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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.

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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)
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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.

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    Re: LT3083 lab PSU question

    Quote Originally Posted by betwixt View Post
    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.
    Hi, I hope you are all well.
    Here is the current schematic of the PSU. The 100R and the 5v incadescent bulb at the output (used as indicator) helps to set the min current limit for the regulators.

    However I cannot make the PSU produce not even close to 10v. The max voltage at the output is 8.8v. I measure the voltage at the tip36 collector and it is about 9.5v whereas the voltage at the bridge output (loaded) is 15v. I measure 10.5v at the collector of the bd139. Why such a drop in the transistor pair? Maybe tre pair is starving current at the base?

    This might explain the inability of the PSU to provide more than 0.5A of current?

    What should I do, maybe change the base resistors but to what values?

    - - - Updated - - -

    I had left the emitter of the powr transistor unconnected by mistake! So I guess we could ignore the max current limit problem now that the problem is corrected. However, I can still get a max voltage output of 8.83v.
    I measure the voltage at the tip36 collector and it is a bit less than 11v whereas the voltage at the bridge output (loaded) is 15v. I measure a bit less than 11v at the collector of the bd139.

    I measure the in-circuit max value of the voltage set pot (200k) and it is 180k. Maybe this is the problem?

    Any clues?

    - - - Updated - - -

    Yes this voltage set pot was the problem. Instead of 200k it was 180k (Chinese crap). I added a trimmer in series (100k) and I am able to precisely set the voltage now to 1.2-10v which was the requirement. I can actually set this to 12v as well when I increase the value of the series trimmer, at the expense of higher lower voltage. It is difficult to access the voltage pot now that the circuit has assembled, but in the future I may replace this 180k pot with a higher value one and remove the trimmer.
    Last edited by neazoi; 3rd April 2020 at 13:25.
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    Re: LT3083 lab PSU question

    If you want to minimize the output current due to using a minimum current bias resistor, you could use a two-transistor NPN current mirror set to the desired current (1mA).
    The mirror current won't increase significantly with change in output voltage.
    The mirror will also draw the minimum current down to 0v output.
    Zapper
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    Re: LT3083 lab PSU question

    Quote Originally Posted by crutschow View Post
    If you want to minimize the output current due to using a minimum current bias resistor, you could use a two-transistor NPN current mirror set to the desired current (1mA).
    The mirror current won't increase significantly with change in output voltage.
    The mirror will also draw the minimum current down to 0v output.
    I thought I have solved the current issue but not. The PSU is not able to provide more than a few 10s of mA of current! the voltage drops.
    Any ideas what should I check?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: LT3083 lab PSU question

    It's a fair bet you have inadvertently built something wrong - or blown a part or parts up thus far ...



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    Re: LT3083 lab PSU question

    Measure the voltage at all the nodes and post them with a schematic with the nodes labeled.
    Zapper
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    Re: LT3083 lab PSU question

    Hi,

    Posting a schematic from a datasheet is useful to discuss the correctness of this schematic.

    But I assume this schematic is correct ... and there is some mistake in your circuit.
    Thus it makes more sense to post your schematic and your PCB layout, additional pictures could also be helpful.

    Klaus
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    Re: LT3083 lab PSU question

    Try these tests:

    1. using a high current range, briefly use the meter to short out the +5V to ground. The 7805 should go into current limit at around 1.5A. This will prove whether the input pre-regulator (TIP36/BD139) are working properly.

    2. Next, repeat the experiment but this time at the output of the FIRST LT3083 (across the 15uF capacitor) and start with the 20K pot at mid position. You should measure approximately 1.5A and the potentiometer should adjust it from a few mA up to nearly 3A. This will prove whether the current limiting stage is working.

    3. If those tests pass, there should be sufficient current supply to the voltage regulator (second LT3083) so the problem lies there.

    Don't measure the short circuit current for too long unless you have adequate heat sinking!
    In test 2, be sure to measure AFTER the 0.33 Ohm resistor or the IC could be damaged.

    Brian.
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