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  • Author Author: oldmanraskers
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CPLD Controlled Door Lock - First Stage Testing

oldmanraskers

image I think this emoticon says it all!

 

Today I got some parts, including the LD1086D2T33TR regulator chips. I connected ground to ground and 24V to the input pin and I expected 3.3V on the output pin but what I got was my bench power supply going into constant current mode (limiting at approx 1A) and a reduction in voltage from the bench power supply output to about 8.5V instead of the 24V that I had set it to?


The output of the regulator started at about 6V and very steadily grew... I stopped this test when the output pin reached about 6.2V.


I am utterly bemused and didn't expect this behaviour at all - I expected that the regulator would accept 24V in and give 3.3V out, the only thing happening that I did expect, is that the regulator gets real warm.


Does anyone have any idea what is happening here?

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  • I have ordered new chips and though I am not treating the new chips any differently they are working perfectly - with cap and without caps image. So, moving on I have tested my two power regulation stages and I am now getting a steady 3V3 and 1V8 output.

  • in reply to oldmanraskers

    Unlikely re either 30V or individual chip miswiring - more likely it's going into self oscillation due to no caps.

     

    MK

  • in reply to michaelkellett

    I tested continuity across the input and ground pins of a new device and found open circuit. I then simply connected ground to ground and input to 24V, the output was around 6 V (not 3.3V) and the bench supply went into constant current mode. I tested continuity across the regulator again and found a dead short that wasn’t there before the test. It’s true that there wasn’t any caps involved in my simple test, but the behaviour still seems incongruous with the datasheet. When you say swapped pins, do you mean that the device might actually have pin1 connected to pin3 internally and vice versa – does this happen?


    I have since test a new device as described and found that the device did work for a time and then shorted. I am beginning to wonder if the device accepts 30V as a max but only as an impulse for very short times, and under normal conditions can only tolerate something less than 20V like the LT1086?

  • Should have worked, my guess is a cap the wrong way round or swapped connections on the regulator.

     

    Can you run your bench supply at lower voltages and current limit.

     

    I would start off with a 50mA current limit and wind the volts up from zero, checking the regulator output as I go. If it takes more than 5V in to get 3.3 out then something is wrong. With no load you shouldn't see more than 10mA input current.

     

    MK