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LED to light when battery drops below 3.5V

JPU

May 19, 2012
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Hi

I am using a LIPO battery when fully charged it is 4.2V and I would like to create a circuit to light an LED when the battery voltage drops below 3.5V therefor warning the user.

Space is not abundant and therefore the circuit needs to be small and simple. Is there an easy way to do this as there is not even enough room for a PCB. I have read about using op amps to achieve this but I think they would be hard and fiddly to wire without a PCB.

If someone could help with this idea or has some pointers I would be greatful.

Thanks in advance

JPU
 

(*steve*)

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There are three terminal devices which change their output state at a given voltage. I'm pretty sure one of these would be just what you're after.

Go here, select 3.5V in the 8th column and start checking out the datasheets of the devices.
 

CDRIVE

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Justin, make sure you select the correct one. There were so many listed that I just may have missed one that will drive a LED directly. I downloaded the MAX6412-MAX6420.pdf but the output sink current was only as follows. The odds are that I'm looking at the wrong model though.

VCC ≥ 1.0V, ISINK = 500μA
VCC ≥ 2.7V, ISINK = 1.2mA
VCC ≥ 4.5V, ISINK = 3.2mA

Chris
 

JPU

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Hi Chris

I was confused by the amount of choices, eventually having being made aware of such a device by Steve I looked at the Rs online website.

I have tagged these parts for order:

The Volt supervisor:


http://uk.rs-online.com/web/p/volta...2267573743D3636322D343230312677633D4E4F4E4526

Transistor:

http://docs-europe.electrocomponents.com/webdocs/078e/0900766b8078ea66.pdf

I intend to use the voltage supervisor with a voltage divider to monitor the voltage and then activate the transistor once the voltage reaches 3.3V (revised figure).

I am not sure if this will all work so its going to be trial and error. As I am a novice, I do find the technical data sheets difficult to decipher.

What do you think of my choices.

FYI: I intend to add the circuit to my headlight to warn the user of a nearing depleted battery. The switch I am using has a built in LED and I thought it would be a neat addition!

Regards

Justin

Edit:

Chris, I have been having a play with Tina tonight, I thought I would mess about with a circuit which I intend to build to carry out the above task. I couldn't find the footprint for the TPS3803 but I got side tracked anyway! This is a TINA problem I have,,please can you explain where I have gone wrong here in this schematic,,shouldn't the LED flash 5 times a second? Can you please explain what I am doing wrong with the voltage generator.
 

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(*steve*)

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It may be worth checking some of the Lithium battery charger ICs. You may find that there are some that will manage the charge, warn you on low voltage, and disconnect the load if the voltage falls too far (there are, but they are almost universally in packages that are a PITA to hand solder).
 

CDRIVE

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Justin, you need a 1K resistor in series from VG1 to T1's Base. In the real world, without it, the base-emitter junction would be destroyed el pronto! You also have to move VF1 to the Collector. You're measuring ground where it is now.

Chris

Edit: I would also set the DC property of VG1 to 2.5V and the Sinewave property to 2.5V. This should give you a VG1 output of 5VPP but will not swing negative, which emitter - base junctions don't like.
 
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CocaCola

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Justin, make sure you select the correct one. There were so many listed that I just may have missed one that will drive a LED directly.

you need a 1K resistor in series from VG1 to T1's Base. In the real world, without it, the base-emitter junction would be destroyed el pronto!

If you use a SOT23 transistor with an integrated 1K resistor (like the NPN PBRN113ET or PNP PBRP113ET) to boost the switching current you hardly increase the size of the final product at all... Just a dab of super glue to mount the transistor on the back of the other chip, some coil wrap wire and you are still in an extremely small integrated package...

Just my 2¢...
 

CDRIVE

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If you use a SOT23 transistor with an integrated 1K resistor
Just my 2¢...

Good tip but I was referring to his Tina circuit, which is unrelated to this topic. I think he's just playing with Tina and components to aid in his understanding of transistor action and Tina too.

Chris
 

JPU

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Thanks all for your input. I was just experimenting to help in my understanding. I have ordered the components so I will probably be back with some more problems when they turn up.:p

Thanks again.
 

CDRIVE

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If you want to play with simulating a battery low indicator create this. It has no temperature compensation but it will give you the basic idea. There's no doubt that the chip you ordered will have far more temperature stability.

I'm going out to get some sunshine but I'll post a BJT version when I return. It will have the same structure as this though.

Chris
 

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JPU

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Chris

I will have a play around with that. Its exactly what I wanted.


I have changed my order with RS and I instead I have gone for the MCP101-270HI/TO. Its in a TO-92 package!! I have also ordered the 2n7000 to play around with.

Thanks again

Justin
 

CDRIVE

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Here's a BJT version with hysteresis. The LED fires when the battery voltage drops <= 3.5V. It will remain on until the battery is recharged to 3.88V. This circuit won't flicker.

