I have a very well made custom-made lab supply given to me by a friend. It is capable of 1.6A max and 32V DC voltage.

I want to add a secondary output to this supply but I have a hard time figuring what point in the system I should choose as input to this buck-boost module.


simulate this circuit – Schematic created using CircuitLab

  • C1 capacitor has 32V across it.
  • The Variable Switched Supply look like this converter or something similar. This module is actually controlled by a voltage pot and a current limit one.

I experienced the full power of the cap when I tried to rush the discharge with a screwdriver. This makes me wander:

Q1: If I were to add the buck converter directly to this cap would it be sufficiently safe? Could I get electrocuted if there was a direct path from the cap to me?

Currently the buck-boost is connected to OUT1 and OUT2 DC outputs which I consider safe (the module I added is capable of step up and step down transforming). The problem with this configuration is that at about 3.5V on the standard output the module triggers the short circuit protection and never recovers. I have to cut the power, wait a few seconds and only then does the output voltage recover.

Q2: Why is the module shortcircuiting? I presume the IC has some undervoltage protection. Is this normal behaviour?

  • \$\begingroup\$ Schematic please! Is your 40 V cap isolated from the mains? What’s the voltage rating on the buck? \$\endgroup\$
    – winny
    Sep 16 '20 at 22:33
  • \$\begingroup\$ The output seems to be isolated. The supply has a big transformer inside and I couldn't see a connection between primary ans secondary. I am sorry about the schematic but I will have to add it later. In my timezone is already midnight. \$\endgroup\$ Sep 16 '20 at 22:58
  • \$\begingroup\$ I was just wondering if the 32V from an capacitor could kill me. Note that all the circuuts pass by a 6.3A and then 1.25A fuses \$\endgroup\$ Sep 16 '20 at 22:59
  • \$\begingroup\$ Highly unlikely. What’s the input voltage rating for the buck? \$\endgroup\$
    – winny
    Sep 17 '20 at 11:58
  • \$\begingroup\$ The rating for the buck that I want to add is 3.8V-32V. What I find weird is that lowering this voltage to 3.5V seems to make the module short circuit. \$\endgroup\$ Sep 17 '20 at 14:41

Q1: Your cap is seperated from main line, so voltages below 48V are no problem to touch. The resistance of your body is too high to let the current flow.

Don't know, what the buck-converter does. If any charge would increase voltage of your whole circiut relative to earth over 48V, then, to be a littly bit more safe, you could connect your secondary GND to Earth.

  • \$\begingroup\$ It seems the cheap buck converter uses the mighty "XL6009 " which has no undervoltage protection. It seems to be very unstable under 3V. owenduffy.net/blog/?p=12435 \$\endgroup\$ Sep 18 '20 at 12:08

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