# How to use ordinary voltage regulator from a high fault current source?

My power source is 12v 40amphour(car battery). The wiring resistance is low so the prospective fault current could be very high . 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?

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

• +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 '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.

"The NTE960 is rated up to 35V and this might easily be exceeded in an automobile"

I seriously doubt that.

Sure there are a load of inductive things like ignition coils and fuel injectors but the switching is low side in my experience. The switching transistors may need protecting from high voltage but the 12volt supply won't see all of that, in fact it is more likely to go negative as the inductor pulls it down at switch off and some current flows through protection diodes [for example TVS].

I built an electronic switcher for the indicators on a motorcycle. It was absolutely great until I started the engine. It went crazy stupid with the engine running. The oscillator / Mosfet driver [IR2153] has its own clamping Zener on VCC so overvoltage was not the problem, it also has Under voltage lockout.

I put a series diode to Vcc and a small capacitor [100uF from memory] from Vcc to ground. Problem solved.

Same goes for LED driver on the same bike. LEDs used for Daytime running lights on front. They were flickering when engine was at idle. Considering the drive circuit is configured as "constant current" the problem had to be voltage dropout, possibly due to ignition coil current.