What resistor would I need to reduce 11 volts to 3 volts?

Question

I am replacing my trucks door buzzer with the guts from a musical greeting card. I am totally lost on how to calculate value of the resistor I would need to reduce the 11 volts that normally powers the buzzer to the 3 volts needed for the greeting card.

Answer 1

A resistor won't work, because dropping resistors only work with constant current. So, when it is not running (or even between notes), it will see the full voltage. When it's running and singing, the voltage will be correct. Seeing the high voltage even part of the time will likely smoke the gizmo.

Use a regulator. Here's a tech document: http://www.national.com/appinfo/power/fi...

Here's three places to look for parts online. Check their minimum order/shipping and handling before ordering. Just look for voltage regulator. I would get one with at least 15V minimum input voltage (Vin).

If you have a Radio Shack, they may have an adjustable one. These use 2 resistors to set the voltage. Make sure it will go to 3V.

Edit: Auto manufacturers almost never use regulators for electromechanical things like buzzers that are robust and not fussy. They can't afford the cost. Likely, it just seems 11V steady. Either way, it's still risky to use a dropping resistor. Most low voltage stuff is easily zapped. It is a disposable device anyway, likely damaged by heat cycling in your truck. If you want it to last at all, use a regulator. Some are small and surface mount, can be soldered to the greeting card thing.

I could not find a 7803 part, it is not standard, but perhaps is made by some obsure manufacturer. The adjustable part doesn't quite go down to 3V, but maybe close enough. The package is kind of large, plus you need external resistors. Better to get a smaller 3V regulator if one is available.

Answer 2

In order to use a resistor to drop the voltage you need to know the current consumption of the greeting card. Also, the resistor can drop the voltage, but the voltage varies with the current consumption of the load.

Use ohm's law to caclulate the resistance:

   Resistance = Voltage/Current

Also, be aware that the resistor must have the proper power rating. The power the resistor must dissipate is:

  Power Dissipated = (11 -3) x Current

Answer 3

vehicle voltage should be 12volts, 15 after starting.

       it's like a voltage divider problem. you want 3 volts across the card, and 9 volts  across the resistor - so the resistor needs to be 3x the resistance of the card. you probably can't just use an ohmmeter - you need to measure the amperage used by the card.  3v/ amps = R, and R x 3 would give you the needed resistance for a 9 volt drop.

    Wattage rating of the resistor = 9 volts x amps.

    there are one - chip voltage regulators available.

I think it would be a  #7803 (3 terminal  package)

maybe #7903?

Answer 4

Use a multimeter to find the resistance of the greeting card (while its running).  Then it's easy:  you just need a resistor (11/3) times the resistance of the greeting card and place them in series.