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  1. #1
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    LT3083 lab PSU question

    On page 19 of the LT3083 there is a lab PSU which I have made. However the voltage varies at a very unpredicted way, like a logarithmic way when completely unloaded. Also when completely unloaded the voltage output climbs much higher to about 14v.
    When I connect a small 12v incandescent panel indicator bulb at the output the voltage setting is smooth as it should.
    But even then i can produce a max of 9v.

    Second problem, I loaded it with a device drawing 250-500mA at 9v and it is like pulsating on/off
    What is the problem?
    Last edited by neazoi; 26th February 2020 at 20:25.
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    Re: LT3083 lab PSU question

    Are you observing the minimum load current requirement as shown at the bottom of page 3. The load current must be >1mA over the whole voltage range you have under your control. Basically, add a resistor across the output so 1mA flows at minimum output voltage, it will waste a few mA as the voltage is increased but you can probably live with that.

    Brian.
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    Re: LT3083 lab PSU question

    Quote Originally Posted by betwixt View Post
    Are you observing the minimum load current requirement as shown at the bottom of page 3. The load current must be >1mA over the whole voltage range you have under your control. Basically, add a resistor across the output so 1mA flows at minimum output voltage, it will waste a few mA as the voltage is increased but you can probably live with that.

    Brian.

    about 1K?

    Also see the second problem I describe. (edited the post)
    Professional engineering is the top, but amateur engineering is more fun.
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  4. #4
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    Re: LT3083 lab PSU question

    1K is fine if 1V is the minimum output. There is an explanatory note at the bottom of page 1 showing 909 Ohms for 0.9V.

    The instability could be due to several things, as it has a floating reference it will be very prone to noise and voltage drops in the wiring but I would first check what voltage is going into the second stage. The first stage is a variable voltage source with its output decided by the drop across the 0.33 Ohm output resistor so it works somewhat like a constant current supply. The second stage is a voltage regulator which gives the selected output provided that the first stage gives it enough current. You have to determine whether the pulsating is caused by the output stage oscillating or it's input voltage varying and going below the level needed to sustain the output.

    Brian.
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