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Registered Member #58
Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317
I am having a problem with getting a 50% duty cycle 555 to run at 540khz, I've heard different ways of doing it and tried em with no luck, and just telling me r1 and r2 values isnt really much help because the variances in the schematics. right now i have a 100k pot going from pin 3 to pin 6 with a 102 (1nF cap) from pin 6 to gnd max frequency is like 10khz, which doesnt seem right at all. This is for a Solid State Tesla Coil to be run open loop. I could use a tl494 but I am not much sure on getting 50% on those either. Thanks for help with my noobish problem . Mike
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
You aren't going to get 50% duty cycle with a 555 timer with a circuit like that. To get 50% duty, you need to put a diode from pin 7 (cathode) to pin 6 (anode) across the resistor, r2.
If you are only getting 10khz, you need to check your values again. you have an error somewhere. Remember, a 555 timer is just an RC circuit charging and discharging. If your cap and resistor values aren't right, they will charge / discharge at the wrong rate.
Registered Member #139
Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358
If you _really_ need 50% duty, then you could do it with 2 555's, or a tl494. Is this for a startup waveform for your SSTC? If it is, you don't need 50%, or to run it at your coils Fres, any old freq'l do :P
Registered Member #97
Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61
When I need to get a 50% duty cycle with a 555 I always make r2 huge and r1 and the capacitor small. If you haven't used it here has a lot of useful 555 stuff.
Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
You don't need to use two 555s or large resistors to get 50% duty. An alternative astable circuit is to connect the output of the 555 via a resistor to the threshold/trigger pins and then have a capacitor from here to ground.
This has 50% duty cycle, although the output is no longer isolated from the internal circuitry (not always a problem).
Registered Member #358
Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28
Posted: Tue Apr 18 2006, 12:19AM You don't need to use two 555s or large resistors to get 50% duty. An alternative astable circuit is to connect the output of the 555 via a resistor to the threshold/trigger pins and then have a capacitor from here to ground.
That's exactly how he's got it set up, or at least how he tried to set it up.
max frequency is like 10khz, which doesnt seem right at all.
What's the minimum resistance of the pot? Try just a 1k resistor in there, that should get you close.
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Help with some foolish 555 duty cycle problems
