# How to use ordinary voltage regulator from a 40amp source?

My power source is 12v 40amp(car battery). My project needs 12v but only 500 or 1000ma max. My 12v voltage regulator (NTE960) explodes with 40amps, so I must reduce the current (too much current for other components anyway).

What's the best way to reduce the current before it enters the voltage regulator?

Is the best way to put a 10 watt resistor between the voltage regulator and the power source? (ex NTE-10W015)

V=I*R

Where V=12v and R=15Ohms shows that the output will be 800ma and 9.6watts.

I assume a 10 watt resistor supports 10 watts of output (not that its 10 watts input)

What about a diode? Except for zener diodes I can't find a 10 watt diode.

What if I get 14v instead of 12v? Output will then be 13 watts. What will happen to the resistor then? Should I use a 15 or 25 watt resistor then?

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Will the 10 watt resistor or diode increase the drain on the battery? –  Matt Feb 14 at 21:21
–  David Feb 14 at 21:38

If it "explodes" you're doing something horribly wrong. That part is basically an LM7805.

Perhaps put a 2A fast-blow fuse in series with the input to protect the wiring (and check the connections- perhaps you have them reversed).

If you're using it in a car, that's a different matter, there are transients on a car electrical system that need to be protected against. A small wirewound series resistor (eg. 1 ohm 5W) and a hefty 14V TVS (eg. 3000W) would help with that.

And don't forget a capacitor on the input, and preferably one on the output (something like 47uF/25V would do for both).

simulate this circuit – Schematic created using CircuitLab

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+1 for transients in a car. From what I understand, "transients" is putting it lightly. I'm not an automotive engineer, but I know they have standards for robustness that include all sorts of crazy cases. Research "load dump" for just one example. –  Phil Frost Feb 14 at 23:22

My 12v voltage regulator (NTE960) explodes with 40amps

40 amps is what the supply is capable of delivering should the load connected to it demand that current. The regulator isn't being forced with 40 amps just because you connect it to a battery/supply that could deliver this current.

More likely the voltage regulator is being destroyed with over-voltage transients when the engine is running and your alternator is churning out pulses to charge the battery. The NTE960 is rated up to 35V and this might easily be exceeded in an automobile.

If you are going to use a series resistor then I suggest you also apply a 5W 22V zener to 0V at the junction of the regulator and resistor. The value of the resistor should allow a few volts to be dropped at the maximum load (1 A max). The regulator requires at least 7V so plan on 8V and assume the resistor could "lose" 4V (from 12V). At 1A this means 4 ohms - choose a 3R9 resistor rated at 7W.

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