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  #1  
Old 11-29-2009, 12:00 AM
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Default Low voltage battery detector

Hi everyone


I want to build a low voltage detector for 12 volt batteries, this is for stand by equipment, generators, welders, light towers. My thinking is if the battery drops below 8 volts on startup then the battery needs to be pulled and charged then tested.

The current draw before it detects a low voltage occurrence has to be under 1mA. When the battery dips below 8 volts a red LED will flash every 5 seconds, the detector will not reset until it is powered down and up again.

I was going to start with these parts, a blinking LED and a voltage detector, from there I need some way to latch the LED on until it is reset by disconnecting it.

http://elexp.com/a_data/08L36BID_08L36BGD_08L36BYD.pdf

http://ww1.microchip.com/downloads/e...Doc/21434h.pdf

First is there better components to use to do this project and how do I latch it on when it dose detect a low voltage.
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Old 11-29-2009, 01:05 AM
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The Microchip part you're looking at certainly has a low current, but they don't have a version that will operate on more than 10v.

Take a look at this: http://www.onsemi.com/pub_link/Collateral/MC34161-D.PDF
A universal voltage monitor IC. Vcc can go up to 40v. It has a built-in voltage reference and a couple of open-collector output comparators.

Mouser carries them in both DIP and SMT packages. Here's a DIP package version:
http://mouser.com/ProductDetail/ON-S...KHqnFrwxMP8%3d

Oh, and your threshold for low battery voltage is off by quite a bit.

A charged lead-acid battery will have a voltage of about 12.7v-12.9v @ 25C when it's fully charged, depending on chemistry.
Once it gets down to about 12.5v, plate sulphation begins. If the plates get heavily sulphated, the battery won't hold or accept a charge.
By the time it's down to 11.4v, it's considered completely discharged.
If it's below 11v, it's likely that it has one or more dead cells.
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Old 11-29-2009, 01:29 AM
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Quote:
Originally Posted by SgtWookie View Post
Take a look at this: http://www.onsemi.com/pub_link/Collateral/MC34161-D.PDF
A universal voltage monitor IC. Vcc can go up to 40v. It has a built-in voltage reference and a couple of open-collector output comparators.
I have some 34161 I will start with them for this project, thanks. This is a starting battery and I wanted to monitor the voltage under load, when the engine is starting. These units are started weekly and the starter can be used as a load tester.

I may up the voltage some, above 8 volts, but using a graphing oscope I see about 9 volts sometimes when the starter first kick in on some of the generators.

The 34161 will draw about 800uA which is good, but how do I latch the out put high or low until I reset it.
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Old 11-29-2009, 01:47 AM
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See the sample schematic on page 1?

Instead of connecting pin 6 to an LED, connect it to pin 2 via a NC pushbutton switch.

When the battery voltage drops below the threshold you've set using a resistive divider, pin 6 goes low, which pulls the input of the comparator low; it's then latched.

Press the pushbutton to turn it off.

Also, the comparator can only sink 10mA per output. Since you want to drive a bright LED that'll take 20mA or 30mA, you can use the other comparator output to pull the base of a PNP transistor (2N3906, 2N2907) low via a resistor; sinking about 3mA current. A 3.9k resistor would do fine.

Then, connect the PNP's emitter to +12v, and a current limiting resistor to the collector. 470 Ohms should limit current thru your LED to about 26mA. Connect the anode of the LED to the current limiting resistor, cathode to ground.
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Old 11-29-2009, 02:00 AM
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Originally Posted by SgtWookie View Post
Instead of connecting pin 6 to an LED, connect it to pin 2 via a NC pushbutton switch.
Wow, I'm impressed, I don't know how you see that but I'm going to try it tomorrow morning.

I think what your doing is pulling pin 2 low to reset it, is that correct? I was looking at Figure 20.

Thanks SgtWokie, more on this tomorrow, it's my bed time.
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Old 11-29-2009, 02:46 AM
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Well, I was close - but it needed changing a bit.

The reference doesn't get used; pin 7 is grounded. That makes both internal comparators function identically.

