Is a slight continuity beep between + and - on a AC/DC PSU normal?

[rthorntn]

Hi all,

I have a Meanwell RT-125A here, it's never been used, I connected it up with no load and I get a flashing green light, I think this PSU needs some amps on the 5V so this may be the problem.

But it got me checking things, so I bust out my U1272A put it on Ω and hit Shift, if I put one probe on common and the other on +5V I get a tiny beep and the display reads 45, it happens to a lesser extent on -5V but the display quickly goes to 0.

I read that this could just be "Most linear supplies will show a short for a few seconds until the caps charge up from the probe's bias current. That is normal" [gearspace.com] but now I'm worried and I didn't think this Meanwell was linear (although I think I can see a transformer inside there)?

Thanks.
Richard

[Kim Christensen]

Unless it shows a dead short, a continuity reading on the output of a power supply is meaningless. (You'd be checking it when it's unplugged/off)
Better to put some sort of a small resistive load on it, power it on, and measure the voltage across the load. Aim for a current draw of around 10% of it's rated output for a quick test. (not super critical)

Doesn't look like that power supply has a minimum load requirement according to this datasheet.

[rthorntn]

I put my re:load on it, set to 2A and it worked fine!

[timenutgoblin]

I have a Meanwell RT-125A here, it's never been used, I connected it up with no load and I get a flashing green light, I think this PSU needs some amps on the 5V so this may be the problem.

According to the datasheet, there is a minimum load requirement of 15% of the rated output current for CH1 (5V @ 12A) which translates to:
15% of 12A = 1.8A. 2A > 1.8A.

I think that the reason the LED flashes is because of the minimum duty cycle capability of the PWM controller; I don't think it's low enough to regulate without a load connected to the output.