Oscilator with Freq+duty cycle control?

[random person]

So I've managed to build a current source that is PWM-ible to about 500khz+ at 2A+. Now i'm just looking at building a Frequency source where I can change both the frequency and the duty cycle independently of each other. Most relaxation oscillators only have a 50% duty cycle and with 555 timers the frequency and the duty cycle are related. Anyone know how to build easy frequency source that can adjust the duty cycle and the frequency independently of each other?

 

[Benm]

You can build something like that using two 555s, or a 556. The first part will act like a normal oscillator. The second one will act as a pulse length corrector. This way, you can set the frequency with the first oscillator, and then adjust the pulse width with the second.

 

[123splat]

read the National Semi specs on the 555, or try an Exar 8038....

 

[GBD]

Heres an oscillator Ive used a couple of times, it will do as Benm described:
Ive just used an LM556 for the ease of a single 14-DIP package.

The first part of the time is just a variable frequency oscillator (50% duty) and it just feeds the trigger of the other 555, which can be adjusted for duty independantly (0-50)

 

[Meatball]

I actually have a pcb ready to go to build such a device. Its uses two 555 timers to interrupt a DRSSTC tesla coil. If you want the PCB, let me know and I'll see if I can't find it somewhere. I'm in the middle of moving so we'll see how that goes!

With this pcb, you can individually control pulsewidth, and pulse repetition frequency.

Here is the schematic:

Here is the circuit for MULTISIM users:

Download, and then change the file type to .ms11

KEEP IN MIND, that the passive component values are set for HIGH frequencies. You will have to adjust them to match what range of frequencies you are looking for.

 

[Cyparagon]

I think those 0.1µF should be 100000pF and the 10000pF should be 0.01µF.

Otherwise, cool!

 

[HIMNL9]

You can probably use a single 555 with a "dirty trick", but this depend from what precision you need ..... with a single 555, you can change some duty-cycle AND frequency, but frequency changes only in a reduced range, and duty-cycle cannot go over 10-to-90 % range .....



EDIT: like this one: (damn coreldraw is not the better program for draw circuits )