I have an 18 V dc linear power supply rated for 7.5 A.

I am applying an electronic load at 2 A constant current prior to applying ac input to the power supply. After applying the ac input, the power supply fails to output a volt. If I repeat this process at 1 A on the electronic load the power supply will power up and output 18 V.

I tried this same set up with a similar switching PS and the PS came up regardless of the load applied.

The linear supply i am working with tests good if i apply no load or up to 1.5 A at initial power up. And after it is powered up i can increase the load to full load (7.5 A) and the power supply maintains 18 V dc.

Question 1 - Does a linear supply have a load rating at initial power on?

Question 2 - Does a switching power supply have a load rating at initial power on?

Also, When the linear power supply is driven to over current (about 9 A) the voltage drops out and the electronic load displays about 1.75 A. Once I reduce the electronic load to 1.75 A the output comes back on.

Question 3 - is this value of 1.75 A significant to the load value that the power supply can operate at initial power on?

Question 4 - Is the difference between the over current trip point and the point where the power supply recovers called hysteresis?

  • \$\begingroup\$ Constant current \$\endgroup\$
    – Bigtoe1982
    Commented Mar 31, 2020 at 19:57
  • 2
    \$\begingroup\$ There is no reason to think that all supplies are the same. Please add links to the manufacturer's datasheets of the supplies in question. \$\endgroup\$ Commented Mar 31, 2020 at 20:00
  • \$\begingroup\$ I would say "Yes" to all 4 of your questions. \$\endgroup\$ Commented Mar 31, 2020 at 20:00

1 Answer 1


With no manufacturer, model number, photos, or schematic, all answers are guesswork. The true answers to the first three questions are maybe, maybe, and maybe.

4 - Yes. In your case, it sounds like a foldback current limiting circuit hysteresis.

2 - Most switching power supplies will power up into a 100% load. Depending on the size and complexity of the supply, it might have a "soft-start" design that ramps up the output voltage to reduce the startup input current spike.

1 - Maybe, but it is not an common industry practice. Not that if the downstream load has a significant input capacitance, this appears to the supply's output stage as a brief dead short. This probably is going on in your situation, but without any information about the load ...

  • \$\begingroup\$ The supply is proprietary. The electronic load is an Instek PEL-3111 \$\endgroup\$
    – Bigtoe1982
    Commented Mar 31, 2020 at 20:12

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