voltage cut off switch - Page 2 - RC Groups
Thread Tools
Aug 10, 2007, 05:50 PM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by kit
Resistance values for 7.8 volts and 11.7 volts would be helpful. Thanks
Sorry, I missed that.

For 7.8 volt cut off in a two resistor voltage divider (no pot) you could use 2.2K for the lower resistor, and 4.7K for the upper resistor. For 11.7 volts you could use 2.2K for the lower resistor and 8.1K for the upper resistor.

Dan
Sign up now
to remove ads between posts
Aug 10, 2007, 06:45 PM
kit
kit
Registered User
Thank you sir.

Need a heatsink for mosfet?
Aug 10, 2007, 09:37 PM
K4UAV
dalbert02's Avatar
Thank you Dan, your time is much appreciated!
-dave
Aug 12, 2007, 07:06 PM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by kit
Thank you sir.

Need a heatsink for mosfet?
Sorry it took so long to get back to you. You probably will not need a heat sink for the FET, but it depends on the particular FET you use, and how much current you draw; if you are only drawing a few hundred ma, the you probably won't need a heat sink even if you use a high RDS (on) FET like the irf520. If you are drawing several amps with that FET, then you probably will need a small heat sink since the FET would be dissipating about .8 watts when it's on if it is sinking 2 amps. If you use a FET with a low RDS (on), then you shouldn't need a heat sink even if you are drawing several amps.

Dan
Aug 12, 2007, 07:07 PM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by dalbert02
Thank you Dan, your time is much appreciated!
-dave
My pleasure. I hope it's been of some help.

Dan
Aug 12, 2007, 11:45 PM
Reduce the drama...
rick.benjamin's Avatar
I've ordered the parts from Mouser.
Thanks Dan!
Aug 12, 2007, 11:55 PM
kit
kit
Registered User
Does this look ok Dan?
Think I'll try to design a little PC SMD board for it, so I don't have to drill any holes. Not going to use any switches, just a strip of tin can for the push button switch that touches the PC copper when pushed, and for the resistor switch, a wire that can be plugged into 2 different sockets.
Aug 13, 2007, 11:02 AM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by kit
Does this look ok Dan?
Think I'll try to design a little PC SMD board for it, so I don't have to drill any holes. Not going to use any switches, just a strip of tin can for the push button switch that touches the PC copper when pushed, and for the resistor switch, a wire that can be plugged into 2 different sockets.

Yup. That looks great.

Dan
Aug 13, 2007, 12:38 PM
Registered User
I would like to use this circuit on a 28 volt battery pack.
Do you have any suggestions on how to modify this design for higher voltage batteries?

Thanks. -John Elliott

[QUOTE=Dan Baldwin]This low voltage cut off device has an adjustment range of about 4 volts to 16 volts as shown. <snip>

Dan
Aug 13, 2007, 04:51 PM
Registered User
Dan Baldwin's Avatar
[QUOTE=jbeanelliott]I would like to use this circuit on a 28 volt battery pack.
Do you have any suggestions on how to modify this design for higher voltage batteries?

Thanks. -John Elliott

Quote:
Originally Posted by Dan Baldwin
This low voltage cut off device has an adjustment range of about 4 volts to 16 volts as shown. <snip>

Dan
What adjustment range are you looking for, or would you just want one set voltage?

Some changes would be necessary for operation at 28 volts.

Dan
Aug 14, 2007, 12:05 PM
Registered User
It would be great if it could have an adjustable cutoff voltage of between 22 and 28 volts. But if the adjustability feature would be too complicated I would settle for a fixed cutoff of 24 volts.

Thanks in advance for any suggestions

-John Elliott

[QUOTE=Dan Baldwin]
Quote:
Originally Posted by jbeanelliott
I would like to use this circuit on a 28 volt battery pack.
Do you have any suggestions on how to modify this design for higher voltage batteries?

Thanks. -John Elliott

What adjustment range are you looking for, or would you just want one set voltage?

Some changes would be necessary for operation at 28 volts.

Dan
Aug 14, 2007, 01:35 PM
Registered User
Dan Baldwin's Avatar
The FET can only handle a gate voltage (VGS) of 16 volts, so I added a voltage divider to reduce the maximum gate voltage to about 12 volts at 28 volt battery voltage. I also changed the transistor to one that's rated for a higher voltage. The values shown in this schematic should give you an adjustment range of about 21.5 volts to about 28 volts. I haven't breadboarded this circuit, so I can't promise you that it will work.

If you wanted to use fixed resistors instead of a pot, you could use 19.8K for the upper resistor, and 2.2K for the lower one.

Let me know if it works.

Dan
Aug 14, 2007, 02:03 PM
AndyKunz's Avatar
A 15V Zener would be better than a R divider.

Andy
Aug 14, 2007, 02:36 PM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by AndyKunz
A 15V Zener would be better than a R divider.

Andy
The first time I drew John's circuit up I had a 9.1 volt zener in the gate circuit, but I realized that a voltage divider would give guaranteed turn on and safe gate voltage, and save one component. I still need both resistors in the divider even if I use a zener.

Dan
Aug 14, 2007, 03:41 PM
Registered User
Thanks very much Dan. I wil try your modified cirucuit with the divider.

-John Elliott

Quote:
Originally Posted by Dan Baldwin
The first time I drew John's circuit up I had a 9.1 volt zener in the gate circuit, but I realized that a voltage divider would give guaranteed turn on and safe gate voltage, and save one component. I still need both resistors in the divider even if I use a zener.

Dan


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