square pulse generator with adjustable duty factor

[hatamata]

Hi
I built the simple 555 circuit that has a frequency range between 1 Hz to 80 KHz square pulse, but I cant find any circuit to allow me to control the duty factor of the output pulse.
I need to be able to adjust the duty factor between .01 to .0001 %
If my 555 circuit will not fit a duty factor controller and you think I should build another circuit from scratch, then there is no problem, I will go for the new circuit.

The other alternative would be to adjust the on times starting from 1ns to as much as 1 millisecond

The circuit will drive several MOSFETs in parallel that will control 60V max from a bank of charged capacitors, I've seen one circuit using 555 astable + 74AS00 as a monostable that was able to control the pulse width to as low as 10ns, unfortunately, I lost trace of it.

Thanks

 

[karesz]

Hi,
Im a little bit confused with your data:
0.01%-0.0001% is a range of 1:100_I think its to much for a 555, but you are speaking over pulse width setting of 1n-1msec; its a range of 1:1`000`000!
What is really needed?
In my opinion is a pulse width of 1 nsec is inpossible to generate with a 555.
K.

 

[hatamata]

Hi karesz and thank you

The .01 to .0001 % is of the output pulse width of the 555 (i.e. 0.01% times the pulse width) , if I'm getting say 100Khz, its pulse period will be 1/100000 = 0.00001 S, and at 50% duty cycle, that will give me the on/off time of 0.000005 S (5 micro second) this I think the minimum pulse width I can get from 555, at 1% duty cycle (.01% ratio that I mentioned in my post) will give 50 ns, and the the .0001% will give me .5 ns.

As you said, no way 555 can give such short pulse, besides, I dont need to go that low in pulse width and even if I did, it will be a complicated circuit to have a mosfet or igbt driver and then the mosfets/igbts + rise and fall times will be greater than this 0.5 ns if we are talking a simple circuit.

What I need is to be able to divide the pulse coming out of 555 (when at 0.01 duty cycle) by 100 and take only one of these 100 parts to work with, but at 100,000 Hz, I will not need shorter than 10 ns, however, at low frequency, I sure will need

OK, let's not work on duty factor, let us do it another way, can I feed the output of the astable 555 to a retrigable monostable chip that will give me an output pulse width starting from 10 ns and wider?

 

[FvM]

As said, it's completely illusional to achieve ns timing with 555 timer. You should have read this from a 555 datasheet before.

Generally, there are different concepts that can achieve the said specification. Personally I prefer an old fashioned analog pulse generator (HP 8082A) for similar purposes. It's mainly using discrete transistor cicrcuits (including some custom manufactured integrated transistor arrays) to generate the waveforms.

Today's pulse generators have partly digital waveform generation. I think a digital design (e.g. based on a FPGA) can be the most simple solution.

 

[karesz]

Hi Hatamata,
In my opinion is your "specification" very untypical constructed & "overs specifyed"! :-(
I would say it simple so: needed is a generator with frequency range of 1Hz--80KHz and duty cycles between i.e. 1...50%. That is all!
Other question is, how can you tune the frequency in the 1Hz-80KHz range, must be it in only one band?
If yes, than is a mus to make it with an DDS_ so it will be practically an arbitrary generator what you need.
You can it set with a PIC (or other micro/PC) easy with your wished _for other circuits very extreme_ parameters...
K.

 

[karesz]

Some old product for your situation:
**broken link removed**

 

[hatamata]

karesz
The astable part is not a problem, 555 with several capacitors on a selector + variable resistance will do it, I built several ones and I think I have one ready some where, I can build a new one in no time if needed.

My problem is with the pulse width, I need to have it between 10 nanosecond and 10,000 nanosecond (adjustable with a variable resistance and if it takes a selector or jumpers then that is fine). I know that in the pulse width with the range of 10 microsecond this might be wider than the pulse width coming out of the 555 and that is why I mentioned in my first post that I prefer to control the duty factor instead of pulse width, of course you know that Duty factor is the ratio of the ON time verses the OFF time of a signal.

I know that controlling the pulse width to the rang of as low as 10 ns can be done with one of the 7400 IC family but I jut gut lost when looking their data-sheets

Thank you also for posting the item on ebay, but I have a problem where I live that electronic items get lost in customs and that shipping may cost 10 times the value of the item, I hope I would be able to build it my self, besides, this one delivers a much wider pulse width than I need.

I dont know how to program a chip so I hope I can get an analog solution.

To phrase it the way you used I need a square wave generator, with frequency range between 1 Hz - 80 KHz (this part I alrady have) and duty cycle between 0.0001 and 1 %

 

[karesz]

Hi,
I have built so 15 years ago some similare circuit/function in ECL, but the minimal pulse width was at 3-5 nsec & adjustable up to (as I remember) ca.usecs(microsec)...
I dont have yet the circuit at hand, but the idea is: a diff line receiver to use where the shortest(!) differentiated input pulse & original are on the both inputs from one line receiver_your residual pulse has a changing width if you doadjust a potentiometer of the differentiator...
After that it has a buffer & finished. With 3-4 line receivers was to realize.
K.

