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Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
Hi everyone, I'm new but I plan to stick around :) My question- I'm trying to create a frequency with the 555 at about 20-30kHz, but when I try at high frequencies, i just get a constant high output, no pulses. I'm using this to drive a flyback, hopefully for a plasma speaker, so I'm not sure if this goes in the high voltage section or here, It's more of a circuit problem, so I'm putting it here but if a mod wants to move it by all means please do.
I'm following this schematic; with a 10nF cap, a 1K resistor for R1, and a 2K resistor for R2, which should be 28kHz if I did my math right; any ideas what I'm doing wrong? can the 555 not handle these high frequencies? because it was working with a 10K pot for R1 and R2 with a 10nF making about a 4kHz arc, I can't think why it wouldn't be working with these values :( (yes, I have a decoupling cap on pin 8)
Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
Thanks guys, using a 10K, 22K, and 1nF gave me a perfect 25kHz :D @proud mary, It would appear there is, I'll keep that in mind from now on. in addition, one of the guys over at AAC pointed out that I need 100 ohms for every volt on the Vcc for R1. @experimentonomen, I have never heard of that, voltage in on the output pin!? @IamSmooth, yep, that did the trick :D
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
The rule of thumb i go by when making a 555 oscillator, is that if you keep R1 1Kohm and R2 some typical value like 10Kohm, the output will be very close to 50% duty. So, if you are not pleased with your duty %, just use 1k for R1 and change R2 to keep the frequency you want.
But, about the circuit experimentonomen is speaking of uses the output of the 555 to charge the capacitor, instead of charging it with the v+ rail, through a resistor and diode. You can also use pin 3 as the source AND discharge, simply by not using a diode. These two circuits have no advantage in the case of performance.
However, there is one more circuit utilizing the output as the charge/discharge source for the cap that is very useful as a PWM control. (will open a java app)
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555 high frequency oscillator question
