Drop down voltage

[mabnz]

Hey guys

I'm trying to wire up a buzzer to my garage door so it will sound when the door opens. The courtesy light puts out 32V and the buzzer will draw about 10mA.

What could I use to step down the voltage to the 15-16V the buzzer requires?

Cheers

 

[crutschow]

You can use an LM317 regulator adjusted with two resistors to give 15V output (if that 32V lamp voltage is DC).

 

[mabnz]

Awesome, I've not heard of an LM317 before. Looks really easy. What role does the 0.1uf capacitor play in the circuit?

 

[jpanhalt]

Cruschow's condition, "if the 32V is DC," may have slipped past you. Are you certain it is DC? Do you know what DC means? What is the source of the 32VDC?

John

 

[mabnz]

Cruschow's condition, "if the 32V is DC," may have slipped past you. Are you certain it is DC? Do you know what DC means? What is the source of the 32VDC?

John

Oops, I just forgot to respond. Yes it's DC, the garage door opener has a 24V DC motor and the LED courtesy light module (which I'm hooking into to run a buzzer) run on what appears to be 24-32V DC.

 

[crutschow]

Both the 0.1uF input capacitor and 1uF output capacitor are to insure that the LM317 remains stable (doesn't oscillate). They should be mounted close the the regulator pins.

 

[Mike odom]

Why not just put a 160 ohm resistor in line with the buzzer (works for AC or DC)... 160 ohm * 10mA = 16V dropped across resistor, 16V across buzzer.
I*I*R (power) would be 16 milliwatts through the resistor.

 

[mabnz]

Both the 0.1uF input capacitor and 1uF output capacitor are to insure that the LM317 remains stable (doesn't oscillate). They should be mounted close the the regulator pins.

Thanks, it's good to understand how it works.

Why not just put a 160 ohm resistor in line with the buzzer (works for AC or DC)... 160 ohm * 10mA = 16V dropped across resistor, 16V across buzzer.
I*I*R (power) would be 16 milliwatts through the resistor.

Will a 160Ω resistor really drop it down from 32-16V? Seems rather low.

 

[dr pepper]

I'll probably get shouted at for saying this, if its a mechanical buzzer that uses about 100ma then why not just wire a 1.2w 12v bulb in series with it, the kind used for backlighting dash's.
If its a low current beeper though it'll probably fry it as the lamp will pass too many ma, you might be able to use a 20v 1.3w zener to lose the volts for a low current beeper, connect the cathode of the zener to the + supply (backwards).

 

[4pyros]

Will a 160Ω resistor really drop it down from 32-16V? Seems rather low.

Do you know ohms law?

 

[eTech]

Thanks, it's good to understand how it works.


Will a 160Ω resistor really drop it down from 32-16V? Seems rather low.

I think that should be 1600 ohms...

 

[dr pepper]

Yes I get that too 16v/0.01a = 1600 ohm, a 1k5 is nearest.

 

[4pyros]

1.6K ohms is a standered value for 5% resisters.
Now if the buzzer draws 10ma how much power do you think the resistor will need to dissipate?

 

[eTech]

1.6K ohms is a standered value for 5% resisters.
Now if the buzzer draws 10ma how much power do you think the resistor will need to dissipate?

For 5% resistor
0.160 watt nom,0 .1597-0.1596 watt or use 1/2 watt resistor...

 

[4pyros]

How did you come up with a 1/2 watt resister thats 0.5 watts you only need 0.16 watts.
A 1/4 watt would work. It is 0.25 watts.

 

[eTech]

How did you come up with a 1/2 watt resister thats 0.5 watts you only need 0.16 watts.
A 1/4 watt would work. It is 0.25 watts.

Yes...1/4 watt would work...I just derated 100%.

 

[Mike odom]

Yes I get that too 16v/0.01a = 1600 ohm, a 1k5 is nearest.

my bad... damn math

that's what I get for using a Canadian calculator

 

[Mike odom]

Yes...1/4 watt would work...I just derated 100%.

Probably don't need derating on this as it's probably not going in a box in an oven....

but a 1/4 watt resistor for 160mW you've already derate 36%.