Thread: LM555 output frequency drops when supply voltage from the regulator

Hi,

I'm using a LM555 timer to produce a frequency of 240KHZ & 50% duty cycle.

The circuit i'm using is Astable. (See Circuit)
RA is 82ohm
RB is 200ohm
C is 0.01uF
RLs nothing

I built up this circuit and got nearly 240K frequency and 44% duty cycle

But when use a normal LM340T5 to produce a 5V to the LM555 timer...
The frequency immediately drop to 190KHZ
This cause my 3rd IC almost burn T^T

anyone can guide me to the solution?

Thanks
Best Regards
Wan Xi

P/S: Regulator Circuit See HERE

Do you have a bypass cap on input and output of your regulator?

When you got what you wanted, was your supply voltage from a PS @5v

Yes! I did put capacitors. (See Picture)

I tested them seperately. Both are ok.
But when i connect Regulator output to LM555 input..
The frequency just dropped to 190KHZ.

Sounds like your regulator is not regulating. What is input voltage source? Have you measured the 5 volts on the regulator output?

Your output cap for the regulator (or more specifically, the decoupling cap for the 555 timer) is much too small. For TTL 555 timers, you need a HUGE decoupling capacitor (like 10uF) because they actually produce a ~150mA short-circuit due to shoot-through in the internal totem pole output transistors whenever they switch (or so I am told). CMOS versions don't have this problem but have lower drive current.

Originally Posted by dknguyen

For TTL 555 timers, you need a HUGE decoupling capacitor (like 10uF).

Good idea, I always just put a 10uf cap + a .1uf on the reg output as a default.

Your input source to the regulator does not have enough current I think. That is why you see big spikes on your 5v. 2v spikes on Vcc are not good. I think you need a stronger supply before your regulator. Like I asked before, what is your input voltage source, and give specs.

Can you power your circuit, including regulator from your bench power supply?

Nice scope by the way

Originally Posted by Wan Xi

Hi,

I'm using a LM555 timer to produce a frequency of 240KHZ & 50% duty cycle.

The circuit i'm using is Astable. (See Circuit)
RA is 82ohm
RB is 200ohm
C is 0.01uF
RLs nothing

Try to use bigger resistors and a smaller capacitor - The current in the RC timing branch is too high for the power supply and for the 555's discharge transistor at pin 7.

Eg: with 820Ω, 2kΩ, and .001µF (= 1nF) you have the same frecuency with one tenth of the current.

Intersil makes a Cmos ICL7555. In its datasheet they say that the shoot-through current when an ordinary 555 switches is 400mA.

Two supply bypass capacitors should be used. One is a 0.1uF ceramic type to stop the very narrow high frequency spike that the 10uF electrolytic type cannot filter.

Originally Posted by Wan Xi

Channel 1 is the Regulator Output: 5V, where my input is 6.8V

The datasheet specs a Vinmin of 7.5V. Your 6.8V may not be sufficient to allow your part to regulate properly...

Originally Posted by Wan Xi

1) I built up this circuit and got nearly 240K frequency and 44% duty cycle.
2) But when use a normal LM340T5 to produce a 5V to the LM555 timer the frequency immediately drop to 190KHZ.

What are you powering your circuit with when it works properly, i.e. number 1 above?

If you need accuracy why are you using a 555 instead of a crystal oscillator?

Originally Posted by ecerfoglio

Try to use bigger resistors and a smaller capacitor - The current in the RC timing branch is too high for the power supply and for the 555's discharge transistor at pin 7.

Eg: with 820Ω, 2kΩ, and .001µF (= 1nF) you have the same frecuency with one tenth of the current.

Bingo!.............