[sdiy] 3340 oscillator - current limiting resistor equation is wrong?
Tom Wiltshire
tom at electricdruid.net
Sat Nov 18 12:25:29 CET 2017
The 3340 VCO needs a current-limiting resistor on it’s negative supply if that supply is greater than -5 volts. The example data sheet circuit shows a -15V supply with a 820R resistor.
The data sheet also offers an equation for working out the resistor for other supply voltages:
Ree = (Vee - 7.2) / 0.008
Which is the same as:
Ree = (Vee - 7.2) * 125
(From http://www.curtiselectromusic.com/uploads/CEM_3340-3345_Long.pdf )
Now, here’s the funny bit. If you put -15V into that equation, you don’t get 820R.
Ree = (15 - 7.2) * 125 = 7.8 * 125 = 975R
So what’s going on? Am I having a bad maths moment or missing something else, or is it just wrong?
Thanks,
Tom
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