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| General Electronics Chat This forum is for general chat about electronics, eg: Dont know what a part does? Dont know how to read a circuit? Want to get an opinion? |
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| | #1 |
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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 Last edited by Wan Xi; 8th January 2009 at 09:50 AM. | |
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| | #2 |
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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
__________________ Pay it forward. Last edited by Mikebits; 8th January 2009 at 08:18 AM. | |
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| | #4 |
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Sounds like your regulator is not regulating. What is input voltage source? Have you measured the 5 volts on the regulator output?
__________________ Pay it forward. | |
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| | #5 |
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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.
Last edited by dknguyen; 8th January 2009 at 10:31 AM. | |
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| | #6 |
| Good idea, I always just put a 10uf cap + a .1uf on the reg output as a default.
__________________ Pay it forward. | |
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| | #7 |
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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% | |
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| | #8 |
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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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| | #9 | |
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Eg: with 820Ω, 2kΩ, and .001µF (= 1nF) you have the same frecuency with one tenth of the current.
__________________ E Cerfoglio Buenos Aires Argentina | ||
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| | #10 |
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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.
__________________ Uncle $crooge | |
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| | #11 |
| The datasheet specs a Vinmin of 7.5V. Your 6.8V may not be sufficient to allow your part to regulate properly...
__________________ Go Trojans! | |
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| | #12 |
| What are you powering your circuit with when it works properly, i.e. number 1 above?
__________________ Go Trojans! | |
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| | #13 |
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If you need accuracy why are you using a 555 instead of a crystal oscillator?
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| | #14 | |
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| | #15 | |
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__________________ I've never advocated drugs, alcohol, violence or insanity, but they have worked well for me!.......Hunter S. Thompson | ||
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| Tags |
| drops, frequency, lm555, output, regulator, supply, voltage |
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