View source for Generate pwm using ic 555: Add a diode

== Method 2; Longer low time: Add a Diode ==

<gallery>
Ic555 diode.pdf| The schematics. 
555 diode Ranalysis.png| The simulations.
555 diode RanalysisContour.png| The contour and surface plot of varying resistances.
Ic555 diode2 R.pdf|Added the second diode. This will make the output identical if R<sub>1</sub> and R<sub>2</sub> are identical.
Ic555 diode2 R3.pdf|One more resistance (or potentiometer) to make it more symmetric. This is discussed in more detailed in the next chapter, because the two potentiometers can be easily replaced by a one.
</gallery>

The added diode ''D'' shorts the out the resistor ''R_2'' during the charging cycle; thus the capacitor charges directly from <math>R_1</math> removing ''R_2'', though a small '''leakage current''' will flow through it. The constants are modified due to the voltage drop of ''D''.

<math>
\begin{align}
T_\text{on} &= 0.8 R_1 C \\
T_\text{off} &= 0.7 R_2 C \\
f &= \frac{1.44}{(R_1 + 2R_2)C}
\end{align}
</math>

<math>
pwm% = \frac{ T_\text{on} }{ T_\text{on} + T_\text{off}} 
= \frac{0.8 R_1 }{0.8 R_1  + 0.7 R_2  } 
</math>

which simplifies to 
<math>
pwm% \approx \frac{R_1 }{R_1  + R_2  } 
</math>

The simulation file: [[File:555 diode 300 300.circuitjs.txt]]

The '''leakage current''' through ''R_2'' can be eliminated by introducing a second diode ''D_2'' in series with that. This will make the output identical if R<sub>1</sub> and R<sub>2</sub> are identical. See the image above. The corresponding simulation file is [[File:555 diodes2 300 300.circuitjs.txt]]. Adding one more resistance (or potentiometer) to make it more symmetric. This is discussed in more detailed in the next chapter, because the two potentiometers can be easily replaced by a one. The file is [[File:555 diodes2 300 300 300.circuitjs.txt]].