Why does my SMPS show poor regulation?

[goldyrathore]

Hi,

I have a PC SMPS (Frontech JIL-2415, 500 Watts) powering a car amp ( Sony XM-504Z, 100Watts RMS ) at home for a sub duty. The SMPS is rated to provide 31 Amps at 12V.

Here is what I found:
1) No load voltage-> 12.1 volts
2) Voltage with amp switched on but no input signal->11.6 volts
3) Voltage with amp fully loaded (about 100 watts) and 20Hz sine played->10.6


Why is the supply sagging so much when the amp is idle? Why is it sagging so much more on load?
Can somebody please explain whats happening?

Thanks,
Goldy

[AP2]

Hi,
the PSU has a normal behavior. because it was developed for continuous load.
then if it is 500w (all voltage) as may 30Amp 12V on output? normally the greatest power is reserved to +5 V.
other reason for the loss of voltage to power is that the other outputs remain unloaded.
(Output circuit has a differential resistance to balance the load across all outputs voltage in relation to the load. (When the load is fixed)

P.S.
You can (only for test) apply an load at +5V (3-4Amp) when amplifier is in idle and see voltage output.
Regards

[wwenze]

That power supply looks like the type that can explode anytime. I reckon the overcurrent protection isn't really working either. And I don't think it has 100W on +12V, regardless of what the label may say.

[wwenze]

Couldn't find a clear picture of the power supply's label. Do try what AP2 has suggested of loading the +5V.

[benb]

My understanding is multi-output "PC" SMPS's don't regulate all the outputs - they just regulate ONE output (the highest power, the one that powers the logic on the motherboard, 3.3V or whatever), and the other outputs just follow along. This works well with variations of line voltage, but not with variations in load current. All these voltages are taken off one transformer, and the circuitry can only reasonably regulate one output (the main 5 or 3.3V) for changing load conditions.

At first I was about to agree with others, load down the 5V or main output and see what happens, but on second thought the 12V output is unlikely to be rated for anywhere near 30A.

I did google 500W PC PS and they (the few entries that give current output of each voltage) do have more current on the +12V line than I expected - this one has two 12V outputs, each at 22A:
Newegg.com - Antec earthwatts EA500 500W Continuous Power ATX12V v2.2 80 PLUS Certified Active PFC "Compatible with Core i7/Core i5" Power Supply
This has three 12V outputs, two at 17A and one at 12A:
Black CoolMax 500W Modular Power Supply CU-500B ATX12V EPS12V Supports SATA PCI-E-Best Computer Online Store Houston Buy Discount Prices Texas-Directron.com

It may or may not be a good idea to connect several 12V outputs together to get a single higher-current output. I suspect this will work only because these outputs are not themselves regulated.

Long story short, you're repurposing this supply to do something it wasn't intended to do. It's rather remarkable that it works at all with such a big load on the 12V line and nothing on the others. If you want truly regulated 12V, you need to use a 12V-only output SMPS that will give the current you need.