# ? using a lm317t voltage regulator

#### james kilgore

Joined Apr 25, 2015
4
I have a project using a 9 volt to power led lights an a 3 volt electric motor.
Someone told me about the LM317t voltage regulator.
I have no electronic experience. The schematics I have seen make no sense
to me what so ever. I have found a page with a resistor calculator.
From it it seems I need a R1 value of 650 ohms and a R2 value 0f 950 ohms
to give me a voltage output 3.08v. My question is if the input voltage id 9v,
can I use 1 resistor between adjustable voltage out and my electric motor.
If so would the resistor need to be the values of R1 and R2 added together?

#### blocco a spirale

Joined Jun 18, 2008
1,546
Your resistor values for R1 and R2 look ok for an output of around 3V, so just use those values.
I'm not quite sure what your question means (as you seem to have a workable solution) but there is no point connecting a resistor in series with the motor.

#### james kilgore

Joined Apr 25, 2015
4
Thank you for your reply. I am building a jetpack like that used in the movie The Rocketeer.
I have bought led lights that work off a 9v battery, and fan motor that runs off of 2 AA batteries.
I want to wire them to a momentary switch. I want both to operate when the switch is pressed.
I need to keep this project simple because I have no experience in electronics.
Again Thank you.

#### blocco a spirale

Joined Jun 18, 2008
1,546
Ok, so you require 2 different voltages. The LM317 is about as simple as it gets, there are 3.3v fixed regulators out there but they're hardly worth it in this case. If it is a standard inexpensive brushed motor (with no electronics) and only needs to operate for short bursts of a few seconds, you could get away with running it directly from the 9V battery... the higher fan speed will certainly give it a more exciting sound.

If you that's no good, why not just run the fan from a couple of AA cells, it looks like there should be enough space in the jetpack.

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#### james kilgore

Joined Apr 25, 2015
4
Ok but that is 3 times its normal input. Can I put a resistor inline to the motor?
If so could I use a 1.6 k ohm resistor?

#### Papabravo

Joined Feb 24, 2006
12,702
Only if you want the resistor to keep you warm in the stratosphere.

#### blocco a spirale

Joined Jun 18, 2008
1,546
Ok but that is 3 times its normal input. Can I put a resistor inline to the motor?
If so could I use a 1.6 k ohm resistor?
It is somewhat higher than the rated input but it is temperature rather than voltage that will kill the motor so for short periods it should be ok.

A 1.6k resistor won't work but another, very simple, way to reduce the power to the motor is by connecting a small torch bulb in series with it; try a 6V bulb to start with.

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