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sparky007
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Join Date: Oct 2011
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Size of a Resistor

01/19/2012 3:59 AM

Good day,

I am currently sitting with a situation regarding my alarm system at my house.

The sensors of the alarm system is operated via two 3V batteries in series and are currently under volt which causes the system not to work. There are no 6V batteries and we must buy two 3V cells in series which the price of one is R80. Each. The price for a new sensor is R200.00 which is close to the price of the batteries. The price for a 9V battery is R20.

1)Is there a way that I can connect a resistor ?

2)And if so, where?

3)At the beginning of the battery or close to the led?

4)How will i Know what size of resistor must i connect?

5)Will there be any damage to the sensor if i connect the 9V battery without an resistor?

Do you guys think it is possible or may the case be?

6)How does the colour coding work with the resistors?

Thanks
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PWSlack
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#1

Re: size of a resistor

01/19/2012 4:03 AM

A1-4) Yes, though a proper voltage regulator would be more efficient.

A5) Yes, there will.

A6) http://en.wikipedia.org/wiki/Electronic_color_code

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SolarEagle
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#2

Re: size of a resistor

01/19/2012 5:15 AM

9vdc to 6vdc regulator....

http://www.aliexpress.com/fm-store/905606/210894723-448177287/LM7806.html

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czgut
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#3

Re: size of a resistor

01/19/2012 5:50 AM

Probably those 3V batteries are bigger (if more expensive) and last longer than 9V battery. May be 9V battery used in this application will last 10 times shorter than original 2 x 3V. What Type are original 3V batteries? How long they worked?

1. buy 3V batteries , the same type as factory supplied.

2. measure current drawn from batteries I [mA] (or ask technician to to this)

3. read capacity of batteries C [mAh] ( 1Ah = 1000mAh)

4. Capacity divided by drawn current will give You discharge time T = C / I [hours]

If calculated discharge time T [hours]/24/30 gives 5..12 months, then probably everything is OK. That also means that drawn current limits battery life.

If calculated discharge time is few years, then may be self discharge of the battery (battery shelf lifetime) is the limit. You can consider buying Lithium Batteries - they have longer lifetime (up to 10 years) but more expensive.

If calculated discharge time is shorter than 1..2 months, replace whole "sensor". Probably something is wrong.
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Yusef1
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#4

Re: Size of a Resistor

01/19/2012 10:11 AM

The batteries normally last a long time because they do not put out very much current. So therefore the resistance the batteries are facing now is very high. You should be able to measure that resistance with a common meter.

You can reduce a 9 volt output from the battery to 6 volts by installing a resistor in series before the load.

The value of that resistor must be one third the value of resistance the batteries are facing now.

just like that

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Randall
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#6
In reply to #4

Re: Size of a Resistor

01/19/2012 3:06 PM

I think you meant half not a third.

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Yusef1
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#7
In reply to #6

Re: Size of a Resistor

01/19/2012 3:14 PM

Good catch. Yes, it should be a half. Sorry OP.

That would divide the load into three parts. one part new resistor, and two parts original load.

I blame it on the lack of coffee....

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czgut
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#20
In reply to #7

Re: Size of a Resistor

01/20/2012 7:31 PM

... but may be sensor is not just switch with 0V on it? ... led is it LED? If so, needs about 3V. If receiver is supplied from the same battery?
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sparky007
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#12
In reply to #4

Re: Size of a Resistor

01/20/2012 12:49 AM

Will the lifetime of the battery be more shorter if a resistor is connected into the system?

Between a regulator and a resistor which will be the best option?
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Randall
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#5

Re: Size of a Resistor

01/19/2012 3:03 PM

Why not use 4 off AAA, AA, C or D cells?

How big are the 3V cells you are supposed to use?

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cuba_pete
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#8
In reply to #5

Re: Size of a Resistor

01/19/2012 4:39 PM

He is stating monetary units in Rupees, I believe.

