# Resistor help!

Discussion in 'General Electronics Chat' started by Tripwr, Jun 4, 2014.

Not open for further replies.
1. ### Tripwr Thread Starter New Member

Jun 4, 2014
2
0
Long story short... I think

I have a electric motor that runs off of 12v DC that draws 2.2Amps. It runs through a 3 way switch... High off and low. The high and off both work, the low setting has the same output 12v 2.2amps. It is missing a resistor... I cannot get a new switch, I need to add the resistor after the switch... no choice. I need to figure out how to cut the motor speed down. I'd like it to run somewhere between 1/2 - 3/4 the speed... How do I figure out what type of resistor I need to use?

Any help would be much appreciated. Thank you in advance!

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,505
512
Is the speed controlled by current?
(More current more speed?)
Is the relationship linear?

3. ### #12 Expert

Nov 30, 2010
16,704
7,351
Assuming this is a fan motor...

Ohm's Law says that the effective resistance of your motor at full speed is 5.45 ohms. Adding 5.45 ohms in series with the motor would apparently cut it to half speed, but motors aren't that cooperative. The Fan Laws say that power used goes up by the cube of the speed, so a little slower uses a lot less power.

I would suggest that you try something in the range of 2 ohms to 10 ohms and see what happens. You can get a pair of 10 ohm, 10 watt resistors at Radio Shack and use them to make 5 ohms, 10 ohms, and 20 ohms by connecting them in series, parallel, and one at a time. That will get you some experience in how THIS motor acts for a couple of dollars.

Safety note: there is a lot of power happening here compared to the usual resistors you find in most electronics devices. Know and use Ohm's Law (E=IR) and Watt's Law (P=IE) to be sure your resistors don't burn up. And remember, cars are designed with their speed resistors in the air stream to keep them cool. You are allowed to use moving air to keep your resistors cool.

and then, it might not be a fan

4. ### Tripwr Thread Starter New Member

Jun 4, 2014
2
0
You are correct it is a fan motor... For a 65 gto rear defogger. I purchased a potentiometer and figured I could wire it in and adjust it to get the speed I wanted off he fan. The problem then is I have to shove the the potentiometer under the dash which I don't want to do... Is there a way to take a reading off the potentiometer when I have the fan at the speed I want to figure out what resistor to purchase?

Plus could you specify a resistor from radio shack? I tried looking and they have so many kinds I want to make sure I get the right one.

Thank you for the information this is very helpful.

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,505
512
10 Ohm, 10 Watts resistor: http://www.radioshack.com/product/i...filterName=Type&filterValue=10-watt+resistors

The rest of 10 Watts resistors: http://www.radioshack.com/family/in...s&pg=1&searchSort=TRUE&retainProdsInSession=1

Apr 30, 2011
1,426
364

7. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
The owners of All About Circuits have elected not to allow discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here: