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Topic: LM317 voltage regulator R2 is getting very hot? why? (Read 10920 times) previous topic - next topic

calvingloster

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

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

polymorph

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.

http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Steve Greenfield AE7HD
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KeithRB


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.

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?

calvingloster

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?

KeithRB

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.

Grumpy_Mike

Quote
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.

JChristensen


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:

http://en.wikipedia.org/wiki/Current_source
http://en.wikipedia.org/wiki/Current_limiter

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.

calvingloster


Quote
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?

polymorph

Regular low pressure lead acid cells cannot be charged by simply connecting them to a constant voltage, unlimited current charge.

I answered your questions about using an LM317 as a constant voltage, current limited charger. Including a link to a website that tells you all about charging lead acid cells. Please study it.
http://forum.arduino.cc/index.php?topic=243044.msg1742390#msg1742390
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

cjdelphi



Quote
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?


It's not a great idea maxing out the power supply all the time but as the battwey voltage rises, the current will drop as it approaches 14.2v, if you have a 100ma supply and the battery is rated 1000mah it will take 10 hours constant current, 11 or higher hours at constant voltage...  (as current deceeases nearer to it's charge voltage 14 v)

JimboZA


Not difficult at all and as ever, GIYF.


How ironic.... I had to Google what that meant.

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Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

fungus

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

fungus


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.


My suggestion: Instead of devoting hour and hours of your energy to something that's never going to work very well, devote that time to finding a way to get stuff from eBay.

You must know somebody who can help...
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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