Switching Power Supplies: please educate me

[sr1200]

I have a project that needs 12v dc power.
After looking around. I found this locally (somewhat): 12v Switching Power Supply

Looked pretty well built, cheap and easy…
However, and this is where I need help, the unit when powered by mains AC, only outputs a very low (mv) square wave voltage. Is there something else i need to do to get the 12v out of this? Is it expecting to see some kind of extensive load (what im putting on here is probably only sucking a few ma at best). If anyone has any ideas or can throw me a schematic for something i need to attach this to, id sure appreciate it, cause im at a loss right now and i really need to get this project completed asap.

 

[Khron]

How are you measuring the output voltage?

 

[sr1200]

Measured on a dmm and put the osc scope on it just to see what was coming out of it since the dmm was giving me a crazy readout.

 

[Khron]

Does it do the same if you load it with, say, 100mA or so? And/or does the power supply itself have an indicator LED of any sort, that doesn't light up?

 

[sr1200]

The power supply has a green led which comes on. Ive tried to run the output to a small headphone amp i have that has a higher load than my project and i get nothing there either.

 

[ruffrecords]

.MeanWell is a well known and much respected manufacturer - I use them myself for heater supplies. However, the one you have chosen is rated at 1.3 amps output at 12V and for this to work as intended the load needs to be at least 10% of this value. So see if you can find a 100 ohm 5 watt or 10 watt resistor and try that right across the output.

For smaller loads, MeanWell do a wide range of small PCB mounting supplies

Cheers

Ian

 

[warpie]

Also, you'll need a very short path between the tip and the GND wire. The typical standard probe will introduce problems.

 

[Bo Deadly]

However, and this is where I need help, the unit when powered by mains AC, only outputs a very low (mv) square wave voltage. Is there something else i need to do to get the 12v out of this? Is it expecting to see some kind of extensive load (what im putting on here is probably only sucking a few ma at best).

SMPS can modulate the output for low load conditions. Usually this results in the correct output but extremely noisy. So if you're really only seeing a few mV, that seems odd. It might be that the load is so low that it's triggering an underload condition and outputting very short pulses just to detect a load.

So I would try loading it. You might need to load it with 20% of it's rated load. So 300mA maybe. That would be like a 40 ohm 5W load. Not ideal. This is why it's important to buy an SMPS that is small enough for the target load.

SMPS can be very sophisticated and efficent. At least most of the MeanWell ones are known to be. Some of the really small ones (like what you would need if your load is really only a few mA) not so much. They can be noisy. This is one area where SMPS are not so useful. They perform best at near 100% load.

 

[sr1200]

Thank you all for your replies. Im going to look into getting something that better suits the project. Maybe just a wall wart or something will suffice for this that doesnt require a heavier load.

 

[ccaudle]

The datasheet indicates really low power consumption with "no load," which may indicate some kind of low power shutdown scheme.
No indication of minimum required load for normal operation, but does indicate the current range is 0-1.3A.
MeanWell RS-15 spec sheet

The datasheet indicates that fault condition causes hiccup mode, which sounds like what you have described. That basically is where if an overcurrent is detected, the supply shuts off the output, then a short time later enables the output and monitors the current; if the current is rising faster than the fault setpoint it shuts off right away again. That cycle just repeats over and over until the fault condition is removed.

So any chance you have something misconnected, a stray wire strand across the output connector, etc.? Really huge capacitors connected to the output?

 

[murrayatuptown]

It is almost surely a DC power supply. If you are measuring output with a DMM on an AC voltage range, you may not see much of anything, especially if measuring it without a load.

Too many things to guess about how the scope and probe relate to the observed square wave.

 

[murrayatuptown]

I have a habit of using an adapter to DMM banana jacks (dual banana to BNC). Then I use a shielded coax cable with short spring clip leads (Pomona, probably). So I have a mostly shielded DMM cable. But I have the clip leads go bad because I use it a lot at work...and have an occasional test cable failure that causes loss of voltage or continuity. Bad test leads happen more often than expected.

And I am not the only person. The metrology/calibration lab doesn't want your probes when they recertify test equipment. They are the first to complain when theirs fail.