LM339 Low voltage alarm help

[SimpleJoe]

So i started to make a portable bluetooth speaker and decided to use some left over laptop L-ion batteries (i do love a cheap and easy way out of things). But of the very little i know of electrical circuitry one of the main problems i could see is over discharging. So i found a simple little circuit using a LM339 to display a low battery alarm (below) but because i plan to be hardcore raving when the battery runs low i don't think an alarm will cut it (i would most likely ignore it). So my question is, is there any way to have a light come on (this part i can manage to get to work) at a little over 12 volts (4 cell l-ion) as well as have a relay that cuts it off at anything below say 11.5 volts. do i need some sort of signal inverter, any that doesn't require more ICs or transistors would be great but not mandatory. Please keep the technical jargon to a minimum, i've lost enough brain cells to inhaling solder fumes already

Thanks for any help
SimpleJoe

 

[CDRIVE]

Hello Joe and welcome to EP!

That simple circuit already has an LED indicator. The LM339 has open collector outputs with a maximum current sink = 18mA. We need to know how much current you'll be pulling from the battery. This way we can determine the most efficient interface, whether it be a relay, transistor or both. At only 18mA my guess would be both.

Chris

 

[SimpleJoe]

Yeah i had the LED working but i thought i could just invert the signals to get the opposite for the relay side but apparently i was wrong I am going to pull about 1 amp if i am charging a phone off the battery as well. I already have a salvaged G5PA-28 relay which i assume is 12 volt switching

Thanks again

SimpleJoe

 

[CDRIVE]

What's the coil resistance of your relay?

Chris

 

[CDRIVE]

Never mind, I looked it up. The coil resistance is 272Ω. At 12V it pulls 44.2mA!

I'm heading out the door so I'll get back to you later. In the interim other members will probably be posting suggestions and possibly even schematics. Either way you can be assured that Electronics Point is the right place to be.

Chris

 

[GPG]

LM339 is a quad comparator 3 sections unused in your diagram. An LM393 is a dual and could be used to give a warning (LED) and cutoff at different voltages. Still need a transistor either as a relay driver or (MOSFET) switch

 

[SimpleJoe]

Thanks for your help Cdrive,
GPG do you think you could make a complete diagram using a LM339 and what ever else i need? i am simple after all

 

[CDRIVE]

I'm feeling quite magnanimous today, so I'm going to volunteer GPG to do it.

Chris

 

[CDRIVE]

Well I was hoping that GPG or another member would jump in and volunteer.
Any takers? Anyone??? Hey! I'm trying to embarrass you! Every schematic you post is another notch for immortality!

I don't have the LM339 in my spice library but I do have the low power version LP339. If I'm forced to I'll use that.


Chris

 

[SimpleJoe]

So from the little research i understand i ended up with the circuit below. It works but i just want to make sure it's the best way to go about things and mainly that it is stable and won't overheat or give me testicle cancer or kill off all the polar bears

Thanks,
SimpleJoe

 

[CDRIVE]

So from the little research i understand i ended up with the circuit below. It works but i just want to make sure it's the best way to go about things and mainly that it is stable and won't overheat or give me testicle cancer or kill off all the polar bears

Thanks,
SimpleJoe

Good initiative Joe. I added two components. The Diode is used to protect the circuit from back EMF that occurs when the coil voltage is removed and can be quite high. The resistor is needed to limit base current. Without it you will destroy the Base-Emitter junction and the LM339.

Don't worry about your testicles. They're safe!
Chris

 

[SimpleJoe]

That one semester of electrical engineering really shone through in this one thanks for all your help

And to be honest with you that was actually my main concern

SimpleJoe

 

[CDRIVE]

Joe, I forgot to mention that you should use the normally (NC) closed set of relay contacts.

Chris

Edit: This is wrong and was corrected in my next post.

 

[SimpleJoe]

The relay is a DPST or a dual SPST (I'm not sure on the terminology) both are normally open, that's why i inverted the signals on the diagram i drew so the LM339 switches to VCC instead of ground when above the set voltage (well, i assume). Yes i know this would be inefficient (constantly drawing power) but the battery pack i made is sightly over 10 Ah so it shouldn't be a big deal. Also replacing it means i have to either talk to a "Jaycar employee" (pronounced self-absorbed asshats) or wait roughly 33 dog years for anything to arrive from ebay (I suspect they actually use snails to carry snail mail).

Thanks again,
SimpleJoe

 

[CDRIVE]

Joe, sorry about that. Yes, you're correct. You need normally open contacts like you have. Not only because your 339 logic is configured that way but also because normally closed contacts would continue draining the battery when the relay hold current drops below the manufacturer's spec'd level.

Chris