HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Aug 09, 2007, 05:23 PM
K4UAV
dalbert02's Avatar
Miami, FL
Joined Jul 2003
1,749 Posts
Discussion
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 http://www.rcgroups.com/forums/showt...50#post7959164
Thanks,
-dave
dalbert02 is offline Find More Posts by dalbert02
Reply With Quote
Sign up now
to remove ads between posts
Old Aug 09, 2007, 06:16 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
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
Dan Baldwin is online now Find More Posts by Dan Baldwin
Reply With Quote
Old Aug 09, 2007, 06:19 PM
Reduce the drama...
rick.benjamin's Avatar
USA, OR, Damascus
Joined Apr 2004
4,044 Posts
Hi Dan;
I'd like to see your design!
rick.benjamin is offline Find More Posts by rick.benjamin
Reply With Quote
Old Aug 09, 2007, 06:41 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
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
Dan Baldwin is online now Find More Posts by Dan Baldwin
Reply With Quote
Old Aug 09, 2007, 07:11 PM
K4UAV
dalbert02's Avatar
Miami, FL
Joined Jul 2003
1,749 Posts
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
dalbert02 is offline Find More Posts by dalbert02
Reply With Quote
Old Aug 09, 2007, 07:31 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
This is what the simplified version would look like to drive your video equipment. I'll post the parts list in the morning.

Dan
Dan Baldwin is online now Find More Posts by Dan Baldwin
Last edited by Dan Baldwin; Aug 10, 2007 at 12:48 PM. Reason: Changed resistor value
Reply With Quote
Old Aug 10, 2007, 11:04 AM
Master of 1 point landing
Naperville, IL
Joined Jul 2005
2,841 Posts
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.
ImaBiggles is offline Find More Posts by ImaBiggles
Reply With Quote
Old Aug 10, 2007, 11:37 AM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
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
Dan Baldwin is online now Find More Posts by Dan Baldwin
Reply With Quote
Old Aug 10, 2007, 12:56 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
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
Dan Baldwin is online now Find More Posts by Dan Baldwin
Last edited by Dan Baldwin; Aug 10, 2007 at 01:04 PM.
Reply With Quote
Old Aug 10, 2007, 01:42 PM
kit
Registered User
USA
Joined Oct 2001
695 Posts
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
kit is offline Find More Posts by kit
Reply With Quote
Old Aug 10, 2007, 04:58 PM
Master of 1 point landing
Naperville, IL
Joined Jul 2005
2,841 Posts
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
Of course there is, damned if I know the math though . Dan, wanna help us out?
ImaBiggles is offline Find More Posts by ImaBiggles
Reply With Quote
Old Aug 10, 2007, 05:01 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
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
Dan Baldwin is online now Find More Posts by Dan Baldwin
Reply With Quote
Old Aug 10, 2007, 05:24 PM
Reduce the drama...
rick.benjamin's Avatar
USA, OR, Damascus
Joined Apr 2004
4,044 Posts
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
rick.benjamin is offline Find More Posts by rick.benjamin
Reply With Quote
Old Aug 10, 2007, 05:34 PM
kit
Registered User
USA
Joined Oct 2001
695 Posts
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
kit is offline Find More Posts by kit
Reply With Quote
Old Aug 10, 2007, 05:43 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,722 Posts
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
Dan Baldwin is online now Find More Posts by Dan Baldwin
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Voltage cut-off for TP 2100? Steviewonder Batteries and Chargers 20 Jan 29, 2004 03:05 AM
Phoenix 20 No Cut Off Switch inedesca Power Systems 3 Dec 02, 2003 02:11 AM
Low voltage cut-off for battery discharger?? Chuck Batteries and Chargers 2 Aug 29, 2002 05:49 PM
Samples of Low Voltage Cut-Off Relay Circuits jrb Control Line 7 Dec 23, 2001 08:49 AM
jeti 140 high voltage cut-off berniep1 Electric Plane Talk 7 Apr 25, 2001 12:34 PM