Simultaneous start of two small switching power supplies

[resca54]

Hello, in a small home project I need two small switching power supplies. I am not an expert and I chosen two
LJH-172 230VAC-15VDC 2A: I have no schematics, they look very basic but acceptable.

Each one works fine by itself. With no load, they draw 2mA (each).

My present testing environment is through a separation transformer 230-230V and a light-bulb 230V 60W to limit the current in case of short.

The problem shows when I connect both the PS in parallel (input only, the output is presently disconnected): 7-8 times over 10 the bulb lights fully. The current drawn reaches 270mA that, in fact, match the 60W of the bulb! Testing with two ammeters, each PS draws 135mA, so the problem is not related to just one. Two times over ten, they power up successfully and they wok fine forever.

Should I power up one and, later, connect the second one, they both wok fine forever.

I tried to place them quite far away (1 meter) and nothing changes.

Any idea what may be gong on?
How to further investigate?
Any workaround?

Thanks in advance!

 

[betwixt]

The trouble with simple switch mode supplies like those is they draw more current at lower input voltages. I suspect the combination of loading your isolation transformer and the series light bulb is causing such a large voltage drop that one or both supplies can't start up properly. Supplies like yours draw a surge at start up until their input capacitor has charged and the output voltage has stabilized, that surge is what causes the voltage drop and of course you have two surges simultaneously when both are connected.

Brian.

 

[resca54]

The trouble with simple switch mode supplies like those is they draw more current at lower input voltages. I suspect the combination of loading your isolation transformer and the series light bulb is causing such a large voltage drop that one or both supplies can't start up properly. Supplies like yours draw a surge at start up until their input capacitor has charged and the output voltage has stabilized, that surge is what causes the voltage drop and of course you have two surges simultaneously when both are connected.

Brian.

Thank you, Brian!
You pointed out the problem: with a 100W bulb, they start with no problems. Of course, in the final project, they will be powered from mains with non bulbs!

I suspected something like that, but I was confused because I wondered where that power is dissipated.