voltage cut off switch - Page 3 - RC Groups
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Aug 16, 2007, 06:08 AM
Registered User
The MAX933 IC has an internal 1.182V +- 2 % reference , make it more easy.

Here is a example, for battery > 6V
http://www.geocities.com/ilufa/discharger_E.htm

just replace "the 390 ohm and LED" with your loading,
and replace the "220 & 22 ohm" with "22k & 2.2K"
It's cut off may adj. by mod the 2.2k
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Aug 16, 2007, 09:21 AM
Registered User
Quote:
Originally Posted by ilufa
Hi Philip,

thanks for pointing out how to do this with a MAX933. I seem to be having some trouble understanding the push-button start switch: shouldn't it connect the cathode of D1 with the positive rail of your circuit instead, while the current start switch contacts are permanently connected?

Best regards,

Fred
Aug 16, 2007, 10:32 AM
Master of 1 point landing
Well, its getting closer and closer to winter and I was thinking about buying pdaniels lipo discharger - but unfortunately the price shot up. Obviously its a great unit, but Im cheap.

In looking at the diagram for this discharger in this thread, but being sort of lazy- I began to wonder . . .


Most of use know that consumer electronics with LIPOs have to have a PC board that prevents over discharge, over charge, and over draw. These boards are dirt cheap and typically have a 3V LVC - too low for storage. However, I am at a bit of of a loss on what the pitfalls are to this logic:

Buy a handful of these boards

Hook a diode up to the input so the board is seeing bat voltage - vdrop (match em so the v drop is the same)

Sink the current using a 10 watt 10 ohm resistor. For about $6.50 - you have a discharger that will draw your cells down to 3.6 - 3.7V/cell and automatically stop. You can put an LED before the resistor to tell ya when its done.


Opinions?
Last edited by ImaBiggles; Aug 16, 2007 at 11:04 AM.
Aug 16, 2007, 10:55 AM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by fredhs
Hi Philip,

thanks for pointing out how to do this with a MAX933. I seem to be having some trouble understanding the push-button start switch: shouldn't it connect the cathode of D1 with the positive rail of your circuit instead, while the current start switch contacts are permanently connected?

Best regards,

Fred
The schematic definitely seems wrong I think this is what he meant.


Dan
Aug 16, 2007, 11:28 AM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by ImaBiggles
Well, its getting closer and closer to winter and I was thinking about buying pdaniels lipo discharger - but unfortunately the price shot up. Obviously its a great unit, but Im cheap.

In looking at the diagram for this discharger in this thread, but being sort of lazy- I began to wonder . . .


Most of use know that consumer electronics with LIPOs have to have a PC board that prevents over discharge, over charge, and over draw. These boards are dirt cheap and typically have a 3V LVC - too low for storage. However, I am at a bit of of a loss on what the pitfalls are to this logic:

Buy a handful of these boards

Hook a diode up to the input so the board is seeing bat voltage - vdrop (match em so the v drop is the same)

Sink the current using a 10 watt 10 ohm resistor. For about $6.50 - you have a discharger that will draw your cells down to 3.6 - 3.7V/cell and automatically stop. You can put an LED before the resistor to tell ya when its done.


Opinions?
The overdischarge voltage threshold for the protection circuit you referenced is 2.5 volts. If you add a diode in series, the threshold rises to about 3.2 volts, which is still too low for lipo storage. You could add two diodes, or some combination of silicon diodes and schottkey's.

Dan
Aug 16, 2007, 11:34 AM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by ilufa
The MAX933 IC has an internal 1.182V +- 2 % reference , make it more easy.

Here is a example, for battery > 6V
http://www.geocities.com/ilufa/discharger_E.htm

just replace "the 390 ohm and LED" with your loading,
and replace the "220 & 22 ohm" with "22k & 2.2K"
It's cut off may adj. by mod the 2.2k
The TL431 I used in my design also has an internal reference. It's 2.5 volts 1%

Dan
Aug 16, 2007, 11:38 AM
Registered User
Quote:
Originally Posted by Dan Baldwin
The schematic definitely seems wrong I think this is what he meant.


Dan
Hi Dan,

I fully agree with your version of the circuit diagram.

Regards,

Fred
Aug 16, 2007, 11:40 AM
Master of 1 point landing
Quote:
Originally Posted by Dan Baldwin
The overdischarge voltage threshold for the protection circuit you referenced is 2.5 volts. If you add a diode in series, the threshold rises to about 3.2 volts, which is still too low for lipo storage. You could add two diodes, or some combination of silicon diodes and schottkey's.

Dan
yep, I confused that with the release threshold value. So if using a combination of diodes to get to 3.6-3.7V, it should work right?
Aug 16, 2007, 04:34 PM
kit
kit
Registered User
Hi Dan,

Could you tell or show me where to add an LED to the circuit in post #22 as a reminder that I have
a pack plugged into it? Got the PC board made, just have to start soldering now.

Thanks
Aug 16, 2007, 06:14 PM
Registered User
Dan Baldwin's Avatar
If you want an LED to show you that the discharge is still taking place, you could put the LED (with a 570 ohm or so resistor in series, of course) across the load. If you want an LED that's on all the time to remind you that you have a pack attached, you can put it (again with a resistor in series) across the battery leads, but remember that it will constantly drain the battery pack even after the discharger trips out. Since the discharger has basically no drain when it's off, it doesn't matter if you forget and leave the pack on it for weeks.

Dan
Aug 16, 2007, 09:02 PM
kit
kit
Registered User
Thanks Dan, no Led needed then.
Aug 16, 2007, 11:52 PM
Registered User
Quote:
Originally Posted by Dan Baldwin
The schematic definitely seems wrong I think this is what he meant.


Dan
Dan,

Thank you
You are right, I loose one connection in the schema,

iLuFa
Last edited by ilufa; Aug 17, 2007 at 12:42 AM.
Aug 21, 2007, 10:08 PM
kit
kit
Registered User
Hi Dan, sorry to bother you again.

Since I bought a couple extra pieces of TL431ACZ, how would I go about using one to make a precision 5 volt, and 10 volt, 1% voltage references to calibrate my DVMs, etc. Thanks in advance.

Soldered my SM PC board discharger together today and after remembering to put the jumper wire in place , it now works fine for discharging 2S and 3S packs.
Aug 22, 2007, 10:24 AM
Registered User
Dan Baldwin's Avatar
Quote:
Originally Posted by kit
Hi Dan, sorry to bother you again.

Since I bought a couple extra pieces of TL431ACZ, how would I go about using one to make a precision 5 volt, and 10 volt, 1% voltage references to calibrate my DVMs, etc. Thanks in advance.

Soldered my SM PC board discharger together today and after remembering to put the jumper wire in place , it now works fine for discharging 2S and 3S packs.
You'll need to use precision resistors for your voltage reference. If you use 1% resistors with your 1% TL431, you could be off by considerable more than 1%, so you will need to use .1%. For the 5 volt reference, the two resistors in the voltage divider will be the same value (perhaps 2.2K). For the 10 volt reference, the upper resistor needs to be three times the value of the lower resistor. You could use 4 of the same value resistors, three in series on the top, 1 on the bottom for the 10 volt reference, and short out two of the three on the top for 5 volts. If you have a whole bunch of the same value resistor (app 1K to 5K), you could sort through them with your ohm meter, and select 4 resistors that are close to the same value. It doesn't matter what the exact value of the resistors are, they just need to be the same.


Dan
Aug 22, 2007, 11:31 AM
Don L.

Another Approach


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.
Last edited by dleroi; Aug 22, 2007 at 12:39 PM.


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