Voltage controlled duty cycle with a CMOS inverter

Sure, except that you need another inverting function. Wire one as an integrator and one as a Schmitt (with the extra inverter/transistor or what have you). use a voltage divider to move the Schmitt's trip points.

Won't be too well controlled, but probably OK for closed-loop use.

cheers

Phil Hobbs

So, you mean a RC oscillator for the first and a resistor with FET to offset shift it for the next section?

Cheers

Klaus

Den torsdag den 18. august 2016 kl. 00.14.35 UTC+2 skrev snipped-for-privacy@gmail.com:

it is a bit more delta-sigma than pwm and I have never tried it, but:

WIRE -32 160 -112 160 WIRE 80 160 80 64 WIRE 80 160 48 160 WIRE 144 160 80 160 WIRE 224 160 224 64 WIRE 224 160 208 160 WIRE 384 160 224 160 WIRE 560 160 528 160 WIRE 384 192 384 176 FLAG 80 224 0 FLAG -112 240 0 FLAG 560 224 0 FLAG 144 176 0 FLAG 384 192 0 SYMBOL Digital\\schmtinv 144 96 R0 WINDOW 3 96 -57 Left 2 SYMATTR InstName A1 SYMATTR Value vhigh=5,td=1n,vt=2.5,vh=0.1 SYMBOL cap 64 160 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL res 208 48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 64 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL voltage -112 144 R0 WINDOW 3 -219 55 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value SINE(2.5 2 10) SYMBOL res 544 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL cap 544 160 R0 SYMATTR InstName C2 SYMATTR Value 100n SYMBOL Digital\\schmtinv 384 96 R0 WINDOW 3 73 6 Left 2 SYMATTR InstName A2 SYMATTR Value vhigh=5,td=1n,vt=2.5,vh=0.1 TEXT -98 264 Left 2 !.tran 100m

-Lasse

...Jim Thompson

But if you want perfection you'll need to use a 555 Timer and a comparator >:-} ...Jim Thompson

You can do pretty good with just a 555 Timer...

;-) ...Jim Thompson

This was last weekend's project for Mo. Well, it's almost done. It's a sprinkler timer and a Rat Light (tm) power supply.

I did a similar thing about 15 years ago, in a steel Hoffmann box, but it totally rusted out in the sea-salt fog. This one is plastic with stainless hardware.

The Rat Light is an LED thing that illuminates the lemon tree, and keeps a big rat from skinning the lemons at night.

Seems to work.

The box is from Amazon, really nice:

Oops, wrong thread. Should have been home improvement.

Thanks, that was sort of what I was thinking about, just a pullup and a FET to pull down :-)

Cheers

Klaus

That's a nice solution also :-)

The lowest price is 6 cents for a 555. Very nice price, but I would be killfiled by all at SED if I use a 555 :-)

Cheers

Klaus

I don't know why there's all this RELIGIOUS bias against the venerable

Did you see my post where I did the duty-cycle with JUST a single 555? ...Jim Thompson

You can do just as well with a much cheaper and smaller footprint single supply comparator:

Please view in a fixed-width font such as Courier.

. . DCM . . . --------[R]---- . | | . | TC75S56F | . | | \ | . +------- |- \ | . C | | >--+-->out . === ----|+ / | . | | |/ | . com | | . | | . Vc >---[R1]--+----[R2]--- . . . . . .

The classical duty cycle modulator is a tri wave generator and a comparator with a bit of hysteresis. With two inverters, you can use one as an (approximate) integrator and use a voltage divider on the input to the other one to move its logic threshold. A third inverter would let you add some hysteresis, which would help the edges a lot.

Pretty crappy, but would work if the requirements on range and accuracy are sufficiently loose and you design around all the corner cases (logic threshold variations especially).

Cheers

Phil Hobbs

Like this...

Or this is even better ;-)...

...Jim Thompson

You can build the classic R-C-schmittgate oscillator, and pull its duty cycle with an added resistor to the gate input. LT Spice refuses to simulate that.

Well, some of us have standards. ;)

The requirement is vague as to whether the OP cares about the frequency going all over the place with duty cycle, but I wouldn't do it in a product anyway--the logic threshold is allowed to vary all over the map, so it would be a potential headache if you got a reel of HC04s whose logic threshold were near the upper or lower limits. That's one that might work better with TTL. ;)

Cheers

Phil Hobbs

But LTspice is free ;-)

However, if you know about .IC, it will run on LTspice >:-}

Best solution is _always_ a 555 Timer...

...Jim Thompson

In real life, HC schmitt oscillators are usably repeatable. The duty cycle thing would change frequency, but that might be OK for something like a PWM temperature controller. The requirement is vague.

If it's not good enough, use something better.