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    • #11056

        Hi all,

        How about creating a constant current source with a Field Effect Transistor (FET) ?

        In the OpAmp labor, we can find the parts needed.

        In the manual of this set, Gerd Kopperschmidt explained this in experiment 9 on page 24 how to make measurements with a FET constant current source.

        Let’s use this technique in a slightly modified version to charge a capacitor with a constant current, thus creating a lineair slope.

        To set the thyristor, made of T1 and T2, its threshold, we use now a zener diode of 2V7 and use a 10K potentiometer.

        We can now adjust the point where the thyristor will start conducting. This will affect the amplitude of the sawtooth.

        On top, you see the FET with a 5,6KΩ and 250KΩ potentiometer. This will adjust the current flowing and thus adjust the frequency if the sawtooth oscillator.

        The FET is set up as a constant current source this way and the capacitor will have a constant charging current.

        Once a certain voltage across this capacitor is reached, the thyristor conducts and the capacitor is very quickly discharged via a protecting 120Ω resistor.

        Here’s the setup picture :

        Many greetings

        FET constant current

      • #11058

          Hi Frank,

          For those with a less advanced understanding of electronics (such as myself), what are the advantages of this circuit?  How does the field effect transistor work and what differentiates from the ‘regular’ transistor?  What is the thyristor semi-conductor and what is that doing for us in this circuit?

          Thank you so much for these educational circuits with descriptions, they are really so helpful.  Especially since 99% of the Lectron System manuals post 2001 are all in German!

        • #11060

            Hi everybody !

            The advantage of using  FET above a normal transistor is the simplicity of the circuit.

            The disadvantages are a very low constant current (depending of FET type), different current depending of FET batch and low temperature stability.

            But most usages, this circuit will do !

            The FET, as connected in our circuit, will deliver a constant current when enough gate voltage is applied. By using a resistor between Gate and Source, the current can be set.

            In its saturation zone, the current flowing from Drain to Source (IDSS) will be almost constant, hence the name constant current source.

            There’s a very good article giving more details : FET as constant current source

            I will explain in an upcoming experiment The Lectron thyristor module and equivalent transistor circuit (like here proposed) to make it all clear !

            Till then !



            • This reply was modified 2 years, 11 months ago by LectronFan.
            • This reply was modified 2 years, 11 months ago by LectronFan.
          • #11064

              Thank you for the detailed explanation Frank!  Always look forward to your next experiment and lesson!!


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