The upper comparator output (pin 6) is used to ground the voltage divider coming from the battery.
The lower comparator output (pin 5) is used to turn on the LED via the PNP transistor.

I didn't have the MC34161 available as a model, so I threw one together out of some opamps, logic IC's and voltage sources.

Like this:


The trip voltage is off by just a tad, but it's right about 8v.
I chose 120k and 22k for the voltage divider to keep the current requirements within your 1mA limit.
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File Type: png MC33161 Battery Monitor - 8v.PNG (18.8 KB, 216 views)
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Old 11-29-2009, 03:03 AM
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Quote:
Originally Posted by Smoke_Maker View Post
Wow, I'm impressed, I don't know how you see that but I'm going to try it tomorrow morning.
I've sorta been at this for awhile.

Quote:
I think what your doing is pulling pin 2 low to reset it, is that correct? I was looking at Figure 20.
I didn't bother looking at all the applications stuff; I saw what I needed to see on page 1 and the electrical specifications.

Took me another minute or two to figure out the logic of using the reference, though - decided you didn't need it, as by grounding the internal comparator's inverting inputs, both outputs function identically, which is what you needed.

When the voltage on the divider R1/R2 falls below the internal 1.27v reference, both output transistors get turned on. The upper output is connected to the middle of the voltage divider via S1, a NC pushbutton. Since the divider voltage is shorted to ground, it won't rise above the 1.27 internal threshold reference, which keeps the output transistors turned on.

Pressing S1 allows the voltage divider to jump back up, which turns off the output transistors.

Total power consumption (with LED off) will be typically around 600uA, but not over 950uA.

Sounds like it'll meet your requirements, right?
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Old 11-29-2009, 07:35 PM
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I connected it several different ways and it still not what I need. I followed your schematic first and there was no LED light, then I wired it like figure 20 to make sure the detector was good. What I ended up with is in the schematic. I connected the LED to pin 5 and grounded pin 6, no change, then the LED to pin 6 and grounded pin 5.

The way it is weird it will light the LED when the voltages to 8 volts, but grounding pin 6 dose not change the operation. If I use pin 6 the LED go's out when the voltage reaches 8 volts.
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File Type: png Low battery voltahe detector 2.PNG (6.2 KB, 70 views)
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Old 11-29-2009, 07:57 PM
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You're showing an MC64161 in the schematic. I don't know what that is.
Did you mean an MC34161?

If so, your R2 is much too low; if R1 is 100k then R2 should be about 18.9k to trip near 8v. If you are testing using a 12v bench supply, you can simulate the voltage dropping to 8v by decreasing the value of R2.

The internal comparators trip at about 1.27v, with some hysteresis. You adjust R1/R2 to match that trip voltage.
18.9k/(100k+18.9k) = 0.159
8v * 0.159 = 1.272v; just about the internal comparator reference voltage.
If you want it to trip at 12v, change R2 to 11.9k. As you decrease R2, it will take less voltage on Vcc to trip it.

Pins 5 and 6 are open collector outputs. They will be either connected to ground via the internal transistor, or floating (open).

Connect output 6 to pins 2 & 3 using a switch. Open the switch to reset the circuit.

You have exceeded the current rating for pin 5 by a considerable amount; it's only rated for 10mA maximum. If you are powering the circuit from 12v and your LED has a Vf of 2v, current through D1 will be > 45mA. You have likely burned up the pin 5 output, if not the LED as well.

That is why I used a PNP transistor to source current to the LED.

If you go back to my schematic, you can eliminate that R4 if you wish; that is simply to make sure that the PNP transistor is turned off.

Pin 7 should be connected to ground. If it is 'floating' or connected to something else besides ground, the logic will not function correctly.
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Last edited by SgtWookie; 11-29-2009 at 11:13 PM. Reason: Corrections; R2 high/low relationship
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Old 11-29-2009, 08:04 PM
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Quote:
Originally Posted by SgtWookie View Post
You're showing an MC64161 in the schematic. I don't know what that is.
Did you mean an MC34161?
Woops, it is a MC 34161
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