Maybe someone can mail him some of these at cost:

http://www.google.com/products/catalog?q=3+volt+battery&rls=com.microsoft:en-US&oe=utf8&um=1&ie=UTF-8&tbm=shop&cid=12787651475944457067#

Maybe the battery mafia in India has driven the cost so high that battery junkies need to find their fixes elsewhere...
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aurizon
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#9

Re: Size of a Resistor

01/19/2012 11:02 PM

The easiest way to do this and be safe it to use series diodes in the forward direction. Each one will drop the voltage by 0.7 volts, so 4 diodes at 2.8 volts will do the job.

The voltage drop is not current dependent. Diodes like the 1N4000 series should be cheaply available. The 1N4000 will do the job

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eltech
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#10

Re: Size of a Resistor

01/19/2012 11:43 PM

R200 is like $10, Dont be a tight arse just go and buy a new one.
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sparky007
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#11
In reply to #10

Re: Size of a Resistor

01/20/2012 12:41 AM

Thanks for that comment ELTECH, it's not about being a tight arse but to pay R200 for 2 batteries is crazy. And this is a way to learn more about electronics.
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Randall
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#14
In reply to #11

Re: Size of a Resistor

01/20/2012 5:30 AM

OK what about a small switching regulator.

http://search.digikey.com/us/en/products/LM2574N-ADJ%2FNOPB/LM2574N-ADJ-ND/212614

OR

http://search.digikey.com/us/en/products/LM2574N-ADJG/LM2574N-ADJGOS-ND/1476675

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Epke
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#13

Re: Size of a Resistor

01/20/2012 2:31 AM

you could use something like this

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Anonymous Poster #1
#15

Re: Size of a Resistor

01/20/2012 8:37 AM

How about a Wall Wort ( AC to DC adapter)

They are available with 6 Volt DC output and a universal input... 50 to 60 Hz & a voltage range of 100 to 240 volts AC.

You might be able to find an old Cell Phone charger with a 6 volt output.

No more batteries required............
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Geoffrey36
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#22
In reply to #15

Re: Size of a Resistor

09/09/2012 11:38 AM

Not a good idea, if I may say so, to have an alarm system which goes off with the mains power!
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WJMFIRE
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#16

Re: Size of a Resistor

01/20/2012 1:16 PM

Did anyone even read post 9. If money is the name of the game, the diode idea is perfect. GA

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Randall
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#17
In reply to #16

Re: Size of a Resistor

01/20/2012 2:35 PM

If he'd suggested 1N4148s I'd have agreed but 1N4000 are going to be pretty expensive.

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34point5
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#18
In reply to #16

Re: Size of a Resistor

01/20/2012 3:13 PM

Diodes are also a good idea as alarms draw different currents depending on what they are doing at the time.

Also meaning it's quite likely it already has a 5V regulator on the PCB. well worth having a look.

I'm not sure if Randal is having a joke on "IN 4000", but, $15 for 175 is hardly expensive.

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JIMRAT
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#19

Re: Size of a Resistor

01/20/2012 6:59 PM

Any country that has a monetary symbol of Rxx has alot of motorbikes with 6v batteries. I dont understand why OP says he can't source 6V batteries.

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Haajee
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#21

Re: Size of a Resistor

01/21/2012 9:53 AM

Regards.

You are giving the prices of batteries but the data of sensors is not known.

Can you find why 2x 3V cells in series can't make 6V. Ate cells placed in a battery case and connected in series through its connecting strips/wire springs

or connected by a wire fitted with push-on connectors.

It may be that you are overlooking the problem in connections anywhere.

or

the cells [if not the exact replacement of originals] are of lower capacity.
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Users who posted comments:

34point5 (1); aurizon (1); cuba_pete (1); czgut (2); eltech (1); Epke (1); Geoffrey36 (1); Haajee (1); JIMRAT (1); PWSlack (1); Randall (4); SolarEagle (1); sparky007 (2); WJMFIRE (1); Yusef1 (2)

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