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

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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?

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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.

 

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

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Hi,
I tried to change both regulator's capacitors to 10uF
But there is no changes. It is still the same waveform.

Below is the image.

Channel 1 is the Regulator Output: 5V, where my input is 6.8V
Channel 2 is the LM555 Output: Freq-194.9KHZ, Duty Cycle-50.1%

 

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

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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.

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E Cerfoglio
Buenos Aires
Argentina

 

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.

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Uncle $crooge

 

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

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

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

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Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com/

 

I agree that the capacitors are too small but I would go with a 250 uf on the input of the reg and a 1000 uf on the output. That should smooth things out

 

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

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