555 duty cycle modulator
555 duty cycle modulator
(OP)
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.
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.





RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
Electrolytic tolerance is often something like +50%/-20%.
RE: 555 duty cycle modulator
[quote Electrolytic tolerance is often something like +50%/-20%.]
And what about ceramic instead?
Thanks.
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
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
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RE: 555 duty cycle modulator
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.
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
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.
RE: 555 duty cycle modulator
I tried potentiometer in pin 5, it changes frequency a litle but messes with duty cycle too, and that was expected.
RE: 555 duty cycle modulator
With diode i can reach min values of 10%-15%.
RE: 555 duty cycle modulator
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.
RE: 555 duty cycle modulator
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.
RE: 555 duty cycle modulator
http://www.edn.com/design/test-and-measurement/436...
To see the circuit, click on Figure 1 that should lead to a PDF of the section of the magazine, then scroll down.
RE: 555 duty cycle modulator
It's not exactly a self-evident circuit.
RE: 555 duty cycle modulator
Everyone has to have their 666 experience, They never listen when you tell them. This will also be a memory carried for life.
RE: 555 duty cycle modulator
"666" = The Sign of The Beast.
"668" = Neighbour of The Beast.
Etc.
RE: 555 duty cycle modulator
VE1BLL in the circuit from the link you posted where does pin 7 connects? An dthe wiper pin of P2 (R1)?
RE: 555 duty cycle modulator
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.
RE: 555 duty cycle modulator
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.
RE: 555 duty cycle modulator
After some research and experimenting i concluded the advices you all gave me, namely:
1. Tried a smaller capacitor (0,1 uF) with the "astable/diode in paralel with R2 configuration" in 555 and the timing equations are much more reliable compared with real world values. However the frequency is afected by R1 and R2 variations. Not good for what i want.
2. Tried this circuit : http://www.rmcybernetics.com/projects/DIY_Devices/... , and voila, independent frequency adjustment and duty cycle adjustment.
Special thanks to VE1BLL and all others.
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
I want to have more or less 500 mA with 12 V on the electrovalve, so i need a transistor since LM393 output is max 5 mA, no problem. But with the configuration "electovalve load in series with transistor colector-emitter circuit" i don t have constant 12 v on electrovalve terminals. When i change duty cycle i change voltage on the electrovalve.
I dont see how to overcome this...
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
As i said i want to test this electrovalves, so i want to provide a signal exactly the same as the ECU from the car provids when these are fited in the car. In the car when i connect osciloscope at electrovalve terminals the "on" voltage is always batery voltage (more or less 14 V) regardless of duty cycle value.
In my aplication as i increase duty cycle the "on" voltage across electrovalve terminals decrease (actualy a 12,5 ohm resistance right now).
My question is: Can i do something to keep "on" voltage constant regardless of changing duty cycle, as hapens in the car?
Thanks.
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
If you want to make it work like when it is controlled by the ECU, you might want to see if you can find out what transistor is in the ECU ouput. The reason this might matter, is that different transistors are going to have varying clamp voltages to protect the transistor. It can be anywhere from 20V to several hundred. This difference in voltage will affect the behavior of the solenoid as it shuts off.
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
Yes, always make sure the transistor saturates at these speeds. The spec sheet of the 2N2222 says you will get 1.6V at 500ma. At an amp that would be a couple volts and probably not for long. If you paralleled up five of them it might work. As far as zener clamping, no clamping is the best. I've noticed in automotive systems a trend to use resistors instead of diodes. My guess is a 100 ohm 1W resistor in parallel with the coil would keep the spikes under 50 volts, be fast and reliable. Once you get over a zener over 30V it likely doesn't matter at these speeds.
RE: 555 duty cycle modulator
1. Use a TIP31C transistor;
2. Increase the current button in my power suply since it was giving me a very low current suply (i think it was the main problem).
You see some times i forget that a power source is just it, a "power" provider and power = U * I not just U...
Sincerely thanks to everyone.
RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
I will try with a FET too.
Thanks.
RE: 555 duty cycle modulator
http://www.onsemi.com/PowerSolutions/product.do?id... Vce.sat=0.135V at Ic=6A
TTFN
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RE: 555 duty cycle modulator
RE: 555 duty cycle modulator
My question is:
Just for curiosity i want to know the power dissipated by the elctrovalve. So i need to know current and Voltage. I searched in some websites for calculations of a mean voltage and current in PWM drived devices.
I found this formula:
Is this right? if now i can i calculate a mean voltage and currente in PWM? Thanks.
RE: 555 duty cycle modulator
I just wanted to add that unless you use a base-emitter resistor on the driver transistor you don't get the specs you think. Of course that also sucks away drive current.