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Strange connection in Low Voltage Disconnect circuit

Posted on 2014-12-04
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Last Modified: 2014-12-04
I've been searching high and low for an ultra-low power LVD circuit (Low Voltage Disconnect) for batteries and found the attached diagram on the web.
 I think I have a reasonable understanding of the working principle, but there is one particular part that looks strange to me:
Take a look at the connection marked in red. Does it pass through the inverter or does it go directly from switch S1 to ground? The latter seems odd, since this would short-circuit the whole thing once you press switch S1.

I do realise that S1 is only a push-button and not a regular switch, but still......

Being a newbie I would love some help in understanding how this could possibly work in reality.

Thanks
ES

http://electronicdesign.com/power/almost-ideal-low-battery-cutoff-circuit-draws-only-12-mua

Low voltage disconnect-circuit
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Question by:EISTO
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8 Comments
 
LVL 24

Expert Comment

by:Eirman
ID: 40481308
I used to build electronic kits etc, so I am somewhat acquainted with electronic circuits.

I presume 1D is a diode which has two connections so it would seem that the red line goes under the diode,

However, there is a clear path from the bottom to the top to put in a straight line.
But as you observed, that would short the power supply when the relay is closed.

So maybe 1D is not a simple diode and has four connections! This seem the most likely to me.
Do you have a list of parts?

I'm wondering if this is a latching relay ... there is no push to break switch.

For more responses I suggest that you "request attention" and ask the moderator to add this question to the
"science & math" zone. This zone is observed by a lot of electronic boffins.

EDIT: I read the article .. they are not diodes  .... I think they are pin outputs on a chip
and there are only two pin numbers on 1D (8 & 9), so the line appears to go under the 1D.

Strange .. the more I look at that circuit, the more flawed it appears.
Once the relay latches closed, the short circuit would be constant.
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Author Comment

by:EISTO
ID: 40481361
Hi Eirman,

Thanks for the reply.
There is no part list, I'm afraid. However, all components are labeled. The relay and the two ICs are even labeled with model numbers.

No, "1D" is not a diode. All 6 triangle symbols are part of an inverter IC referred to as "IC1" in the diagram. It is still available on the market.

I think what you write in your edit is correct. All the pins on the inverter are clearly labeled in the diagram and there are no labels or connection symbols where the red line touches "1D"
You are also right about the latching relay. The idea is to eliminate continuous current through the coil, as you would have in a regular SPST/NO relay.

Looking closer at the circuit, I can see no reason why the red line is needed at all. Bypassing the relay should be enough to kick-start the comparator and inverter so that the relay latches on....don't you agree?

ES
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LVL 24

Expert Comment

by:Eirman
ID: 40481387
Silly me ... a diode is a triangle and a line!

Looking closer at the circuit, I can see no reason why the red line is needed at all. Bypassing the relay should be enough to kick-start the comparator and inverter so that the relay latches on....don't you agree?
I don't have enough knowledge of electronics to agree or not, but that red line definitely seems totally wrong.

Why not ask a question the forum that hosts the diagram
http://electronicdesign.com/forum

You might be lucky and get to communicate with the original designer.
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Author Comment

by:EISTO
ID: 40481425
Thanks again, Eirman. You're definitely not more silly than me. This is my first encounter with ICs and it's been a steep learning curve to put it mildly.

I'll try to ask a question at the original forum, but the (anonymous) person who posted the circuit seems to have left around 2005. The post itself is nearly 15 years old. Internet is getting old :-)

ES
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LVL 83

Accepted Solution

by:
Dave Baldwin earned 500 total points
ID: 40481435
The red line is just the power connections for that IC, nothing more.  Pin 14 goes to the positive supply and pin 7 goes to ground.  It could have been drawn better.
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Author Comment

by:EISTO
ID: 40481463
Aaaahhhhhhh.....
You're absolutely right. Checked against the data sheet i found on Mouser, and pins 7 & 14 are indeed GND and V+ respectively. They just aren't labeled on the diagram.

Thanks for your help!

ES
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Author Closing Comment

by:EISTO
ID: 40481464
Quick feedback. Completely solved.
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LVL 83

Expert Comment

by:Dave Baldwin
ID: 40481710
You're welcome, glad to help.
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