tirsdag 1. august 2017

Yusynth VCO core discharging

I may have written about this before, but here goes:


The Yusynth saw core ramps down from 5 to 0v, then resets to 5v. the top of the charging cap is always at 5V while the bottom drops towards zero as current is sunk through the expo converter (u4).

The LM311 comparator has its positive leg grounded (when sync is not used). The LM311 is an open collector type comparator. For these, the rule, as written here, is that: 


"Current WILL flow through the open collector when the voltage at the MINUS input is higher than the voltage at the PLUS input.


Current WILL NOT flow through the open collector when the voltage at the MINUS input is lower than the voltage at the PLUS input."


When current does NOT flow, the output is pulled towards 15V via R20. When current flows, the output is pulled towards -15V which is connected to pin 1 of U5.


Pin 1 is called ground, but it can be connected to a lower voltage if necessary.


During capacitor charging, the voltage at the minus input is positive and thus higher than the positive input. In this case, current flows and the output is negative.

Once the negative input reaches 0V, for an instant, the input is lower than the voltage at the positive input, and current stops flowing. The output is then pulled towards +15V.

This would mean that as long as the output is negative, the JFET transistor is switched off, and once the output is positive, the JFET conducts, resetting the cap.

This is in accordance with what wikipedia says about an n-channel JFET:  "To switch off an n-channel device requires a negative gate-source"

 The source and drain of the transistor will always be between 0V and 5V, thus a -15V gate voltage assures that it is turned off. Similarly, source and drain will never be above 5V, so a +15V will always turn it off.

Since the source/drain may however reach 0V, the emitter of the transistor cannot be connected to ground - this would leave the comparator output and thus the JFET gate at about 0.6V, which is not enough to keep it pinched off.

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