# Are 555 PWM circuit resistor and capacitor values independent of the voltage applied to a 555?

I am new to electronics so my question sound dumb.

I am using this diagram to build a 555 timer IC PWM circuit:

The above circuit operates on 9V but I have 5V.

Do I need to change capacitor and resistor values also, or are these values independent of the voltage applied?

From the below picture, it doesn't looks like I need to change a capacitor and resistor values.

• um, what sense do the two diodes make there? This "schematic" (it's not a schematic, really) does not seem to be quite sensible! So, start with a better schematic. The Texas instruments NE555 datasheet has an example "9.2.2 pulse-width modulation", which makes very much more sense. Commented Jul 26, 2021 at 11:42
• @MarcusMüller The diodes allow independent adjustment of the charge and discharge. Commented Jul 26, 2021 at 11:58
• @HandyHowie but it's but one potentiometer, so these are inherently linked anyways Commented Jul 26, 2021 at 11:59
• How accurately do you need to know? Do you see voltage shown in the equations for T? Well then... to a first approximation, no. But read the fine print in the datasheet ... if you need precision, you may notice a percent or so drift when changing voltage. But then, if you need precision, a 555 is the wrong tool for the job.
– user16324
Commented Jul 26, 2021 at 12:06
• @MarcusMüller This answer explains the diodes - electronics.stackexchange.com/questions/563128/… Commented Jul 26, 2021 at 12:12

Yes, the frequency of oscillation is independent of power supply voltage $$\V_{CC}\$$. Almost. It's impossible to make it completely independent, but the designers went to great lengths to keep $$\V_{CC}\$$ out of that equation.

This is because the 555 timer IC design relies on the time it takes for a capacitor to charge and discharge to some fraction of the supply, not to any particular specified voltage. Whether your supply voltage $$\V_{CC}\$$ is 6V or 12V, the timing capacitor will charge to $$\\frac{2}{3}\$$ of that supply in the same amount of time; it's just that by doubling the voltage, you also double the charging rate. The time constant $$\\tau = R \times C\$$ of the resistor+capacitor system remains the same in all cases:

simulate this circuit – Schematic created using CircuitLab

On the left the supply is +6V, and on the right it's double that at +12V, but the resistors R1 and R2 have the same values, as do capacitors C1 and C2. Therefore the time constant in both cases is the same:

$$\tau = R \times C = 1k\Omega \times 1\mu F = 1ms$$

Here's a graph of the voltage across the capacitors, at X and Y as they charge, from the moment power is applied at time $$\t=0s\$$:

The horizontal green markers represent $$\\frac{2}{3}\$$ of the supplies, 4V and 8V. Notice how both outputs X and Y reach their respective $$\\frac{2}{3}\$$ point at exactly the same instant, $$\t=1.1ms\$$. Therefore, the time constant represents just that; the time it would take for the capacitor to charge to about $$\\frac{2}{3}=67\%\$$ of its final value. More precisely, it's 63.2%, but in any case, it's the same regardless of supply voltage.

There will be a tiny variation of oscillator frequency with $$\V_{CC}\$$, because there are other elements inside the IC which are not quite so independent of that supply, but that variation won't be nearly as significant as frequency variation with time and temperature, both of which cause capacitor and resistor values to change. This should be of greater concern.

If you want an oscillator that is truly immune to power supply voltage and component degradation, you should to use a crystal, which can guarantee oscillation at a frequency to within about 0.01% of of its rated frequency, over a larger temperature range, and a much longer lifespan.

The PWM CTRL signal works best from 0.6V to Vdd and is independent of supply voltage for duty cycle as the pulse ratio is determined by the dual thresholds of 1/3 & 2/3 V+ but low side is about 1 diode drop above ground due to Vce(sat) on reset.

This will work the same on 9V as 5V for f and d.c. but if you want full PWM range, I suggest a better way.

For a broad list of dimmer, ESC motor control , read lots more here.