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• Rigol DP832 Power Supply set for 20 mA can kill a LED

  RoGeorge • 09/18/2015 at 08:54 • 6 comments

  I was demonstrating the Constant Current capabilities of my new power supply, Rigol DP832:

  NOTE: This picture is from another project, please ignore the DP832 settings from this picture.

  
  

  My intention was to demonstrate that, if the current is set for (limited to) a 20 mA, the voltage can be set for up to 32 V (which is the maximum for this power supply), without demaging the LED. So I did like this:
  

  • Set the maximum current to 20 mA
  • Set the maximum voltage to 32 V
  • Hook a LED to alligator clips
  • Enable power

  The LED started to lit. The power supply indicated a drawn current of 20 mA and a 1.7V voltage drop on the LED. All good.

  Then I wanted to hook a blue LED, in order to demonstrate that the voltage drop is dependent of the LED color. I did like this:

  • extract the first LED from the alligator clips (while the LED was on)
  • pick another LED from the table and attach the first terminal to the alligator clip
  • attach the second LED terminal to the other alligator clip

  LED FLASHED & POPPED OUT DEAD!!!

  The LED was brand new, without any scratches. The strips that looks like scratches on the LEDs plastic body are from the stress during the pop.

  WTF? Can a LED pop like that with the power supply set for 20 mA?

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• Assumption is the mother of all fuckups

  RoGeorge • 09/07/2015 at 21:17 • 6 comments

  Yesterday I was building a headphone amplifier, and was looking for a differential voltage source. Something small, preferable a wall adapter. Any fix voltage in the interval +/-5V ... +/-15V would be just fine. Expected current 100...150 mA.

  Most wall adapters from the scrap box were from old mobile phones, single voltage, 5V or less, with glued cases and no way to adjust the voltage. Still, there was one with a screw-fastened case, single voltage, 8.4V/500mA:

  It has a 4 wires RJ9 female connector (like a LAN connector, but with 4 pins only), which was perfect, because I needed at least 3 wires, but preferably 4. Even more, the schematic was very hacker friendly:
  

  
  All it needs to add a negative voltage was another rectifier circuit, like the one in red:
  
  Even more, the PCB had an un-populated area in the lower voltage part, more then enough to add the 4 red components.

  That was too nice to be true!
  When something is too nice to be true, most of the time it's because it's not!

  After the red part was added and everything was double checked, it was time to plug it in.

  Does it worked? No.
  Instead of +8.4 / -8.4V, the output was +8.4 / -25.2V. Exactly 3 times more negative voltage then expected.

  Can you spot the mistake?

  Read more »

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