555 duty cycle modulator 1

[serbar]

Hi.

Wanted to build a circuit with a NE555 that provides modulated PWM.

After some research I found that in astable mode with a diode in paralel with the discharge resistance of the capacitor i can do this.

I made the circuit and i can modulate PWM (16-88% Duty cycle). My problem is the calculations i did first don t match with the real world values, and i would like to know why.

Hope some one explain to me why...

Ok i will try to post and explain what i did step by step.


1. My goals with the circuit:

Test control vacuum electrovalves, this ones work with control signal of more or less 300 Hz frequency PWM (variable duty cycle of course) and 12 V.


2. Calculations i made:

If I want more or less 300 HZ freq i will try to calculate R1 and R2 (see atached file of hand made circuit) based on this and on the duty cycle i want every time:

Frequency expression based on my calculations:

f=1/( ln2*R2*C + R1*C*ln[(0.66*Vcc-Vdiode)/(0.33Vcc-Vdiode)] )

For my aplication the fixed values are:

f=300Hz
Vcc=5 V
Capacitor capacitance C= 47 uF
Vdiode=0.7 V

Duty cycle expression arranged in function of R2/R1 ratio:

R2/R1 = ((1-DC)/DC)*ln[(0.66*Vcc-Vdiode)/(0.33Vcc-Vdiode)]

For my aplication the DC fixed values i want are:

DC
0,99
0,85
0,8
0,75
0,7
0,65
0,6
0,55
0,5
0,45
0,4
0,35
0,3
0,25
0,2
0,15

Vcc=5 VVdiode=0.7 V

Now for every different duty cycle value, i have a different R2/R1 ratio, combining the DC equation and the frequency equation i have two equations with two variables for each DC value, so i can calculate R1 and R2 for every DC value i want and 300 HZ frequency right? At least on my Excel sheet.

3. Real World conclusions:

The R1 and R2 resistance values calculated don t match the DC values i calculated, and the frequency changes as i change the value of R1 and R2 (both 470 ohm pot).

Why this happen? What am i doing wrong?

Thanks in advance.

 

[OperaHouse]

I avoid anything with a 555 in the same way I dont eat insects even if they are chocolate covered. You could do the same thing with a TL494 or later variations of similar PWM regulators. Duty cycle will be completely variable and frequency is totally independent. Personally if you are going to make a test jig, use something like a UNO processor that will give you repeatable values without a whole wall of pots that always need adjusting.

 

[VEBill]

"...Capacitor capacitance C= 47 uF..."

Electrolytic tolerance is often something like +50%/-20%.

 

[serbar]

uy9yuiy

And what about ceramic instead?

Thanks.

 

[OperaHouse]

I think it has to do with that darsh gone diode thingamajigie.

 

[IRstuff]

Why do you have the diode there? The basic charging configuration of C would have about 3ma of charge current, but your diode limits the drop across R2 to 0.7, when it should go up to about 1.7V.

http://www.fairchildsemi.com/ds/LM/LM555.pdf

Also, your circuit diagram corresponds to the diagram for pulse position modulation, not pulse width modulation.

TTFN
faq731-376

 

[serbar]

The diode is to bypass R2 when capacitor charge happens, in this mode R1 plus voltage drop across diode are the only things that limit charge time, and duty cycle can be less than 50%. in astable DC= (R1+R2)/(R1+2R2). If lower R1 to very small values aproximate from the DC= R2/(2R2) that is never less than 50%.

http://www.williamson-labs.com/555-circuits.htm

I don t understand very well the circuit diagram for PWM with LM555 on the data sheet you posted, do i have to generate a sine wave or square wave in pin 5? How can i do this in my aplication? And the trigger have to be continous?

Thanks.

 

[btrueblood]

Agree with operahouse, never seen the 555 wired with a diode across the setting pins...just resistors. What happens when the pin-to-pin voltage is below the diode Vf?

 

[serbar]

As i said i can modulate PWM with this configuration, but i want to do this keeping frequency constant.

