Author Topic: Two-Transistor Oscillator  (Read 2372 times)

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Offline cyborgemuTopic starter

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Two-Transistor Oscillator
« on: April 09, 2010, 10:58:13 AM »
I'm trying to integrate an oscillator into a circuit powered by a 4V, 100uA PV cell.  I built the right hand circuit here, which claims to draw only 1uA.  But it caused the circuit voltage to drop to 0.5V, obviously pulling far more than 1uA.  Why might this be?  I triple-checked the circuit: no short circuits or crossed leads.  Is there a reason why using a PV cell as power source would affect this?  Thanks!

Offline waltr

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Re: Two-Transistor Oscillator
« Reply #1 on: April 09, 2010, 01:56:04 PM »
Did you actually measure the current of that circuit to see if the claim is true?

PV specs are at two extremes:
100uA shorted, ~0 Ohm, ~zero V
4V open, infinite Ohm, zero current
They don't do both under the same conditions.

Offline Soeren

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Re: Two-Transistor Oscillator
« Reply #2 on: April 09, 2010, 01:57:23 PM »

The current drain of 1µA is the average drain of the circuit.
A capacitor over the oscillators supply terminals is needed in your case.

The formula for capacitance is:
C = As/V [Farad]
 A = Ampere
 s = seconds
 V = voltage drop allowed

If we assume each pulse is eg. 1ms in the 500ms period, the current drain will be:
  500ms/1ms*1µA = 500µA.

500µA in 1ms assuming an allowed voltage drop of 0.5V, will take a capacitor of:
 0.0005 x 0.001 / 0.5 = 1µF

A tantalum capacitor will be best, since it has lower ESR than an electrolyte. You might need a bit higher value than calculated, as they have a large tolerance on the marked capacity.

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives


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