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What are the "steering diodes" next to the pot for?

Schematic
Source: http://www.youtube.com/watch?v=OXsu29K_Ap4

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It looks like you're running an older version of DaveCAD. The newer version... oh wait, there is only one version. :) –  JYelton Aug 28 at 17:26
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@JYelton There is a widescreen version too: youtu.be/2vkkaygR4HE?t=22m1s –  Armandas Aug 28 at 18:05
    
@Armandas Hah! I've got to get a copy of that version! –  JYelton Aug 28 at 18:23

4 Answers 4

up vote 7 down vote accepted

When the capacitor is charging, the current is "steered" through the right diode and the portion of the pot element to the right of the wiper. If the pot is R ohms and the position is a (where a = 0 is full counterclockwise and a =1 is full clockwise) then the resistance seen is a*R.

When the capacitor is discharging, the current is "steered" through the left diode and the portion of the pot element to the left of the wiper. The resistance is (a - 1) * R.

Thus the RC product over a total cycle is the same regardless of A, and PWM percentage can be adjusted without affecting the total period much (it is not quite symmetrical because of the 1K resistor).

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A useful video that illustrates this excellent answer I think: youtube.com/watch?v=YBNIq_d56sA –  David Aug 28 at 17:47
    
I've seen the same term applied to op-amps when they have two feedback paths with diodes in opposite directions to change the gain depending on pushing or pulling. +1 –  ACD Aug 28 at 20:17

I have no idea why they are called steering diodes, but the aim is to that the two 'sides' of the potentiometer control the high resp. the low part of the output waveform, with a wide range (almost 0% to 100%).

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They're to allow control of the duty cycle of the pulse train without modifying the frequency. The capacitor charges and discharges through the same 10k total for both phases, therefore the 555 is configured to work as a constant-frequency PWM generator.

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They're used to adjust the output's duty cycle by "steering" the timing cap's input and output currents through different resistances - depending on whether pin 7 is switched ON or OFF - altering the input's charge and discharge time constants.

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