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555 PWM Circuit

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codex653

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ok so i just built this circuit here:

Pulse Width Modulation - Electronics in Meccano

but i made a few changes to accommodate some things...
1. "frequency preset" pot is a 100k
2. 0.1uF cap on the first timer (connected to frequency pot) is changed to 0.015uF
3. delete 1k output resistor on the second timer
4. Replace the TIP31C with an IRF640
5. Have the source of the IRF640 connected to ground, and the gate is connected directly to the output of the second 555
6. Have the motor and diode in the diagram moved up to be between the +12v and the drain of the IRF640
7. I used a NE556 instead of 2 NE555's
8. caps connected to pin 5 are 0.015uF since i didn't have any 0.01uF's on me

Ok so here's the problem...it all turns on, but i get no pwm. I unfortunately don't have a scope so i can't see if i'm getting any oscillation. The motor spins at full speed and no matter how much i turn the "speed" pot, i don't get any change in motor speed at all. I used my multimeter on the DCv setting to check the output pin on the second 555 but all it shows me is 12v....with pwm i should be able to use a multimeter cause the meter will be able to see the average voltage caused by the duty cycle of the pwm correct? like if i had the duty cycle set at 50% my multimeter would read roughly 6 instead of twelve....anyways, the voltage at the output didn't change what-so-ever...just a steady 12v....

for the mosfets, they are brand new and i found on one of them that the Dsr was effectively open circuit, while the other was infinite resistance on my meter....tested them both out on my pwm circuit and i got the same results with each one.

One thought i had was that the output voltage from the oscillation stage was too high for the input of the pwm stage? I would be putting ~12v or so to the input of the second 555....

any help ppl????
 
One possibility is that when you changed the pot to 100K and the capacitor to .015uF, you simply made everything go faster (frequency is higher, thus so is the speed). It might just be that you cannot notice any speed change (?).
Just a thought, anyway. Perhaps you can try playing around with different pot and cap values to see if you can change the speed?
Der Strom
 
hi codex,
A quick check for the IRF640 would be to disconnect the Gate from the 556 output then connect a 1K from the Gate to 0V, the motor should not run.
If it does the IRF is shorted, [assuming you have it correctly connected ie; Source to 0V, Drain to the motor]

Then connect the 1K up to +12V the motor should run at full speed.

Lets know what you see.
 
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ok yah i'll try that out when i get home after school today

i knew it was going to change the frequency as i'm wanting this thing to eventually try powering a flyback...i was just testing it with a motor first...but even still, the pwm should have limited the power....
 
ok so i tested it out and found that one of my mosfets was shot.....then i replaced the mosfet with another one and found that the pwm still didn't work! i tried adjusting the frequency and everything, but for some reason this stupid circuit isn't working
 
hey i was running some simulations on the circuit in multisim and found it doesn't even work....ummm but i did find this new one that is even more simple and works in simulations

**broken link removed**
 
If you've got a 393 comparator, this one is a great circuit that I have always had great luck with. Probably one of the best ones I've ever found: RMCybernetics - DIY Homemade Power Pulse Controller
You can drive motors, transformers, lights, etc with it. It works great. Just thought I'd mention it... ;)
Der Strom
 
Why did you change the frequency in the first circuit? You gotta slow it down by using it's original component values for the circuit to work. And yeah, I'd suggest building the secone circuit, I've built these for light dimming, and they work fine. The frequecy will be set only by the timing cap, and so the only way to change the frequency will be to change that cap.

I"m pretty sure you don't need D1 in the second circuit, and the 470uF cap seems unreasonably large.
 
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i changed it cause the intent wasn't to run a motor, but to pulse a transformer with a ferrite core, so i needed the higher frequency....well it's good to know that the circuit works tho! its just too bad it's not for my application. i don't really know why D1 is there either.....maybe it's if you are running the 555 from an ac supply and the diode and cap serve to rectify and filter it....
 
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Well, it's OK to want a higher frequency, but you have to compensate for that in the 2nd timer. Just deleteing the 1K resistor in the 2nd timer won't make up for the nearly 5 fold increase in frequency. The way this circuit works is the first timer creates the frequency, and the second one creates the mark/space ratio. In order for that to work, the pulse width of the second state must be the same as the 1st. If you increase the frequency, but don't make the 2nd pulse smaller, then neither the resultant frequency nor the ration will be correct.
 
ahh ok! i never knew that! thanks! but i think i'm gonna go with the second schematic i put up now...it does the frequency and pwm all in one, much more simpler circuit...i don't really need the variable frequency as much as i need the pwm

and plus to change the frequency all i need to do is change the capacitor and that's not bad at all...granted having the variable frequency is a bit more convenient, but i don't mind not having it there
 
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yah i just tried out the second circuit i posted and it works great!! i found tho that when i had a motor connected i needed a much smaller frequency if i wanted to get accurate speed control...but a motor isn't what i'm gonna control in the end anyways...i'm going to be controlling the voltage to a HF transformer for my amplifier :)
 
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