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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Complete UVLO power cut
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Author Topic: Complete UVLO power cut  (Read 5584 times)

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Offline Claviger

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Complete UVLO power cut
« on: July 07, 2014, 11:15:53 PM »
Will this work to completely cut the power from the battery if it hits 6.6 volts? Had an incident where my LiPOs went down into dangerous territory this morning, thankfully they came back to life, but I would like to make it impossible in the future.  Since I am running out of space in the box for my next mod, I need to figure out a way to do this that is very small, this is what I came up with.  NFET, so while it may be a tiny drain while its above 6.6, when the battery is under 6.6 it will cut all power.

« Last Edit: July 07, 2014, 11:23:07 PM by Claviger »

Offline CraigHB

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Re: Complete UVLO power cut
« Reply #1 on: July 08, 2014, 07:10:09 PM »
Rather than just say it won't work I'll show you with a quick simulation.  BTW, I did this with LTSpice which is a free circuit simulator you can download from Linear corp.  It's actually quite good as far as circuit simulators go. 

Attached is a screenshot of the simulation.  Click on the image to make it bigger.

Notice that the gate-source voltage in red is simply clipped by the Zener voltage.  Then the gate-source voltage falls off with source voltage in green.  So, you would never actually get to a gate-source voltage low enough to turn off the transistor until battery voltage is close to zero. 

Even if you do set up a Zener circuit that limits gate-source voltage enough to shut off the circuit, you get into the transistor's active region where it behaves more like a resistor than a switch. 

To do what you're trying to do requires a logic signal on the MOSFET gate.  That's a signal that is either on or off with no voltage levels in between.  It's possible to do this most simply with a voltage detector.  It can be done discretely with a Zener and a network of transistors, but it's much easier to use a single voltage detector chip, that's what they're designed to do.

Offline CraigHB

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Re: Complete UVLO power cut
« Reply #2 on: July 08, 2014, 07:59:48 PM »
Oh, BTW, you mentioned an NMOS, but that can only switch the low side.  Switching the high side as shown in your diagram requires a PMOS.  I used a PMOS in the simulation.  Even so, you'd run into the same issue with the circuit set up similarly on the low side. 

Any field effect transistor has a range of voltages between the the gate-source threshold and the "saturation" voltage where it's neither fully on or fully off.  To be used as a switch, it has to be fully saturated with a high enough gate-source voltage level.  Otherwise it's like putting a resistor inline with the battery.

Offline Claviger

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Re: Complete UVLO power cut
« Reply #3 on: July 09, 2014, 03:24:46 AM »
Well, while I am not terribly happy with your answer, I am very glad you supplied it!

Going to have to go dig for a new (to me) part to use now :P thanks craig!

Also, thanks for the heads up on the simulator!
« Last Edit: July 09, 2014, 05:51:15 AM by Claviger »

Offline CraigHB

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Re: Complete UVLO power cut
« Reply #4 on: July 09, 2014, 04:30:21 PM »
Check out the adjustable voltage detectors on TI's web site.  They have some that can handle voltages involved with 2 series cells.  It's a bit hard to find voltage detectors above 6V.

http://www.ti.com/product/tps3700

Offline Claviger

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Re: Complete UVLO power cut
« Reply #5 on: July 09, 2014, 05:26:52 PM »
Ya I noticed that this morning as I was looking through them.  Considering using a simple 2 resistor voltage divider, using 2 4.7 kohm resistors, to provide the input at 1/2 the actual voltage and getting a detector with a 3.1 - 3.3 volt cutoff. Not sure if it will work the way I am hoping, but the logic works in my head lol.

Irrelevant, the one you linked is perfect!
« Last Edit: July 09, 2014, 07:01:49 PM by Claviger »

Offline CraigHB

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Re: Complete UVLO power cut
« Reply #6 on: July 09, 2014, 08:52:38 PM »
I've looked at the TPS3700 before.  Voltage detectors under 5V are a dime a dozen, but the higher voltage ones are hard to find.  Looks like it should do the job.  If I was to do a mod with series cells, I'd probably use that detector.  I haven't looked into using that one in any detail.  Hopefully you can set it up so there's no draw or only a small draw when the mod is idle.

I always do stuff with a single cell since USB charging is a big priority for me.  Single cell makes that a simple addition.  I could do a series cell mod myself and still have on-board charging, just use a 9V charger plug with two charger controllers, been temped to do that, maybe down the road.  Higher input voltage is better for a number of reasons.

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