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Apr 13, 2009, 02:06 PM
Stetson Flyers

2 cell lipo cutoff


Hello Dan,

Thanks for the great info you've posted here. I hope you can help me modify one of your designs.

My application is to provide a simple low voltage cutoff for my smoke pump. I'll be running it on a 2s lipo.

How could I modify your design in Post 6 to not have the adjust pot and a start switch?

Thanks,
Scotty
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Apr 13, 2009, 03:03 PM
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Dan Baldwin's Avatar
Quote:
Originally Posted by Soclarke
Hello Dan,

Thanks for the great info you've posted here. I hope you can help me modify one of your designs.

My application is to provide a simple low voltage cutoff for my smoke pump. I'll be running it on a 2s lipo.

How could I modify your design in Post 6 to not have the adjust pot and a start switch?

Thanks,
Scotty
The circuit in post #6 does will not work without a start button, but one of the circuits in a later post could probably be modified to work for you. What cut out voltage are you looking for?

Dan
Apr 24, 2009, 03:11 AM
Registered User
Quote:
Originally Posted by hazyh
I found a simple design posted by someone in another forum.
It only use 6 components including a p-channel mosfet.
Here is a modified version of Post#96. I think it has a sharp cutoff now.
Apr 18, 2010, 03:39 PM
Registered User

Discharger works with 1s


Just finished building this for storage discharging single LiPo cells.
Working from the schematic in post #6, I had to change the 1k resistor to 270ohm so that the 3906 would turn on and off properly. The voltage divider resistors for the 431 was 8.2k over 18k, (no pot). Added an LED and 220ohm resistor in parallel with the output for visual indication. Did not have an IRF520, so I used an IRL530 I had on hand, both overkill at my low discharge rates. The output load is switchable between a 27 & 62ohm resistor, which equates to about 140mA & 65mA respectively. The 65mA rate was added to accommodate cells 100mAh and smaller, which would rebound to over 4volts after a quick discharge on the 140mA setting. At 65mA, the smaller cells rebound to between 3.7 & 3.8v.
Nice little circuit.
Thanks Dan and everyone else that contributed.

Regards,
hubgar.
Last edited by hubgar; Oct 31, 2012 at 03:46 PM. Reason: Adding photo
Jan 13, 2011, 02:55 PM
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Almaz's Avatar
Quote:
Originally Posted by dleroi
For the sake of discussion, here's another approach that uses a PIC. This is a prototype that I built for someone that wanted to protect the Lipo packs in garden railroads from over discharging.



Can anyone explain how come Motor (+) connected to P-channel Mosfet Gate. Shouldn't it be connected to battery positive?
Jan 13, 2011, 04:46 PM
Registered User
Motor plus is hooked to B+. The output fet supplies the ground.

They are missing some "junction dots" at the left of the drawing
where you are talking about.
Jan 13, 2011, 06:21 PM
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Almaz's Avatar
Quote:
Originally Posted by jglenn
Motor plus is hooked to B+. The output fet supplies the ground.

They are missing some "junction dots" at the left of the drawing
where you are talking about.
Thank you for clearing this up. A missing dot made me confuse.
Sep 26, 2012, 07:29 PM
Registered User
I built two of these and they work as expected when I use a light bulb as the load. But when I use a 3 or 6 Ohm 50 watt resistor the thing never turns off. If I place the bulb in parallel with the load it shuts right off. Anyone have any idea of what would cause this. Has me stumped and they both do the same thing. The only thing I used different then the schematic was that I used a 510 ohm resistor in place of the recommended 520 ohm resistor.

Thanks,
Dale
Sep 30, 2012, 05:39 PM
Registered User
For anyone that runs into this same problem I solved it by adding a diode accurst the load in the circuit. The wire wound resistor was acting as an inductor and is what was causing the problem.

The cathode of the diode goes to Load Positive.

Dale
Oct 31, 2012, 09:09 AM
Registered User
Dan was gracious enough to answer some additional questions by PM, which I'll post here for the benefit of others:

Quote:
Originally Posted by Dan Baldwin
Quote:
Originally Posted by tonyyng
I want to thank you for the information you provided in this thread. I'm planning to build it and have some questions about optional components.

Is STP30NE06 an appropriate substitute for the MOSFET?
Data sheet here:
http://www.mouser.com/ds/1/389/CD00010548-51064.pdf

Also, is the 2N4403 a suitable replacement for the 2N3906TA?
Yes, the FET and the bipolar transistor you asked about should work fine for that project.

Dan
Oct 31, 2012, 11:57 AM
Registered User
This is probably a dumb question, but if I have a logic level MOSFET, shouldn't the comparator be able to drive the MOSFET directly, that is, without the bipolar transistor?
Jul 10, 2015, 11:56 AM
Registered User
Hi guys, I think this is my first post in RCgroups, and I hope I will not set a bad reputation from the beginning. I am flying a micro quad, on brushed motors and 1s 600mA battery. I know it isnt a big deal for 1s to get too low in voltage, because quad will not fly anyway when voltage is low. But i am getting quite excited when fly and I thought fool-proof auto cut-off will help.

I found these guys on robotics forum, and this is the schematics that I found (which is simplier than these here):
https://www.ez-robot.com/uploads/lip...6712656250.jpg


Now, they used it for robots with much more power than my little quad, but I dont know whether I can down scale it proportionally. I found these components:

http://uk.farnell.com/diodes-inc/dmg...323/dp/2061403

http://uk.farnell.com/nxp/bzx84j-c3v...-9b10-00002068

But I dont know what resistor I will need.

Also it isnt clear why it is so much simpler than the schematics that you have???

I want to make it as small as I can, because for this litle size quads, every gram matters.

THANK YOU!
Alex
Jul 10, 2015, 03:00 PM
Registered User
vollrathd's Avatar
Quote:
Originally Posted by garald
Hi guys, I think this is my first post in RCgroups, and I hope I will not set a bad reputation from the beginning. I am flying a micro quad, on brushed motors and 1s 600mA battery. I know it isnt a big deal for 1s to get too low in voltage, because quad will not fly anyway when voltage is low. But i am getting quite excited when fly and I thought fool-proof auto cut-off will help.

I found these guys on robotics forum, and this is the schematics that I found (which is simplier than these here):
https://www.ez-robot.com/uploads/lip...6712656250.jpg


Alex

Just looked at that parts layout. Don't think it will work, since it appears to use a non-standard MosFet. Wish they'd provided a schematic drawing of it.

99.99% of the MosFets I've used have the far left pin as the MosFet gate. This drawing shows that gate hooked up to the plus battery. And, no current path exists to the load.


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