protection from surge current and surge voltage

[Afzal hossain]

Dear experts,
I need to protect a lm-7805 from 24V or12V car battery surge current and surge voltage.Is there any simplest solution to serve my purpose ?
As far I know a car battery produce around 200A while getting start and about 1000V at stop . I studied about MOV and NTC thermistor ,but I am not getting clear concept about choosing the accurate value of these stuffs.waiting for your valuable response .

 

[kam1787]

a zener diode and LC circuit will stop/limit most voltage surges/transients. I think there is the opposite problem with current surges, in that the voltage will drop sharply - use a capacitor [say 1000uf per amp drawn] to maintain the voltage.

 

[goldsmith]

I need to protect a lm-7805 from 24V or12V car battery surge current and surge voltage.Is there any simplest solution to serve my purpose ?
As far I know a car battery produce around 200A while getting start and about 1000V at stop . I studied about MOV and NTC thermistor ,but I am not getting clear concept about choosing the accurate value of these stuffs.waiting for your valuable response .

Hi Afzal hossain
For surge voltage you can simply use a varistor or perhaps a spargap . ( this is one of the thousands ways ! )
For surge current , there are a lot of way too . using a resistor and triac / using NTC / using an inductor an RLCD network ....


Best Wishes
Goldsmith

 

[treez]

I don't think you will have a surge current problem.

For surge voltage, just have transistor in line and turn it off via a zener in shunt.
To make it better, put a TVS downstream of the inline fet

 

[westom]

Dear experts,
I need to protect a lm-7805 from 24V or12V car battery surge current and surge voltage.Is there any simplest solution to serve my purpose ?

Your concern is something called load dump. Both SAE and ISO define this threat on 12 volts with numbers. Automotive transients up to 270 volts and energy up to 50 joules. SGS Thompson defines a smaller concern: Peak voltage 80 to 100 volts.

Semiconductor companies make other (equivalent) regulators designed to make such transients non-destructive. Search on National Semiconductor's (and others) automotive regulators.

 

[treez]

Yes, good point, and "load dump" is that weird and wonderful thing that certainly happens in theory....but in reality?...I doubt it. I used to work in an auto warning beacon place and none of the beacons were load dump protected, and it never gave a problem.
To get a load dump, you need the engine to start up, then the alternator to get running at full power, then somehow the battery connection terminals have to somehow disconnect themselves, and then the energy in the alternator goes down all the circuit wires and makes a big voltage there since it no longer has a big battery to clamp it.
Alternately, it could be by somebody jump starting a dead battery, and then of course when the jump start is removed, the alternator is running into a dead battery which won't clamp it.
In truth, "load dump" only occurs with very poorly serviced vehicles.

I reckon your best bet is just to use a zener diode which activates on an overvoltage and switchs off an inline transistor before the voltage gets too big across your circuit. By all means use a tvs downstream of the inline fet to make it more snappy.

 

[Pulsetronics]

Using this method overcome over current and voltage

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Using this method overcomes over current and voltage

 

[treez]

...that TIP41 is a good solution for low power......I think tip41 is a darlington(?)...which means you have about 2V between external base and emitter......which also means you have at least 2v across Vce.......so whatever you load current multiplied by 2v is the power loss in the darlington. (could get hot dep on load current)
Is that ok for your application?
Is your application also capable of having the 2v loss?

The alternative is to have a P channel fet normally on, but switched off by a zener when overvoltage comes.

 

[D.A.(Tony)Stewart]

You need to define your environment.

If the car battery is inside a car then the automotive specs are quite stringent such as -24V to +48? and a high Joule load dump rating to simulate the large inductive load from the electric clutch in an ACU, which can be easily protected with an MOV disc.
If the input voltage drops to 0V with low impedance while the output has a large capacitor , then a reverse protection diode is needed from output to input.

A car battery and a charger with no other loads might need some series inductance such as a wire wound series power drop resistor to input cap or a ferrite donut.

If there are other switched loads between the LDO and the battery that might dump other loads, then this is part of your system environment spec.

WHat is your application? Solar panel?