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# VCO current drive troubles

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#### smitchell82

##### New Member
Hi, Im having trouble with a voltage controlled oscillator circuit that im trying to get going. basically the only thing that is holding me back, is I need to somehow convert an input of .25V to a minumum of 70mA to charge a capacitor.

the V/I converter must be linear, so the current has to increase from 70mA - XmA as the voltage increases from .25V - 7V.

This is my first major project, and im not that good with electronics. any suggestions would be much appreciated, because im starting to lose my mind trying to get this VCO going.

Thanks a lot.

Sam

Are you sure you don't mean 70uA? With a linear V/I conversion, 7 volts will give you almost 2 amps!

hmmm

Um, yep, good point. Im using the current to charge a capacitor who's voltage is read off by a schmitt trigger. I have to get the capacitor to somehow charge to 7V, then the schmitt trigger is arranged so it will short the cap through a digital switch and discharge it creating a sawtooth waveform. the lowest input voltage to the V/I converter from the MIDI circuit on the synth is around .4V and the highest is 7V. so I need the capacitor to charge up to 7V for the whole input voltage range.

Any ideas?

Thanks.

Sam

Zero crossing ?

There is no evidence that .25v -> 70mA will make 7v -> 2A.
There is no requirement for 0v -> 0mA

If a circuit was devised such that .25v -> 70mA and 7v -> 170mA then it would follow that an increase of .25v will cause an change of 100mA/27 (about 3.7mA)
0V in would therefore result in a current of 66.3mA

This gives .25v -> 70mA and is linear (assuming my hasty maths is correct).
For small, well defined ranges like this a simple, single common emitter transistor will be near enough to linear, good enough for a couple of octaves. I'm sure I have synth circuits on file that use this technique.

You need to define the maximum current you require from a 7v input, try to imply this from how many octaves this .4 to 7v range represents and the kind of rise-times (cap charge times) this requires.
the lowest input voltage ... ... ... is around .4V and the highest is 7V. so I need the capacitor to charge up to 7V for the whole input voltage range
doesn't make sense to me.
Surely the cap will charge to some (constant) target voltage regardless of input voltage - only its time will alter, thus resulting in some voltage to frequency law.

Also you refer to
the V/I converter from the MIDI circuit on the synth
Surely this will be a MIDI-to-voltage converter?

Quite true, Mechie. I was wrong in assuming that the zero volt intercept has to be zero current.

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