Cross Breeds

Am building a spotlight cct for a dog racing helmet, do I need a resistor?

The Power supply is currently 12 V 7.0Ah and the Spotlight is rated 12v 55W, am also placing in a switch. Been a few years since I did this, do I need a resistor/fuse and how do you know. Also would it be better if I used a different power supply eg 4.0Ah 12V power supply etc....

Public Comments

1. 12 v and 55 w comes to a current of 4.6 amps. Power supplies are rated in volts and amps, not amp-hours. Batteries are rated in amp-hours as they have a limited capacity before they discharge. So I assume you are talking about batteries, not power supplies. A 12 volt 7 amp-hour battery will power a 4.6 amp lamp for 7/4.6 or 1.5 hours. Although i would not count on more than an hour. a 4 amp-hour battery will last for well less than an hour. You need a switch to turn it off and on. You don't need a resistor unless the light is too bright. But in that case you would be better off switching to a lower wattage bulb. A fuse will protect you against shorts, and doesn't use any power. Use a 8 or 10 amp fuse.
2. When you say "7.0Ah" I am assuming that your power supply is a battery. In that case, the more Ahr you have, the longer the battery will last. For a 7Ahr battery, a 55W load (4.6A) will last much less than 7/4.6 hr. The fuse is determined by your load, here you are using 4.6A. You generally want to add some leeway to your fuse (1.5 to 2 x your load is a good rule of thumb). So, a 7A to 10A fuse should work for your light. The other factor is inrush current. This is the current a device draws when it is first turned on. For many devices, this is much higher than the normal load current. Lamps are a good example of devices which have significant inrush current. For these kinds of devices, a slow-blo type fuse is recomended. The only reason you would want to consider a resistor is if you had too much voltage for your lamp. However, for high current devices like lamps, resistors are a bad way to try to reduce voltage. You are much better off making sure your power supply or battery is the correct voltage in teh first place.