# 12v dc motor, 18v lith battery, speed controller? help

#### ninjaman

Joined May 18, 2013
339
hello
i have a 12v dc motor and two brand new 18v lithium ion batterys. i would like to make a wood lathe and use the batteries. can i use the 18v on a 12v motor? also, how could i make a speed controller with virtually nothing, or least amount of money spent?
thanks
simon

#### Tonyr1084

Joined Sep 24, 2015
6,896
I've been working on solving the same problem to some degree of comparability. This is my thread; and you'll see that I've had a lot of trouble. You can read all 8 pages if you care to - but if you skip to page 7 you'll see where I'm beginning to consider using a 555 chip to control speed.

One thing to keep in mind is that if you have an 18 volt source, using PWM - you can inadvertently apply 18 volts. Unless you figure into your circuit a way to prevent an over-voltage situation. In light of my thread - I'm not qualified to give you a good answer. I learn from my mistakes. There's no reason others can't learn from them too. Good luck.

Joined Jul 18, 2013
25,469
If you have a pot controlling the PWM, you just need a series resistor with the pot to limit the range to the the required PWM percentage.
Max.

#### Tonyr1084

Joined Sep 24, 2015
6,896
@MaxHeadRoom Yeah, I applied that in my drawing (post 138) but I don't want to hijack this thread.

PWM - done right - will provide varying power to your motor. However, like I said, 18 volts can be an over-voltage situation. As Max is pointing out - you can limit the output to 67% so you don't over-voltage your motor. But I'm sure your motor can handle a little over voltage. The thing with PWM is that it doesn't limit the voltage. PWM means 18 volts on for X% of time and off the balance of that time, thus producing an average voltage. For instance, with PWM "High" for 67% of the period (period is the time between on-going wave form to the next on-going wave form) while 33% of the time it's off averages out to about 12 volts.

I've heard comment that PWM actually causes an averaging of current. Like I said initially - I'm not qualified to make such statements. I'll speak only in general terms of relativity. In short - it can be done. But consideration needs to be taken as to how much current the motor will draw at full speed startup, and having components that can handle that current. Hence, in my circuit you see I'm using an IGBT to handle that much current. Some here - who know or have more experience than me - think I'm going for overkill. Perhaps way-overkill. My philosophy is - better to have a robust design than one that might struggle with "just enough" ability. I reference the Tacoma Narrows bridge in Oregon (or was it Washington?) as an example of "just enough". It wasn't.