LM555 output frequency drops when supply voltage from the regulator

[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 **broken link removed**)
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 **broken link removed**

 

[Mikebits]

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

 

[Wan Xi]

Yes! I did put capacitors. (See **broken link removed**)

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

 

[Mikebits]

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

 

[dknguyen]

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.

 

[Mikebits]

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.

 

[Wan Xi]

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%

**broken link removed**

 

[Mikebits]

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

 

[ecerfoglio]

Hi,

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

The circuit i'm using is Astable. (See **broken link removed**)
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.

 

[audioguru]

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.

 

[saturn1bguy]

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...

 

[saturn1bguy]

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?

 

[blueroomelectronics]

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

 

[Mike_2545]

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.

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

 

[GonzoEngineer]

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!.............

 

[Hero999]

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

1000µF won't make much difference.

Anything over 100µF on the output and 1µF on the input is excessive.

I echo some of the comments above:

Increase the values of R by a factor of 10 and reduce C by a factor of 10 and the current consumption will be reduced.

What sort of regulator are you using?

You didn't say what the part number is?

Make sure the input is high enough so it isn't dropping out.

 

[saturn1bguy]

What sort of regulator are you using? You didn't say what the part number is? Make sure the input is high enough so it isn't dropping out.

He did say: LM340T5. Which is why I was able to quote Vinmin from the datasheet in a previous post, and why I suggested his 6.8V input may not be enough to allow his part to regulate.

(You must have missed that part of the discussion )