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Author Topic: 9V batteries keep blowing up  (Read 1372 times)

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Offline ShagsTopic starter

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9V batteries keep blowing up
« on: February 22, 2009, 09:09:50 AM »
Ok I have a problem that just does not make sense but since I am a newbie I thought I would throw this up and see if I can get some pointers.

I had and I say HAD two 9V Duracell batteries (Alkaline not NiMh) that I hooked up to my bread board.  In line I have either a SPDT or a DPDT switch (how do I tell, it has three leads but three positions one marked 1 the other marked 2 with the center position as the third).  Wierd thing is that as long as the switch is in the 'ON' position and my circuit is receiving current, the 9V battery is just fine but once I turn the switch to the 'OFF' position it is only a matter of time before the 9V blows.  I don't think it is a Duracell thing (I read some posts on the Net that said they were prone to spontaneous blowing up) because when I hooked up the second Duracell it also blew.  Again if the switch is in the on position everything is hunky dory but as soon as it is switched off it is like the battery is shorting somehow and getting really hot and then boom (sounds kind of like a firecraker).

I checked voltage coming back to the battery and got back a very minute amount of voltage (I thought ok its somehow getting charge back to the battery and trying to recharge an alkaline).  I am not sure if this is because of my voltmeter or if I can truly trust it.

One last idea, is it possible the switch is defective or is there a limit on the amount of voltage that can be passed through some of them.  Could current somehow jump the open circuit.

I have since taken the switch out of the circuit to protect my new NimH batteries.

Matt (aka Shags, hey you have to keep at least one nickname)

Offline paulstreats

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Re: 9V batteries keep blowing up
« Reply #1 on: February 22, 2009, 09:52:51 AM »
So, I bet you have both of the batteries terminals connected to the same switch

Offline ArcMan

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Re: 9V batteries keep blowing up
« Reply #2 on: February 22, 2009, 01:21:47 PM »
This is definitely the problem.

...SPDT or a DPDT switch (how do I tell...

You are, no doubt, short circuiting your battery.  I don't mean to be harsh, but you need to at least learn basic electricity before you hook up a circuit.  Not even knowing what kind of switch you're using to switch power is a sure-fire recipe for disaster.

Offline Soeren

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Re: 9V batteries keep blowing up
« Reply #3 on: February 22, 2009, 01:50:36 PM »
Hi,

In line I have either a SPDT or a DPDT switch (how do I tell, it has three leads but three positions one marked 1 the other marked 2 with the center position as the third).
SPDT = Single Pole, Double Throw
DPDT = Double Pole Double Throw

Single Pole: only one lead/wire is switched
Double Pole: Two poles are switched
Double Throw: It can be switched to one out of two contacts
Center position can be off or something else - like ground, if the switch metal is grounded.

What you have might be a SPDT with Center Off, but since it obviously connects to something, you probably have a Single Pole Triple Throw.


[...] once I turn the switch to the 'OFF' position it is only a matter of time before the 9V blows.

Measure the resistance between the pins and between each pin and the metal on the switch - do it for each switch position.
It sure sounds like you are shorting the battery when you put the switch in the middle position.

Could you post pictures and/or drawings of what you did?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline ShagsTopic starter

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Re: 9V batteries keep blowing up
« Reply #4 on: February 22, 2009, 02:24:45 PM »
Ok I was wrong on the three positions (sorry about that). It definitely has two positions with three terminals.  I have since seen the error of my ways but I am man enough to post it here to the ridicule of all just so maybe it will help another electronics newbie.  Please correct me if I am wrong here.

I initially had the switch wired like this:  1. I had the positive hooked to the same terminal on the switch, lets call it the left terminal.  To correct this I really needed the positive from the battery hooked to the left terminal and have the positive going to my first component hooked to the center terminal to allow the positive to be connected only when the switch was in the left position.  This issue led me to my second mistake.  2. I had the negative hooked up to the switch as mentioned in two of the replies.  This appeared to work because as I had it hooked up if I moved the switch to the right (positive were both hooked to the center terminal the light would come on my led and if I moved it to the left it would go off.  Unfortunately when it was off the switch was just looping the current back to the battery.  When it was on it was ok because the current was eaten up by the components and the resistor.

Now the question for those more astute in electronics.  Would the circuit have been fine if I had hooked the negative to the left terminal on both the component and the battery side and staggered the the positive by hooking the positive from the battery to the left most terminal and the component positive to the center terminal.  in the position to the right there would not be a completed circuit, but to the left it would complete the circuit.  Because the net back to the battery is always 0 wouldn't this work correctly as well. (I am not saying it is correct but would it work, remember this is just for educational purposes only don't just say don't do it.)

Thanks all the terminal was correct but I also needed to stagger the positive or negative in order to make the switch work correctly which brings up one more question for you all, is it more correct to connect your switch to the negative side or the positive side?

Thanks again
Matt (aka Shags, hey you have to keep at least one nickname)

Offline paulstreats

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Re: 9V batteries keep blowing up
« Reply #5 on: February 22, 2009, 06:28:15 PM »
Quote
when it was off the switch was just looping the current back to the battery.

I think it was the other way around. when the switch was off, it prevented the electricity looping back into the battery causing the circuit to work correctly. when the switch was on it connected the outputs of the battery together. ssssssssssssssssssssssizzle ;D

Offline Soeren

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Re: 9V batteries keep blowing up
« Reply #6 on: February 22, 2009, 06:50:17 PM »
Hi,

I don't quite get what you wanna do, but you should not connect both positive and negative to the switch, if that is what you are trying.

You should connect it up like this:


It is a common convention to switch the positive, since we use the 0V as common/ground and you should "allways" keep the common plane connected (in some cases this is your protective earth). That said, when working with a battery circuit of your own that will never have other people working with it, there is no difference in switching one or the other, so go ahead and break convention if that will amuse you :P
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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