PWM with 555 problem

Thread Starter

oopsSmoke

Joined Jun 16, 2009
23
Hi all,

I am working on a PWM circuit using a 555 timer to drive a 3.6V DC motor from a Black and Decker Versapak screwdriver. My power source is a 9.0V, 100mA wall wart. The circuit itself is breadboarded and outputs about 8.5V and .15mA with the potentiometer turned up. The problem I seem to be having, though, is that I can't even get it to run a cheap hobby DC motor.

This is what my circuit is based on: http://www.dprg.org/tutorials/2005-11a/index.html. I've substituted the 100K potentiometer with a 35K pot and the IRFZ46N with an IRF640. I really don't know what all of the components do, I'm just putting them together to get a project to work. My best guess at the moment is to replace C1 with a larger one like a 470uF capacitor.

If anyone can offer any assistance, insight, education, etc. I would greatly appreciate it.

Kevin
 

Wendy

Joined Mar 24, 2008
23,415
Part of the problem is you are trying to use a MOSFET on a 9V circuit. The 555 also doesn't go to rail, with 9V in it can only put out 7.8V, even worse. MOSFETs, unless they are design for it, need 10V gate voltage. There are MOSFET families that will do it, which is what they showed.

You could replace the MOSFET with a BJT NPN power transistor, something like this...



I just noticed their design left out the swamping diode across the motor, a major oversite.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,248
I think your little power supply provides not nearly enough current to run a motor. Its voltage drops too low when the motor is powered. Measure it to see.
 

Thread Starter

oopsSmoke

Joined Jun 16, 2009
23
Thanks for all the replies! I didn't get a chance to check this thread yesterday evening, but I will try some of the suggestions after work today.

Bill_Marsden, I have a couple of TIP31B's. Will they work in the diagram you provided?

Kevin
 

Wendy

Joined Mar 24, 2008
23,415
Actually their schematic will work, as will mine. Pin 3 is the output pin for a 555, pin 7 is the capacitor discharge pin for monostable timing, which goes to ground exactly in sync as pin 3. Their schematic works by grounding the gate of the FET, which will turn it off. They are using a logic FET, which takes much lower gate voltage to turn on. I prefer my technique, it uses less power overall.
 
Last edited:

ale2121

Joined Mar 20, 2009
71
i was referring to the pins in the original schematic, not the one you posted. someone explains why in the link i posted, and this is how i got the original schematic to work.
 

Wendy

Joined Mar 24, 2008
23,415
I understand, but the original schematic will work. Pin 7, while not an output, does have ½ of driver in the form of a switch going to ground. They use a pull up resistor on the gate of the MOSFET. The same scheme would work on a BJT. Not the way I would do it, but it would work.

The only thing I see wrong with their design is the lack of a back EMF diode across the motor, a major problem. The diode will keep the motor from blowing the transistor, which can happen to either MOSFETs or BJTs.
 

Thread Starter

oopsSmoke

Joined Jun 16, 2009
23
I did some fiddling with it last night, but ended up throwing my hands up in frustration again. I swapped out the pot with a 100K, replaced the IRF640 with a TIP31B, and checked the votage between drain and source on the MOSFET and then again on the transistor between the collector and emitter (not sure if I should have done that, but it didn't seem to hurt anything). I don't think I checked the current, though.

In the end, I still ended up getting 8.5ish volts on the leads to the motor with the pot turned up. Still no motor spin. I don't understand how I can hook a battery pack up to the motor and that works just fine, but the same motor hooked to this circuit does nothing.

Oh yeah, when I was checking the voltage on Q1 my multimeter started giving me flakey readings after a while. Could be time for new batteries. Just one more thing to do...
 
Last edited:

bertus

Joined Apr 5, 2008
22,270
Hello,

It is called dirty circuit because they use the discharge transistor to drive the motor.
I have not found the maximum allowed current in the datasheets.
If the current gets to high the chip will go hot.

Greetings,
Bertus
 

SgtWookie

Joined Jul 17, 2007
22,230
ST Microelectronics' datasheet for the NE555 gives a current limit of 15mA for the discharge pin 7. Attempting to sink current from a motor using pin 7 would not be a good idea.
 

Wendy

Joined Mar 24, 2008
23,415
Use the transistor. The 555 will thank you, and if you oops it the transistor is cheaper (though not by much).
 

Wendy

Joined Mar 24, 2008
23,415
ST Microelectronics' datasheet for the NE555 gives a current limit of 15mA for the discharge pin 7. Attempting to sink current from a motor using pin 7 would not be a good idea.

I had never noticed that spec. Seems a capacitor would dump a lot more than that on the discharge.
 

Thread Starter

oopsSmoke

Joined Jun 16, 2009
23
I swapped pins 3 and 7 so that now 7 goes to ground and 3 goes to the motor, per Bill's diagram. I assumed that 5 and 7 went to ground since they didn't seem to go anywhere. This changed did give me some positive results. I was able to get a small hobby motor to run, but the 3.6V motor that I am shooting for still isn't doing anything. At this point, I'm really tempted to pull it all apart and start over. Good thing the weekend is right around the corner!
 
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