I don't believe either one of the circuits I gave you will be superior to the MAX chip though. I posted them for their instructional value.

Chris
 

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JPU

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Chris

Thanks for that. I will have a play with those circuits. They will be useful in helping me to understand, as you say.

Thanks for your time.

Justin
 

CDRIVE

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Justin, do you know how to set Tina up for a DC Transfer Characteristic? Also, you'll note that the plot is measuring LED current yet there's no ammeter or current arrow in my circuits. That's because the LED current limiting resistor (R2) is a macro I created. It's actually a 220Ω resistor with a current arrow in series with it. So, when you draw the schematic you'll need a current arrow or ammeter in there to measure LED current.

Chris
 

JPU

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Hi Chris

Thanks for that. I have had a play with the DC transfer Characteristics analysis tool today. I was wondering how I could get Tina to do that and noticed it in your graph.. I didn't spot the lack of the ammeter but I have inserted a meter now and the circuit works. I didn't get the exact results you got and had to change the resistor values somewhat?? But it works.

In the first example you posted (ie post #11), I can see that the voltage to Q1 would drop to 2.08V at a battery voltage of 3.5V, can you explain to me how this then causes the LED to light. (I have looked up the technical specifications of the 2n7000, what do I need to look at to figure out when a mosfet will switch to allow the current to flow.)


This is another problem you might be a able to help with if you have time, please.
I have contacted TINA technical about it and they said to use the library manager but I'm finding it a nightmare. I wish to add components to TINA. e.g TIs TLV810M The link to the spice is http://www.ti.com/litv/zip/sbvm039

I would like to add this component so that I can play around with the voltage supervisor in TINA,,,surely this can be done??

Do you know how to do it and can you help me or am I getting the wrong end of stick and this is not how adding components to TINA is done?

Justin

PS, apologies for so many questions!
 
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JPU

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Chris

REF post #11

I have spent the last 4hrs reading all about Mosfets and how they work. I think I now have a better understanding of how your circuit works. However, can you explain why the mosfet Q1 switches on at 2.5 V. I have read the the technical data sheet and it says that gate threshold is typically 2.1V.

http://docs-europe.electrocomponents.com/webdocs/0469/0900766b80469a75.pdf

Thanks for your help.

Justin
 

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However, can you explain why the mosfet Q1 switches on at 2.5 V. I have read the the technical data sheet and it says that gate threshold is typically 2.1V.

That circuit is quite sensitive to the Vgs(th) of Q1. The actual value can vary significantly between devices, so you may find that the switching point varies with the device chosen.

You would alter the ratio of the voltage divider to set the point at which it operates.

Vgs(th) is also dependant of temperature (see here) so expect some variation with ambient temperature.
 

CDRIVE

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Chris

REF post #11

I have spent the last 4hrs reading all about Mosfets and how they work. I think I now have a better understanding of how your circuit works. However, can you explain why the mosfet Q1 switches on at 2.5 V. I have read the the technical data sheet and it says that gate threshold is typically 2.1V.

http://docs-europe.electrocomponents.com/webdocs/0469/0900766b80469a75.pdf

Thanks for your help.

Justin

I don't know if you're referring to simulating this or a real world circuit. I mention this because you said that you simulated one or both of my circuits and you had to change resistor values to duplicate my results. This shouldn't be at all. Unlike the real world all the component values, characteristics, voltages. instrument accuracy, everything should be identical down to a gnat's ass. There are three 2N7000's in the spice library. I used the second one on the list in the search box, who's Vgs(Th) = 2.1V. Also, and this isn't relevant to the 2N7000 circuit because it possesses no hysteresis, but the DC Characteristic run for the BC546B's should Start at the highest voltage and End with the lowest voltage. If you check off Hysteresis Run it won't matter but if you don't you will get totally different results if the Start value is the lowest. .. Capice?

Now, regarding the discrepancy for the 2N7000 Vgs(Th). The data sheet specs Min = 1V, Typ = 2.1V, Max = 3V. So what's confusing about that? Typical does not mean guaranteed. By the way, those specs are given with a mere 250uA Drain - Source current.

Chris
 

CDRIVE

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This is another problem you might be a able to help with if you have time, please.
I have contacted TINA technical about it and they said to use the library manager but I'm finding it a nightmare. I wish to add components to TINA. e.g TIs TLV810M The link to the spice is http://www.ti.com/litv/zip/sbvm039

Only once did I manually add a spice model directly to the library. Nothing about it is intuitive and I've bItched to Tina about it. It was so damn confusing that I don't remember how I did it. They need a good wizard. Because it's so much easier, I just make a spice macro. The macro wizard is pretty good and easy to use.

Justin, I looked at the pcode in the.lib file with notepad and I've never seen pcode like that. I'm far from a pcode guru but I have been learning to read it. I at least have become familiar with what I expect to see in it and this isn't it.

Chris
 
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