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Old Aug 09, 2007, 05:23 PM
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voltage cut off switch


I accidently over discharged my lipos while they were connected to some video gear. Does anyone know how to make a low voltage cut off switch that would completely disconnect the battery when the voltage falls below certain point? See https://www.rcgroups.com/forums/show...50#post7959164
Thanks,
-dave
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Old Aug 09, 2007, 06:16 PM
Dan Baldwin is offline
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I built a battery discharger a while back that worked similar to what you are looking for. I have it discharging an A123 pack right now. It has a pushbutton to turn it on, and turns off when the voltage gets down to the selected voltage. I'll modify it and post it if you think it will work for you.

Dan
Old Aug 09, 2007, 06:19 PM
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Hi Dan;
I'd like to see your design!
Old Aug 09, 2007, 06:41 PM
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Here it is as I built it (I think), but I just realized while looking at it that I should be able to GREATLY simplify it. I'll post the simplified one too, but I need to breadboard it first, just to make sure I didn't overlook something.

Dan
Old Aug 09, 2007, 07:11 PM
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Thank you very much Dan! Your time is much appreciated. May you please provide a part list as I can't quite make out the transistor numbers and I'm not electrical savy enough to guess a possible substitution. Thanks!

-dave
Old Aug 09, 2007, 07:31 PM
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This is what the simplified version would look like to drive your video equipment. I'll post the parts list in the morning.

Dan
Last edited by Dan Baldwin; Aug 10, 2007 at 12:48 PM. Reason: Changed resistor value
Old Aug 10, 2007, 11:04 AM
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Quote:
Originally Posted by Dan Baldwin
This is what the simplified version would look like to drive your video equipment. I'll post the parts list in the morning.

Dan
my only suggestion would be to add a cap close to the video unit on the power lines. If it is wireless, many of them are very sensitive to voltage ripple.
Old Aug 10, 2007, 11:37 AM
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Quote:
Originally Posted by ImaBiggles
my only suggestion would be to add a cap close to the video unit on the power lines. If it is wireless, many of them are very sensitive to voltage ripple.
Yup. Wouln't be a bad idea.

I'm working on a writeup and parts list now. I should have it posted within an hour.

Dan
Old Aug 10, 2007, 12:56 PM
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This low voltage cut off device has an adjustment range of about 4 volts to 16 volts as shown. None of the components are critical. If the values of the fixed resistors around the voltage devider (8.2K, 3.3K) are changed, the upper and lower cut off voltages of the adjustment range will change. The 520 ohm resistor and the 1K resistor should be held fairly close to the values shown. The 4.7K resistor can be ANYTHING, but I suggest a value from 1K to 10K. The 2N3906 transistor can be any PNP small signal transistor. The IRF520 is not the best FET to use in this circuit (RDS ON app .2 ohms), it just happens to be what I had in my junk box. I suggest using a lower RDS FET such as the STP55NF06L(Mouser #511-STP55NF06L). I changed the value of one of the resistors in the above schematic (was 520 ohm, now 1K) to give a lower quiescent current (Now about 6 ma at 8.4 volts). Off state leakage current is extremely low. I can't read it on the microamp scale on my DVM.

Dan


Mouser part numbers

Code:
660-CF1/4C511J   510 OHM RESISTOR
660-CF1/4C102J   1.0 K RESISTOR
660-CF1/4C472J   4.7K RESISTOR
660-CF1/4C822J   8.2K RESISTOR
660-CF1/4C332J   3.3K RESISTOR
511-STP55NF06L   60 VOLT 55 AMP MOSFET
31VA401-F        10K 24 MM POT
512-2N3906TA     PNP SMALL SIGNAL TRANSISTOR
511-TL431ACZ     ADJUSTABLE PRECISION SHUNT REGULATOR
103-1012-EVX     N.O. PUSHBUTTON SWITCH
Last edited by Dan Baldwin; Aug 10, 2007 at 01:04 PM.
Old Aug 10, 2007, 01:42 PM
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Dan,

If I build your discharger circuit in post #4, is there any way other than trial and error to determine the cutoff voltage? Tnx
Are those 12v lamps, that can be switched either series or parallel?

Go Univ. Guelph!! lol
Old Aug 10, 2007, 04:58 PM
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Originally Posted by kit
Dan,

If I build your discharger circuit in post #4, is there any way other than trial and error to determine the cutoff voltage? Tnx
Are those 12v lamps, that can be switched either series or parallel?

Go Univ. Guelph!! lol
Of course there is, damned if I know the math though . Dan, wanna help us out?
Old Aug 10, 2007, 05:01 PM
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Quote:
Originally Posted by kit
Dan,

If I build your discharger circuit in post #4, is there any way other than trial and error to determine the cutoff voltage? Tnx
Are those 12v lamps, that can be switched either series or parallel?

Go Univ. Guelph!! lol
The discharger was intended to be used to discharge any size pack from about 5 volts to over 20 volts, so I designed it with a pot. When I built it, I put a scale on the front of the discharger so I could see what voltage I was setting. Although charging voltage is critical, discharge voltage isn't as critical, so I thought that a pot with a scale was close enough for my purposes. If you want it to discharge to a specific voltage, I can give you the value of the resistors to use to set that voltage without having to resort to trial and error.

I used 2 12 volt light bulbs, and they could either be put in parallel, or in series; parallel for high current, series for low current or high voltage (over 12 volts). If you are going to build a discharger, I suggest using the last schematic I posted. It does the same thing as the first one, but much simpler. Just hook the light bulb circuit to the connections labeled "video equipment". You can use the light bulb circuit I show, or just hook a single bulb up to it. If you want, I'll post that schematic.


Dan
Old Aug 10, 2007, 05:24 PM
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Quote:
Originally Posted by Dan Baldwin
If you want it to discharge to a specific voltage, I can give you the value of the resistors to use to set that voltage without having to resort to trial and error.

If you want, I'll post that schematic.
Dan
Please share for the benefit of the math challenged
ME
Old Aug 10, 2007, 05:34 PM
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Another schematic isn't necessary. I'm pretty sure I can hook a light bulb to the video equipment terminals without needing a diagram. Resistance values for 7.8 volts and 11.7 volts would be helpful. Thanks
Old Aug 10, 2007, 05:43 PM
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Quote:
Originally Posted by rick.benjamin
Please share for the benefit of the math challenged
ME
In a circuit using a tl431 where there are two resistors setting the cut off voltage such that one goes to the positive rail (resistor A), one goes to the negative rail (resistor B), and the junction of the two is connected to the reference lead of the TL431
cut off voltage = ((resistor A + resistor B)/ resistor B) * 2.5

Or

cut off voltage = ((resistor A / resistor B) * 2.5) + 2.5

Both formulas should give the same answer.


Dan


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