need a 9v cut off switch, any ideas?

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dalbert02
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need a 9v cut off switch, any ideas?

Post by dalbert02 » Tue Mar 21, 2006 11:56 am

Hi,
I need a circuit that will turn off if the battery reaches 9v. Specifically, I am using 3 lipo batteries to power video equipment and I don't want to over discharge my lipos. Any ideas? Thank you very much, your help is much appreciated.
-dave
"one carefully controlled experiment is often times worth more than a thousand expert opinions"

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Chris Smith
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Re: need a 9v cut off switch, any ideas?

Post by Chris Smith » Tue Mar 21, 2006 12:23 pm

A zener circuit will do the trick.

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Edd
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Re: need a 9v cut off switch, any ideas?

Post by Edd » Tue Mar 21, 2006 2:08 pm

Digging down deeeep within my circuit archives:

http://www.elecdesign.com/Article/Artic ... /1823.html

Look at this … lauded for simplicity EDN circuit… and trim in to YOUR voltage parameter specs and see if it might fulfill your needs.


73's de Edd
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<img src="http://www.hdtvoice.com/voice/images/smilies/worthy.gif" alt=" - " />.....Opportunity is missed by most people because it is dressed in overalls and looks like work........ Thomas Edison


HOLD IT...the key sentence being a disconnect as in "I need a circuit that will turn OFF...(misread as WHEN)... if the battery reaches 9v"... I will look a bit further for a disconnect circuit...ZUJ ...but hold onto that charger circuit link if required in the future !

<small>[ March 21, 2006, 02:40 PM: Message edited by: Edd Whatley ]</small>

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philba
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Re: need a 9v cut off switch, any ideas?

Post by philba » Tue Mar 21, 2006 3:26 pm

I'd use a PMOSFET and a comparator with a precise reference. Here is something I worked up a while ago. it simulates ok but I've never actually built it. sorry for the ugly picture:
<img src="http://www.geocities.com/pcbs4less/batprotect.gif" alt=" - " />

the comparator takes as input a 2.5V reference and a divided down battery voltage. When the divided down Vbat falls below 2.5V the comparator switches on and turns the PMOSFET off. The advantage of a PMOSFET is you can get one with a very low Rds and thus avoid much voltage drop.

This was set up for 12V as the switch point so you will need to change r1 and r2 (9V in, 2.5V out). also, you will need to find a voltage reference chip. I'd use as low power one as I could find. You can use any value you want but there are lots of 2.5V ones around. The PMOSFET can be almost any but it should have a low Rds(for a Vgs of -9) and be able to handle the load current.

R5 and C1 aren't necessary. I use them to buffer the Vbat sense in case there are large transients. If your gear doesn't have sporadic transient current draw, you can eliminate. I think it would be a better idea to add schmitt feedback on the comparator to create a little hysteresis to avoid transients from causing chattering the power line. I was trying to avoid current drain in the off state.

Note that this will continue to draw current on the order of 20-30 uAs so you might want to move the cut-off a little higher. Comparator should be as low power as you can find.

[edit] More on component selection.

The MOSFET and comparator part numbers are just what my SPICE simulator (switcher cad) has in its library, pay them no heed.

I like the TI nano power comparators like the TVL3701 - 560 nA current draw.

The voltage ref could be a zener but they are not super accurate and being off by 5% means you could be 18% off (because of the divider). Maxim has a lot of low power V refs but you could probably live with the LM336. The AME385-2.5 looks pretty good - claims to go down to 20 uA but I've never used it.

The PMOSFET can be almost anything. I use a lot of the Si4401 but you might want something with more current capacity like the STD10PF06T4 (about .2 ohms Rds in a to220), FDS6675 (less than .19 Rds), and so on.

A note on battery voltage. To use this circuit below around 8V will require a "logic level" PMOSFET as the normal ones need around 10V to switch on fully.
[/edit]

<small>[ March 21, 2006, 09:52 PM: Message edited by: philba ]</small>

Engineer1138
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Re: need a 9v cut off switch, any ideas?

Post by Engineer1138 » Wed Mar 22, 2006 12:31 pm

Won't that circuit oscillate? The scenario I'm thinking of is batt. voltage drops below threshold and the load disconnects. Now that the load's gone. battery drifts up a bit and the circuit reconnects the load, causing the battery voltage to drop. Lather, rinse, repeat.

Probably want to latch the off state.

Robert Reed
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Re: need a 9v cut off switch, any ideas?

Post by Robert Reed » Wed Mar 22, 2006 1:39 pm

I agree with 1138. A GENEROUS amount of hysteresis might do the trick here.

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philba
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Re: need a 9v cut off switch, any ideas?

Post by philba » Wed Mar 22, 2006 2:36 pm

yes, that is why I suggested the schmitt feed back.

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Re: need a 9v cut off switch, any ideas?

Post by Gorgon » Wed Mar 22, 2006 3:27 pm

Why not power the circuit from the load side, and 'kick-start' it with a pushbutton when the battery is charged. To make the driver hi-z, just add a transistor, a resistor and invert the comparator output. This should make a stable off condition, the transistor will release the gate when power disappear.

TOK ;)
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Chris Smith
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Re: need a 9v cut off switch, any ideas?

Post by Chris Smith » Wed Mar 22, 2006 3:27 pm

Reverse Crowbar circuits.........

Use a SCR as the connecting piece.

Once it fails, it cant reconnect.

All you need to reconnect it is a momentary switch to latch it, and the voltage does the rest.

The zener [or which ever circuit] sets the fail point.

Also....Check out the MC3425 and see...

http://www.datasheetcatalog.com/onsemiconductor/199/

<small>[ March 22, 2006, 06:08 PM: Message edited by: Chris Smith ]</small>

dalbert02
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Re: need a 9v cut off switch, any ideas?

Post by dalbert02 » Thu Mar 23, 2006 7:28 am

Thanks guys, I appreciate your fast and detailed response!
-dave
73
KI4HVT
"one carefully controlled experiment is often times worth more than a thousand expert opinions"

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