Electronics > Beginners
555 timer astable questions.
(1/2) > >>
MarkusAnd:
Hello!
As I'm continuing my testing of simple circuits I have some questions about using the 555-timer in astable mode. In most of the circuits that use a 555-timer to output PWM-signal, there are two diodes connected between R2 and the pot. When I remove the diodes as in the second picture, i can only adjust the output between 2v and 3v(5v input), and when i use the diodes as in the first schematic i can perfectly dim a LED. Why are the diodes used and why can't I get PWM-output without them?

//MA
LateLesley:
In the first circuit, the way it is wired, you are changing the mark-space ratio of the oscillator. In the second, you are only changing the frequency of oscillation, not the mark-space ratio, so it's always roughly
50%, no matter how fast it oscillates.

The 555 is a relaxation oscillator, it works by charging and discharging C1. When charging, it goes through R2 and R1, when discharging it'll go through R1.

Where the diodes come into play, is to help increase the variation in charge and discharge time.
when charging, it'll go through R2, D2 the right half of R1 to the cap C1.

When discharging it'll go through the LEFT half of R1 and D1 and to ground through the discharge.

as the wiper moves left, it takes longer to charge and discharges faster, giving short pulses.

as the wiper moves right, it charges faster, and takes longer to discharge, giving longer pulses.

The second circuit can vary the mark space ratio, but not as much. it'll just vary the frequency more as both the charge and discharge times get shorter with decreased resistance.
rdl:
Dave built a version of the first circuit in EEVblog #392 and there is some explanation of how it works.
MarkusAnd:
okay, so when I don't use the diodes I'm changing the frequency. So the only reason I'm using the diodes is to give myself a bigger range or? And do I change the frequency when I'm using the diodes?
LateLesley:
It might change the frequency a little, but not by much at all.

With the diodes, the frequency hardly changes, but the mark-space ratio will.

Without the diodes, the frequency changes, but the mark-space ratio hardly changes.

Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod