can draw any value below the limit. So if I set an LM317 to a constant current source of 100mA the device can never draw above 100mA but can draw 5mA or 20mA or any current bellow 100mA

I think the wording is getting in the way. What you want is a power supply that is voltage regulated, current limited.

A single LM317 can do this with a transistor for feedback to limit the charging current. Try this one, with a 5.6 ohm resistor in place of R1:
http://www.sentex.ca/~mec1995/circ/labc3.html

Keep in mind that that website uses 2.45V per cell. That is only good for quick charging, short term connection. For long term slow charging such as what you are doing, use 2.35V per cell.

So without a load, adjust for 6x2.35 = 14.1V as you have said. When the voltage drop across R1 reaches about 600mV, the transistor begins turning on, reducing the voltage, which decreases the charge current. That will be slightly greater than 100mA.

The charge curve: Initially, with current limited to 100mA, voltage across the battery will be less than 14.1V. As it charges, eventually voltage rises to 14.1V and charge current begins dropping.

Keep in mind that 100mA is not a float charge. If you keep it connected for long periods, it will damage a lead acid battery. Float charge is done at a constant voltage lower than the charge voltage, current should be on the order of 1/100th the Ah (amp hour) rating or less.

calvingloster: I don't know if you guys are maybe getting confused. A constant current source means that the device can never draw current above the set limit but can draw any value below the limit. So if I set an LM317 to a constant current source of 100mA the device can never draw above 100mA but can draw 5mA or 20mA or any current bellow 100mA

Don't teach your grandmother how to suck eggs. You are the one who is confused. A constant current source puts out a constant current. You are talking about a current limited source.

KeithRB:

calvingloster: I don't know if you guys are maybe getting confused. A constant current source means that the device can never draw current above the set limit but can draw any value below the limit. So if I set an LM317 to a constant current source of 100mA the device can never draw above 100mA but can draw 5mA or 20mA or any current bellow 100mA

Don't teach your grandmother how to suck eggs. You are the one who is confused. A constant current source puts out a constant current. You are talking about a current limited source.

Let's not get personal please? An LM317 wired up with one resistor between the output and the adjustment pin is a current limiting device. I can supply any amount of current as long as it's not above the set limit. Then the thing limits the current and sais " wait now I will not allow more than 100mA" a constant current source is exactly the same thing is it not?

Why can you not just charge a lead acid battery with a constant voltage of 14.1v until the battery reaches the end end of the "top up" charge. I mean, isn't a bulk charge constant current to just speed things up? I mean, u can charge at a constant voltage it will just take much longer? But who cares if it takes longer? I certainly dont?

You started “making it personal” by declaring we were confused. You might be starving your constant-current-source LM317 for voltage so that it can’t supply the current it wants to. But that is making it a current limit not a constant current.

Why can you not just charge a lead acid battery with a constant voltage of 14.1v

Because it will then take too much current.

You need a current limiting source until it reaches 14.1V AND then you do not increase the voltage any more.

calvingloster: Let's not get personal please? An LM317 wired up with one resistor between the output and the adjustment pin is a current limiting device. I can supply any amount of current as long as it's not above the set limit. Then the thing limits the current and sais " wait now I will not allow more than 100mA" a constant current source is exactly the same thing is it not?

Incorrect. An LM317 with a resistor between the output and adjustment terminals is a (constant) current source. That is not the same thing as a current limiter. Words mean things and these terms have precise meanings in the field.

Since you apparently won't read the datasheet, and continue to argue against what very knowledgeable people here are telling you, try Wikipedia. In particular, note Figure 8 in the first link:

Have an open mind and try to learn the applicable theories. Rather than continuing to be obtuse and getting all hot under the collar, redirect that energy to the challenge I gave earlier: Design, build and test the thing, then bring it back and bask in the glory of proving us all fools. The proof is in the pudding, so go make some pudding.

Grumpy_Mike:

Why can you not just charge a lead acid battery with a constant voltage of 14.1v

Because it will then take too much current.

You need a current limiting source until it reaches 14.1V AND then you do not increase the voltage any more.

Ok I get what your saying. But if I have a power supply that is 14.1v and supplies a maximum current of 100mA then it will be fine provided the 100mA is not more than 20% of the battery capacity? The charger might just get hot because it's working at its maximum rated capicity?