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  #1  
Old 09-23-2008, 12:13 AM
kawadave kawadave is offline
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Default 555 timer as a 12 volt low voltage shut down

Hi everyone,

Is it possible to use a 555 timer to open a small 12VDC circuit when the system voltage drops to 9 volts. The system normally operates between 13V down to 10V under load (1.5amp motor).
I'm using a small rechargeable .8 amp sealed lead acid battery as the power source, and a small 12V receiver for wireless power connection to a micro motor.
The receiver has a constant power draw on the battery.The constant power draw during operation of 3 hours is no problem. However people forgetting to disconnect the power after use is. The receiver drains the battery (in around 24 hours) to 0V. Everyday recharging is not possible after that.
Can a 555 timer be wired to shut the system down around 9V, and then resupply power when the battery is recharged? Is there something even simplier I could use? These will be used in large quantities and need to be easy to assemble and inexpensive.

I greatly appreciate any help.

Dave
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Old 09-23-2008, 03:02 AM
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SgtWookie SgtWookie is offline
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If you want the SLA battery to really last a long time, you shouldn't let it discharge more than 30%.

Many systems allow the battery to discharge down to 50%. That's a trade-off between capacity and battery life. Basically, the deeper you discharge it, the shorter the life will be (the fewer discharge/charge cycles you'll get out of it).

A brand-new SLA battery will charge up to around 12.7v-12.8v. They might take a "float charge" up to around 13.4v, but that goes away about an hour or two after the charger is removed, with no load applied.

Many manufacturers consider them discharged at 11.4v-11.7v.

Batteries begin sulphation at around 12.4v-12.5v. It's a white gook that gets deposited on the plates, which effectively reduces their surface area. The longer the battery stays below the sulphation voltage, the more time the gook has to accumulate.

SLA batteries should be charged at a rate of around 1/10 or less of their AH rating. If you charge them too fast (like by using a charger for car batteries) you will create bubbles in the gel that won't ever go away, and that will reduce the cells' capacity.

All that being said, the 555 draws 3mA to 10mA quiescent current depending upon the voltage that's powering it. Not really something you want to have dragging the battery down more quickly.

Something that could extend the battery life dramatically is an astable timer that would run the transmitter for a relatively short period of time, and then power it down for a relatively long period of time; just transmit occasionally. Much of the time this is sufficient.

You need to check the FCC regulations (or authority in your country, if not in the USA) to verify what the duty cycle is that you are permitted to transmit on the given frequency; they are extremely harsh if you violate the rules. Some fellow awhile back wanted to put GPS location transmitters on rescue dogs; turned out that they could only transmit up to 1 second out of 30 seconds for the frequency they were using.

In the USA, fines can range up to $10,000 per incident, confiscation of all transmitting equipment and jail time. They do not "play nice"; ignorance is no excuse.

I realize that this doesn't exactly answer your question; however there are some things that need to be answered before proceeding.
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Old 09-23-2008, 03:25 AM
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thingmaker3 thingmaker3 is offline
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I think you would be better of with a comparator & zener than with a 555. The zener will give you a reference point which can be compared with 1/2 or 1/3 or whatever fraction of the system supply voltage.
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Old 09-23-2008, 04:17 AM
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I agree with Thingmaker about the Zener/comparator combo.

You could use something like a 1N4689 5.1v 50uA Zener in series with a 160k resistor to set a reference level. You could use the comparator output to hold a P-channel power MOSFET's gate held low; have the gate tied to a 20k resistor to the + terminal. When the comparator trips, the gate of the MOSFET gets pulled high which disconnects the battery.

You'll need a reset button to pull the gate low after charging the battery back up.

Use a Shottky diode to supply a capacitor that powers the receiver. Monitor the transmitter's supply voltage. That will help eliminate false triggers due to temporary surges from the motor load.
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Old 09-24-2008, 01:59 AM
kawadave kawadave is offline
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Default a comparator & zener

Quote:
Originally Posted by thingmaker3 View Post
I think you would be better of with a comparator & zener than with a 555. The zener will give you a reference point which can be compared with 1/2 or 1/3 or whatever fraction of the system supply voltage.
Thanks for your reply.

