PWM Circuit and DC motor problems (for NYE ball drop project)

[froggy365]

Hi all,

This is my first time making a somewhat complicated circuit, with the help of an online schematic. (http://www.dprg.org/tutorials/2005-11a/index.html )

My goal is to make a New Year's Eve ball-drop, which has a motorized controlled-release lit-up ball (on a separate power cable). All of the mechanics are in place, so the only issue I currently have is troubleshooting the electronics on the circuit. I have a variable power supply from 3V-12V at 1A. The motor I am using is a high-speed 18,000 RPM motor that operates from 9V-18V, at a maximum of 1.9A.

I initially used the schematic that I have provided the link for, but the 100K pot burned out, so I replaced it with a 25 ohm 3-watt pot, and that seems to be handling the current better; although, when I turn this pot up, it gets very hot, and apparently can only work at lower resistances.

The voltage input I am using is 12V, with 1A of current. It takes about 40 seconds for the ball to drop currently, so I would like to modify the circuit so that the frequency of the motor turning on would be about 4x greater than it is now (now I think it's turning on around every 1.5 seconds). My setup is a DC motor connected to a large pulley by a rubberband, which is on the same axel as a smaller pulley, which has the string holding the ball wrapped around it. The string goes up about 7.5 feet, through another 2-pulley system (so that the ball is easier on the motor).

It looks great so far, but some help from the gurus would be greatly appreciated!

 

[es1]

This looks like a variant of a pretty standard circuit. However, usually the FET is wired to 555's output pin not discharge like this:
http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm

As reset is tied in your example I think connecting the FET's gate to output is the right thing to do. There is also usually a current limiting resistor on the pot's wipe like on the circuit I linked to.

I suspect what is happening is once discharge let's go of the gate the FET just drives the motor full blast due to R2 so it is not actually PWM'ing the motor.

I have no idea why the pot would get hot enough to burn out though. If you pushed the wipe all the way to one side it would be a short and Q can sink and source a lot of current but TR and THR should be high impedance. My only guess is there is a wiring error somewhere.

Good Luck

 

[es1]

Oh I should've mentioned that I think the schematic you linked looks like it should work fine (Not sure why they use R2 instead of just wiring to Q though) that's why my only guess is a wiring error.

 

[vk6kro]

You might be having trouble because of the way that circuit is drawn.

Where you have wires that cross and make contact, they draw a small circle at the joining point.
Where they cross, but don't make contact, then there is no circle.

So, for example, there is no connection between pin 1 and pin 6. If there was one, then this would explain your pot burning out.

There is also no connection between pins 3 and 8.

Also, the 25 ohm pot is not a suitable substitute for a 100K pot. You need to get a pot that is similar to the one that burnt out.