anybody know of an oscillator or multivibrator than I can crontrol both frequency and duty cycle. I tried to find a chip like the CD4047 but with variable duty cycle. My output needs to be about 12v. I also looked at the 555 timer, but it is hell to tune the duty cycle as the frequency changes. Maybe there is a circuit out there with a 555 that has fixed frequency and variable duty cycle?
You can add a couple of steering diodes (in series with Ra and Rb), the right direction, of course, and have independent control of Ton and Toff.
Or, for one, where I needed a fixed freq. but variable duty cycle, I made an ordinary astable with discretes, but used a pot for the base resistors, with the supply to the wiper, and each end of the pot to a 1K to the base of each transistor.
On 2008-09-19, Electronworks.co.uk Electronic Kits wrote:
it won't go close to 100% duty cycle reliably. (it'll tend to break into frequency divider mode)
the 555's got 2 comparitors in it and generates a triangle (ish) wave on the timing cap.
could possibly use the second 555 to measure the first's state.
advantages of the 555/556 - stronger totem-pole output drive than common op-amps, stronger open-collector output (on the discharge pin) than commom comparitors. disadvantages 555/556 is larger and consumes more power than low-power op-amps. input impedance on the CV pin is rather low (about 6K to 2/3 vcc).
eg: here's an all-555 variable frequency capable PWM 0-100% circuit I drew up in ltspice
228-231. The basic idea is to use a simple 2-gate (inverter) oscillator. Inverter 1 provides the output, which goes through C to a node that feeds the input of inverter 2 through 470K, and also connects the feedback (frequency setting) resistor R from the node to the output of inverter 2. The output of inverter 2 connects directly to the input of inverter 1.
That's the basic oscillator, where R and C set the frequency. To vary the duty cycle as well, you use a pot for R, with the wiper connected to the output of inverter 2. The legs of the pot connect back to the C node through diodes with reversed polarities.
One problem with this is that R sets the frequency, and the duty cycle is set by the proportion of R to each diode. Just changing the single pot setting only changes the duty cycle, but not the main frequency. To vary both, you need a second pot in series with the R pot, but then you reduce the range of duty cycles.
On the other hand, if you don't need intuitive separate frequency and duty cycle controls, just replace the R pot with two separate pots as adjustable resistors. Each one will affect both frequency and duty cycle, but together you will get a huge range of both.
Best regards,
Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis
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The usual way to do this is to make a triangle or sawtooth oscillator whose amplitude is constant but whose frequency can be varied; a 555 does that. Then run the sawtooth into a comparator working against an adjustable DC voltage level. The comparator output is a constant duty cycle independent of the sawtooth frequency.
Or do it digitally, in an FPGA, with a DDS frequency-set loop and a counter/comparator for the duty cycle.
Click on '555_timer.jpg'. You can use any old comparator, but high input impedance ones, like FET input comparators, will be best. The two pots control the frequency and the duty cycle. You can control duty cycles between 0 and 100% using the comparator input pot.
If you like programming, a microcontroller can do this kind of thing quite easily, and is trivial to program for these sorts of applications.
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