WOW! Thinkmaker and SgtWookie, have left my head spinning!

I understand about 20% of what both of they have explained (That being the battery charging and discharging). Please be patient with my situation and bear with me. A "low voltage shut down" will work just as well as the timer.
What I need is a very simple device (maybe something like some sort of a transistor soldered into the positive side of the battery wire), that will allow 13V down to 9.1V to operate the system. When the voltage in the battery drops below 9 volts, the transistor opens the circuit and prevents any current drain below 9v.
After the battery is recharged, the transistor allows current flow again until the voltage falls below 9.1V again. The motor draw is 1.5 amps (if that matters). So on and so on.
Is this what is meant by a comparator & zener?? If so can you tell me exactly what to purchase to get to a 9V shut down. Any part numbers and simple installation instructions would be great.

Producing the system in numbers, I need to purchase a transistor device that will open the circuit, solder it into the system and go.

You guys (and gals) on this site are great!!

Dave
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Old 09-24-2008, 04:51 AM
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SgtWookie SgtWookie is offline
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It's not quite as simple as you'd like it to be.

Refer to the attached schematic.

Q1 is a P-channel enhanced power MOSFET, good for up to about 8A current, maximum Vdss is -55v. There's a 20k resistor that holds the gate at the same potential as the drain by default, which keeps the MOSFET turned off. You'll notice that right off the drain of the MOSFET is a "+Supply" label - that's where the receiver and motor supplies get connected.

S1 is the "reset button" - it gets everything rolling. When it's pressed, power flows from the battery + terminal through R4 to charge up C1, and D2 to provide power to the comparator and the Zener reference divider.

As soon as the comparator "powers up" and sees that it's noninverting (+) input has a higher voltage than the inverting (-) input, the output is pulled low, which turns on the MOSFET, connecting the battery + terminal to power the rest of the circuit. D2 prevents the MOSFET from back-charging C1 through R4 after S1 is released.

C1 and R3 are rather large values; that causes them to average out the available voltage over time. Otherwise, sudden motor loads might cause the comparator to trip early, shutting the whole thing down. R3 is a pot used to set the precise voltage at which the comparator trips.

D1 is a standard Zener diode. It requires about 20mA through it to maintain a reasonably steady voltage reference; that's why R1 is so low in value. A 1N4689 5.1v 50uA Zener in series with a 160k resistor would be a much better arrangement (far less drain on the battery) but the simulator I was using didn't have one, and I simply didn't feel like tracking down a Spice model for it.

The fuse is necessary. Without it, you could wind up having a fire.

The simulation shows an LM339 quad comparator. You only need a single comparator. However, you could use something like an LM393 or LM2903 dual comparator, and tie one input to the +V in (Vcc) and the other input to ground; that way it won't be triggering on it's own.

Once the comparator trips due to the battery being low, it's output basically floats. The 20k resistor pulls the gate of the MOSFET up to whatever the drain voltage is, effectively acting as a switch - anything connected to the drain is isolated from the battery.
Attached Images
File Type: png ComparatorPowerOff.PNG (14.6 KB, 165 views)
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Last edited by SgtWookie; 09-24-2008 at 04:59 AM.
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Old 09-24-2008, 11:18 PM
kawadave kawadave is offline
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Thanks again SgtWookie,

Can the attached schematic be purchased as a completed unit or does it require assembly?

David


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Old 09-24-2008, 11:59 PM
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Quote:
Originally Posted by kawadave View Post
Can the attached schematic be purchased as a completed unit or does it require assembly?
You would need to purchase the individual components, have circuit boards made, assemble the boards, test them, and calibrate via R3 for power-off threshold.

I do not know of an existing single-IC solution for what you seek.

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  #9  
Old 09-25-2008, 02:43 AM
kawadave kawadave is offline
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I really appreciate all the info you and others have provided. Thank you.

I never intended to do any advertising with my signature.

Thanks again.

David
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