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.

 

[IRstuff]

Nonetheless, there are bipolar transistors that can do the job:

http://www.onsemi.com/PowerSolutions/product.do?id=ECH8102

Vce.sat=0.135V at Ic=6A

TTFN
faq731-376

 

[OperaHouse]

Yes, even the venerable 2N3055 will get you to less than .2V satyration at 1A. I use the 10X rule, 1A use needs a 10A device. I do suspect that BC???? transistor will not be able to drive it. Use the 2N2222 for the driver,

 

[serbar]

Don t know if i have to create a new tread for this, if yes tell me please.

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.

 

[OperaHouse]

The voltage should remain constant, the current will change. It is just a solenoid. Why does it matter if you are not driving it with exceptional voltage.

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.