Advice on 555 PWM led control

[GadgetBoy]

So, I've been messing around with the venerable old triple nickel timer, never used it for a variable PWM before, every application I've used it before was for astable or stable multivibrator.

So, here I am, cursing at it because I can't get it to work in PWM to save my life. Think maybe my components are bad, throw in some standard values for a 1Hz blink, and it's working fine. Put my components back in for PWM, nothing.

Fast forward several hours of experimentation, and I've determined that the values of resistors I'm using are too low - it seems the triple nickel can't function if the values are too low. I redo my math to incorporate higher values, and it starts working right away (sorta... the range of adjustment isn't as wide as I'd like, but I'm figuring it out).

So, I have it working, but my question is this - is there a sweet spot for the resistor values you use?

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[Zero999]

Please post a schematic.

The 555 timer can be damaged if too lower resistor values are used, because they allow too much current to flow, resulting in too higher power dissipation and overheating.

Try replacing the 555 timer and trying again with the high value resistors.

[GadgetBoy]

The 555 is fine, after I started using higher value resistors, the PWM started working, I also figured out the range issue. My question was if there was a specific base resistor value I should work from. I'll whip up a schematic tomorrow, if you really want it.

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[Zero999]

As with many things, there's no hard limit for the minimum value.

Refer to figure 12 on page 11 of the data sheet. Current flows through R_A into pin 7 of the 555. If R_A is too low, the voltage on pin 7 will be higher, than it should be, when the discharge transistor is on, which will affect the stability and to the extreme, will stop the circuit working properly and could damage the 555. Unfortunately I didn't see a maximum rating for the current into pin 7, but the saturation voltage is specified at 8mA and going from the graphs, the minimum recommended resistor is 1k.
http://www.ti.com/lit/ds/symlink/ne555.pdf

In general, it's good to keep the timing capacitor small, because large capacitors have generally poorer tolerance, leakage and stability.