I searched a litle more and seen in a book of signal conditioning that placing a potentiometer between pin 5 (control pin) and the capacitor can modulate output signal frequency.

Anyone allreday tried this?

Thanks.

 

[serbar]

Ok i now tried without diode, in astable configuration, and guess, i can have less duty cycle than 50%. I am very confused now. Nothing matches the 555 theory i consulted.

I tried potentiometer in pin 5, it changes frequency a litle but messes with duty cycle too, and that was expected.

 

[serbar]

Regarding my previous post, i have min duty cycle in astable mode of 47%, no less, so i guess this is aproximately 50%.

With diode i can reach min values of 10%-15%.

 

[jimkirk]

I agree with OperaHouse, there are probably better ways to do this, but if you really want to use the 555, the timing components you have are very low, and that can affect circuit operation, as well as electrolytic capacitor tolerance.

Consider increasing the resistor values by a factor of 100 and reduce the capacitor by the same. So 47000 Ohms (or a 100 kOhm pot, one end to Vcc, other end to pin 6 and wiper to pin 7. also, put a 1 kOhm or so resistor in series with each end so the value never goes to zero) and a 0.47 uF cap, mylar would be good, X7R adequate. The 555 will be much happier with these values.

That said, with a $25 or so Arduino you can vary the duty cycle in 1/256th increments with crystal controlled constant frequency. It would be simple to have a pot drive an ADC input to control the variation, so it would act in a very analog manner.

 

[VEBill]

Circa 1980, for a microcomputer programming project at university, my buddy and I teamed up to program the Single Board Computers (8080?) to measure and display engine RPM and ignition dwell using a simulated ignition primary signal (minus the real world noise). To simulate this signal I built a 555 based astable with two pots, one for "RPM" (frequency) and one for "Dwell" (duty cycle).

If I recall correctly (?), the only solution required a comparator. The 555 provided the Astable. The (only) clever bit was that the circuit "stole" the near-linear triangle waveform from the 555 (the node that oscillates from 1/3 to 2/3 Vcc), and compared it to an adjustable reference made from two 4.7k resistors and a 5k pot (these values ensure complete 0 to 100% duty cycle range with a bit of extra pot travel at each end to make sure). The circuit was published as a Design Idea in either EDN or Circuit Design magazine sometime in the late-1980s or very early-1990s.

I distinctly recall determining that it needed a second building block (the comparator stage) to provide the duty cycle adjustment. I concluded that there was no practical solution to have independent adjustments for frequency and duty cycle just using one 555. Feel free to prove me wrong, but that was my very carefully considered conclusion at the time.

It's sad that I can still recall these details 32 years later. I think I'll go have a drink.

 

[VEBill]

Hey, I found this:

http://www.edn.com/design/test-and-...nerator-has-independent-pulse-width-frequency

To see the circuit, click on Figure 1 that should lead to a PDF of the section of the magazine, then scroll down.

 

[VEBill]

PDF LINK Page 6.

It's not exactly a self-evident circuit.

 

[OperaHouse]

"It's sad that I can still recall these details 32 years later. I think I'll go have a drink"

Everyone has to have their 666 experience, They never listen when you tell them. This will also be a memory carried for life.

 

[VEBill]

Everyone has to have their 666 experience

"666" = The Sign of The Beast.
"668" = Neighbour of The Beast.
Etc.

 

[serbar]

Thanks for the ideas everyone.

VE1BLL in the circuit from the link you posted where does pin 7 connects? An dthe wiper pin of P2 (R1)?

 

[VEBill]

The answers to your questions would come from one "Darvinder Oberoi, CEDTI, Jammy, India".

That circuit by Mr. Oberoi is too mysterious for me to comprehend. I still like my idea better as it's perfectly straightforward and yet just a wee bit clever.

 

[serbar]

http://www.ehow.com/how_5992626_build-pulse-width-generator.html

VE1BLL i think this link express your design idea. I will try this but would like to know too if the circuit design from mrs Darvinder works too. I will dig a litle more.

Thanks.