 

[xaccto]

perhaps, using your Osc which you claim already have, use it to trigger like 74LS123
(retriggable-monostable) to generate the pulse width, for each cycle of your
oscillator.

But even 74LS123 I think can generate pulse 22nS (spec)
I really really think your spec of 1ns, you are DREAMING!!

good Luck

:|

 

[karesz]

Hi,
If monostabil, think pls on 74AC123=from 5nsec is possible, but from 4.5ns garanted_ i.e. by Hitachi...
It was surly product by Moto & Hitachi, but seems me to be obsolated! :-(
K.

 

[FvM]

I dont know how to program a chip so I hope I can get an analog solution.

O.K., but many specification points are still unclear:
- how do you want to control the pulse width (e.g. voltage, variable resistor)
- if we imagine control by a variable resistor, what's the required variation without range switching
- what's the intended PW accuracy

 

[hatamata]

Xaccto, the 555 timer I'm using is from here: **broken link removed** which as you see, is a very simple circuit but its minimum pulse width is 10 us. I dont think 555 can reach a pulse width shorter than that. I built it with a selector to chose the capacitor size so i can set the range of frequency along with a variable R.


karesz and FvM, I will have to re study the data sheet of these two ics, even if obsolete, I may still find it locally. the circuit I saw was built with 555 and 74AS00, it had jumpers to each 10X range which will use a 10 times bigger capacitor than the one before it and it had a small variable R to vary the pulse width within this range. Accuracy of 5 - 10% is fine with my application. I dont need to go as narrow as 1 ns as I already said in an earlier post, starting from 10 ns would be great, if not possible, then as close as possible to this range. This will fit my needs very well if I could find this circuit again. It doesnt of course have to use the 74AS00, any other chip if from the same family will do as I think this family is available here.

 

[hatamata]

Guys, thanks, a friend of mine found the circuit and gave it to me, I will post it for every one as soon as I draw it with eagle

 

[karesz]

Fein for you!
I have the said & scanned circuit_as idea is maybe interesting for somebody...
5.6pF/560 Ohm Trimmer has had pulse width of 5--50nsec, as minimal version.
K.

 

[hatamata]

Yep, that is a good one. Thanks karesz

Here is a scan of the one I have, a friend built it and gave it to me last night, I tested the actual circuit last night and is working fine, I cant tell the exact shortest pulse width as I only have a slow usb scope, but it can go below 1 micro second as I could see it's effect on the system I need it for.

As it is shown, it is triggered from pin 3 of a 555 (the circuit in my earlier post will work fine as a trigger in the range of 1 Hz till 100KHz - Astable) you dont need to vary the pulse width of the 555 as a 50% duty cycle will work.

The attached circuit is the retrigable monostable using 74AS00, varying C and the variable R (between pins 3 and 4+5) will vary the pulse width, changing C will change the range, however, my friend used the SMD version of this IC and he advises that C and R would be as close to your printed circuit as possible.

The power source that I'm pulsing is a 60V/ 10A charging a 47000 mf capacitor bank and I think it is handling it fine till now.


PS: this circuit needs regulated 5V

For C: you can use a 47 pf capacitor, but then the circuit will be very sensitive for electric ‎distortion/ interference, so a 100 pf is better for the lowest pulse width (10 ns).‎
Also, you can not use a selector to change capacitors in this part of the circuit (not like ‎the 555), if you need more than one range, use jumpers to select a capacitor that fits your new range, you can use: 100 pf, 1 nf, 10 nf, 100 nf and 2.2 uf to get 10X range from one capacitor to the next.

For R: a 100 ohm + a 4.7 k ohm variable. the V Resistance must be a small one mounted ‎as close as possible to the board (knob type will not work here), use a non metallic screw driver to vary it's value.‎

 

[karesz]

Hi!

Best tnx!!
Over the circuit: I would use a 74 ac00!!
It has similar rise/full times as AS00 (1.5-2nsec with 10pF Cload & ca. 4.5nsec with 50 pF Cload...), but with a small portion of power consuming, than its not a TTL component, its level are not so depending from 5V +/-5% supply & has not only 16ma output current capability but typically +/-50mA...
https://www.fairchildsemi.com/ms/MS/MS-542.pdf
With CMOS (AC00) you can have bigger value of resistor/potentiometer too_maybe you need only 2 different capacitors or resistors..
BTW; I can not agree with your friend; its possible to use a range switch & poti too, but you should route very short to the IC, but its really possible!
K.