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I am a programmer with only basic understanding of electronics/electricity.

I am trying to install this device into my vehicle. The device accepts a wide DC power input (10V to 30V), but has a maximum power supply rating of 12W (Edit: had typed 1W).

I am assuming that means the input current should be somehow controlled, external to the device.

What do I need to do this? A current limiting circuit? A fuse for extreme situations?

A current limiting circuit as described in wikipedia will dissipate power? Are there off-the-shelf components for this?

Is a separate spike guard required?

HRJ
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    Not sure if it's the site or less likely my IT guys, but the link didn't work for me. – kenny Aug 14 '12 at 12:54

3 Answers3

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I am mostly self taught on this stuff so someone please correct me if I am wrong.

I think you have a slight misunderstanding as to how current flow works. Unless there is something seriously wrong with your circuit, that rating means that it will use at most 1W of power from whatever supply system it is hooked up to (so don't hook it up to a 0.5W supply). I guess the best way to think of it is that the device is responsible for how much current it draws; the PSU will not "force" extra current down into it.

Current limiting circuits and fuses are used to protect against short circuits because when you short out the power supply (connect the two terminals together), the load resistance drops to essentially 0 and current flow theoretically approaches infinity. You can see how infinity amps of current could break something!

Think about it:

V = I * R

if V = 30V and R = 0 ohms,

V/R = I (rearranged equation)

30/0 = I (a divide by zero situation)

jeremy
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  • Hey thanks! I hope you are right. But there is no regulated PSU in the vehicle AFAIK. It's just a feed from the battery.

    Does that make a difference?

     – HRJ Feb 11 '10 at 14:49
  • It seems like your device doesn't need a regulated PSU, if it accepts between 10V and 30V. Your car battery should always be above 10V I think (except maybe during the few seconds when starting your car) – davr Feb 11 '10 at 15:38
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    I wanted to add that everything connected to a car battery should have a fuse. I recommend something like http://www.sunlightauto.co.nz/pe/media/p430690ed8648a.jpg – ppvi Feb 11 '10 at 18:14
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You are good to hook it up without any other circuit, although a fuse is never a bad idea. It is actually to prevent damage to the wiring or other devices in you car in the event your device shorts out.

Unless there is a defect in your circuit it will never consume more than the 1 watt it is rated at. Are you sure the rating you looked at didn't say a "minimum power supply rating" instead of maximum? Most likely it was stating the maximum power the unit would consume.

Don't loose sight that your unit will be draining the battery if you are not running the car unless you hook it up to a switched circuit.

Good luck.

Mark B
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  • It definitely says 12W "maximum". That's what is troubling me. – HRJ Feb 12 '10 at 06:48
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    It's speaking of the maximum the unit will draw then. You are still good to go with out the need for a current limiting circuit. The only time you ever need a current limiting circuit is with something like Laser diodes that will consume enough power to burn up if they aren't regulated. I am sure there may be a few other odd devices as well but your device is definately not one of them. – Mark B Feb 13 '10 at 03:59
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Have you checked with the manufacturer for installation instructions and application notes?

Since the flyer lists a wide range input ( and has a big picture of a car ;) you may not need anything. A fuse won't hurt but you will need the manufacturer to tell you the value. The device may have a high in-rush current that could blow a fuse that is sized for the maximum operating current.

Also an EMI filter or transient protection component may be recommended. Again the manufacturer will tell you this.

The device may have an internal fuse and filtering.

Daniel Grillo
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jluciani
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  • I am checking with the manufacturer already, but timezone differences and mis-communications are taking time. I will update the status of the question once I get more info. Thnx! – HRJ Feb 12 '10 at 06:50