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Calculate Fuse

Suraj143

Active Member
I'm charging a 1.2Ah sealed Lead Acid battery from a LM317 Regulator (constant Voltage 13.6V configuration ).The problem is after few months the battery weaks & acts like a short circuit.The regulator heats so much & burning.To protect my regulator I'm Planning to put a fuse in series with the battery.

In shops the fuses are rated in 1.25A/250V or 1A/250V.Is this rating is for 230V AC or DC? Can I apply above fuses?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The voltage is irrelevent for your purpose, it's actually 'up to' 250V - you can happily use such fuses.

However, I'm somewhat disturbed by your batteries failing after only a few months?, and in that fashion - such batteries are out there in their millions, last years, and when they do fail it's simply that they won't hold charge any more.

But you should have had a fuse in circuit 'somewhere' in the first place, a basic safety precaution.

Probably the best place is before the bridge rectifier?, as it then protects against the rectifier going short as well.
 

JimB

Super Moderator
Most Helpful Member
I'm charging a 1.2Ah sealed Lead Acid battery
What is the charging current?
How long is the battery charged?
ie how much time is the charger supplying current.

the battery weaks & acts like a short circuit.
Short circuiting is an unusual failure mode for a battery.
Could it be that the plates are bucking due to severe overcharge?

( A note on English grammar, "the battery weaks" is a strange expression, it would be better to say "the battery weakens", or even better "the battery fails" )

To protect my regulator I'm Planning to put a fuse in series with the battery.
If you read the LM317 datasheet it states that the regulator is over current and over temperature protected by its internal design.
I doubt that a fuse would be any improvement.

As suggested by Nigel, a fuse may be better placed earlier in the circuit, before or just after the transformer.
My choice would be to put the fuse in the incoming mains supply.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
What is the charging current?
How long is the battery charged?
ie how much time is the charger supplying current.


Short circuiting is an unusual failure mode for a battery.
Could it be that the plates are bucking due to severe overcharge?

( A note on English grammar, "the battery weaks" is a strange expression, it would be better to say "the battery weakens", or even better "the battery fails" )


If you read the LM317 datasheet it states that the regulator is over current and over temperature protected by its internal design.
I doubt that a fuse would be any improvement.

As suggested by Nigel, a fuse may be better placed earlier in the circuit, before or just after the transformer.
My choice would be to put the fuse in the incoming mains supply.
You should have one there as well - but often shorts on the secondary side don't blow the mains fuse, which is why I like to place one after the transformer as well.

A classic example was microwave ovens, where shorts on the secondary didn't take the mains fuse out, killing the transformer as well.

The 'solution' was to add an over-volts diode, which fails S/C in such a case - and places a much heavier short on the secondary, blowing the mains fuse.

However, these were expensive, and often blew for no reason.
 

Suraj143

Active Member
Ok Thanks,

What is the charging current?
How long is the battery charged?
ie how much time is the charger supplying current.
I do a constant voltage charge.So I do not know what amount battery is taking.I have plug the battery permanently.
Not sure the thermal shutdown properly happens in cheap Chinese LM317 versions:(

It will be nice if I can place the fuse just before the battery.Because I do not have to change the PCB, I can buy a Wire type Fuse & easily solder to my PCB.
 

JimB

Super Moderator
Most Helpful Member

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