Blowing a 3A fuse with an AA battery?

[Esoremada]

I'm completely new to circuit building and am starting off with this book, but I'm already having trouble with experiment 2, which seems pretty straightforward
https://docs.google.com/file/d/0B5Gl6fAWcS-_bFVrODRwSm5ZMUE/edit?pli=1

On page 11 it says to connect a AA battery directly to a mini blade fuse to see the filament break, but it just isn't happening. I know the current is going through the fuse because when I disconnect the blade fuse from one battery terminal and measure the potential difference it is there. I even tried a 9V battery and nothing

 

[Anon_LG]

Try putting your multimeter in series with the battery and the fuse, check the current using the current setting. There could have been a manufacturing fault with the fuse so try a new fuse, if there is current the fuse was made too thick if there is no current then either you need a new battery or a new fuse. And you measure potential difference, (voltage) in parallel. See here for how to place your multimeter when measuring.

 

[KJ6EAD]

Two possibilities in order of likelihood:

1. You used a battery that is dead or nearly dead such as the one intentionally shorted in the previous experiment;

2. You have a fuse of a higher rating than specified for the experiment.

 

[Externet]

If the fuse does not melt, the energy contained in that AA cell is not enough to reach 3 Amperes for the lenght of time needed for melting. Can be a cheap cell, a depleted cell, a poorly connected cell...

 

[flippineck]

I noticed once that cheap zinc-carbon cells were only capable of producing tiny sparks in pitch darkness with very finely stranded wire, whilst nicad rechargeables could smoke the plastic off a length of fairly thick insulated wire. I think batteries vary quite widely in their ability to supply large currents. I think the technical term is 'internal resistance'. For this experiment to work, you need an AA cell with sufficiently low internal resistance. I'm not sure which types of non-rechargeable, off the shelf cells would be best..

 

[Anon_LG]

It is possible that the fuse does not blow in time and the battery short circuits, or (more likely) there will be internal resistance along with stray resistance from the wire and fuse, this would come to over 0.5 ohms meaning the battery is not outputting 3 amps.

 

[KrisBlueNZ]

I noticed once that cheap zinc-carbon cells were only capable of producing tiny sparks in pitch darkness with very finely stranded wire, whilst nicad rechargeables could smoke the plastic off a length of fairly thick insulated wire.

Yes. Rechargeables (when fully charged) have a much lower internal resistance than zinc-carbon cells. And 3A is quite a lot of current to expect a little AA battery to supply. A 9V PP3-type battery certainly can't supply that much - rechargeable or not.

I think batteries vary quite widely in their ability to supply large currents. I think the technical term is 'internal resistance'. For this experiment to work, you need an AA cell with sufficiently low internal resistance.

Exactly. He would be more likely to succeed with a lower current fuse - 2A or 1A.

 

[Esoremada]

Try putting your multimeter in series with the battery and the fuse, check the current using the current setting. There could have been a manufacturing fault with the fuse so try a new fuse, if there is current the fuse was made too thick if there is no current then either you need a new battery or a new fuse. And you measure potential difference, (voltage) in parallel. See here for how to place your multimeter when measuring.

Okay I put my multimeter in and I'm getting a reading of 44 mA, so I guess the battery just has a really high internal resistance. Now I have another question, is it normal for the multimeter reading to start off high and then go down for a long time? When I completed the circuit it started at 120 mA and then after about 5 minutes it said 44. Now after typing that paragraph it says 42. Now if I break the circuit and reconnect it again it goes down from ~120 to 42 in a matter of seconds and continues dropping slowly again. Is this the battery running out of charge?

 

[BobK]

That is a dead battery.

Bob

 

[Esoremada]

That is a dead battery.

Bob

Alright thanks

 

[(*steve*)]

Now I have another question, is it normal for the multimeter reading to start off high and then go down for a long time?

Perfectly normal.

This may happen as the battery discharges, or with some battery types a rapid discharge will cause the cell to (in a fashion) be depleted because you're trying to extract energy faster than the chemical reaction can create it. Once you remove the load the reaction can (again, in a fashion) catch up and the battery appears to have "refreshed" itself. This "refreshing" tends to happen more with primary cells than secondary (rechargeable) cells.

 

[KJ6EAD]

Be careful using your meter to measure high currents. You're likely to blow a fuse in your meter.

 

[BobK]

I risked it and I get 6A from an AA cell.

Bob

 

[cjdelphi]

Most have 10a current measure when you move the lead on your multimeter, and well you should be safe up to a 10amp fuse...

If the battery can't blow the fuse, it's not likely to do much harm in a short either..

 

[(*steve*)]

In many cheaper multimeters the only fuse on the 10A range is the current shunt itself.

 

[BobK]

Mine has two fuses, one for the 10A input and the other for the mA input. I have blown the mA one several times.

Bob