# The simple way to make a analog speed control for a DC motor - AKA why dont this work?

#### Connor Rasilainen

Joined Jun 13, 2015
1
Hi everyone,

I'm a mechanical engineering student, so it goes without saying my knowledge of electricity and circuits isn't the best.

I'm working on my capstone project for my graduating semester. For this project we need to build a variable speed control for a DC motor. I took a motor out of a cordless drill (17V). Hooked it up to a power source as it ran fine, however we need something with better control for speed than the trigger switch. I tried to replace the trigger switch with a on/off switch and a potentiometer but hooking the motor up now, it doesn't even move.

I know I made a pretty huge error but I'm just not sure why this doesn't work. If someone could explain that to me and what I can do to fix it that would be awesome. thanks. I have access to many electronic components.

I've attached the schematic of what I tried.

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#### MikeML

Joined Oct 2, 2009
5,444
Try a power rheostat that has an adjustment range from 0Ω to about 50Ω (looks like this) in place of the 500kΩ pot. It will have to be rated for power dissipation of 25W to 50W.

Ohms law: To drop a few volts (reduce the voltage to the motor) at an amp or two: R=E/I = 10/2 = 5Ω.

Power dissipation: P=IE 10*2 = 20W

#### #12

Joined Nov 30, 2010
18,223
MikeMl got there first, but...
The obvious answer (to me) is to use a variable pulse width (at the full voltage) to slap the motor into movement, in tiny increments. If you need better control than that, there are stepper motors that will move only a few degrees with each pulse. Name your poison.

#### cmartinez

Joined Jan 17, 2007
7,768
Hi everyone,

I'm a mechanical engineering student, so it goes without saying my knowledge of electricity and circuits isn't the best.

I'm working on my capstone project for my graduating semester. For this project we need to build a variable speed control for a DC motor. I took a motor out of a cordless drill (17V). Hooked it up to a power source as it ran fine, however we need something with better control for speed than the trigger switch. I tried to replace the trigger switch with a on/off switch and a potentiometer but hooking the motor up now, it doesn't even move.

I know I made a pretty huge error but I'm just not sure why this doesn't work. If someone could explain that to me and what I can do to fix it that would be awesome. thanks. I have access to many electronic components.

I've attached the schematic of what I tried.
Mike's right on the money on this one.... that's probably the simplest solution to your problem. But #12 has a point, and a PWM circuit would be the most elegant way of doing what you want. And although that would involve more complex circuitry, the learning experience from that little project would be far more profitable.

Joined Jul 18, 2013
25,459
The other simple way is a SCR bridge off of a AC supply.
Or a triac in the AC line before a regular bridge.
Max.

#### ScottWang

Joined Aug 23, 2012
7,237
You didn't say that how is the voltage you tested for motor and how is the current the motor draw?

MikeML mentioned that it was a easy way if you can buy a Wirewound variable resistors to match what you want.

If you want to using other way then you can using the NE555 + N mosfet, here is the circuit NE555 Clock Generator and PWM adjustable circuit, you can using two different Vcc for ne555 and N mosfet to drive the motor, how many volts you want to use in your project for motor?