Problem with a power supply using PWM

[Krafter]

I'm having some issues here that I don't quite understand. I've been working on a RGB LED project for an annual Halloween party. I have an old PC power supply that I've modified with a toggle switch so I can use it as a project power source. It great because it has multiple voltages and high current ratings.

Now for the problem. I built my setup using three PWM outputs on a 20M2. Those outputs are wired to three MOSFETs for the high current draw that I'm expecting. Nothing fancy about that. Every thing works great with one or two LEDs strips (5M with 300 LEDs). I can turn on and mix of colors and fade them at will. The problem comes into play when I start adding more strips. When I get to about three strips, I start hearing a high pitch sound coming from the power supply. Logic says that it's noise from the PWMs. The more strips I put one the louder it gets and soon after the power supply shuts down completely. It has to be caused by the noise being generated but why? I'm positive that it's not a current issue. Each strip with all three colors on at 100% is about 0.7 amps. Thats a total of 2.8 amps on a 17 amps (at 12V) power supply. Any ideas of a solution or in sites on a suitable power supply that with handle the possibility of 14 amps (yes I know that a lot of strips)?


Image of my project board.

 

[techElder]

Many of those "PC power supplies" are really crappy power supplies, and one would expect them to show their crappiness at higher current levels.

 

[oracacle]

what are all the specs on the power supply, if its multi rail you need to load balance it. the high pitch sound could well be inductor whine, not normally an issue but is a sign of low quality.
how long do you run this system for, a car battery will supply that current load but for how long will depend on the battery. or a better quality computer PSU.

 

[Krafter]

Many of those "PC power supplies" are really crappy power supplies, and one would expect them to show their crappiness at higher current levels.

It works great for everything else. It just doesn't like the PWM cycles. I've watched it shutdown several times and it's always on the up cycle when I'm fading. Maybe I'll just buy this (20A version) and be done with it. I still don't have a clue on why noise would cause a power supply to shutdown like it is.

what are all the specs on the power supply, if its multi rail you need to load balance it. the high pitch sound could well be inductor whine, not normally an issue but is a sign of low quality.
how long do you run this system for, a car battery will supply that current load but for how long will depend on the battery. or a better quality computer PSU.

It doesn't run long at all. It seems to have gotten worse the more times it happened. Four stands will shut down during the 4th or 5th cycle. Here is a snapshott of the side of the power supply giving the ratings.

 

[oracacle]

low quality PSU, the fact that its getting worse indicates that something is failing, maybe a regulator which is why you get the inductor whine. why was it removed from the PC in the first place.
the fact that it rates 2 voltages to not exceed a combined wattage is a bit weird. if at your intended full load you should still be well within the supply tolerance. scrap is my honest opinion, I had an old PSU (don't know where it came from) that I used for a while which start not being able to supply 2 amps on the 12v rail, swapped it for another cheap one that came with a tower which worked fine for a while but was rate for 200 extra watts.

 

[sghioto]

How did you calculate the current draw on the strips? 700 ma seems low for 300 LEDs. Are the strips commercial or homemade?

Steve G

 

[Krafter]

low quality PSU, the fact that its getting worse indicates that something is failing, maybe a regulator which is why you get the inductor whine. why was it removed from the PC in the first place.
the fact that it rates 2 voltages to not exceed a combined wattage is a bit weird. if at your intended full load you should still be well within the supply tolerance. scrap is my honest opinion, I had an old PSU (don't know where it came from) that I used for a while which start not being able to supply 2 amps on the 12v rail, swapped it for another cheap one that came with a tower which worked fine for a while but was rate for 200 extra watts.

I think I'll take your advice and get rid of it. I have a few old PCs laying around and I just stole it from one of those but the next one will be something like I linked above.

How did you calculate the current draw on the strips? 700 ma seems low for 300 LEDs. Are the strips commercial or homemade?

Steve G

I measured it with a current meter and watched it during the "white cycle" with one strip hooked up. It was actually 0.6 something but I rounded up.

 

[rossko57]

On PCs using proprietary PSU & motherboard, like Dells, the mb is often packed with bulk smoothing capacitors. Running such a PSU on its own is likely to show up the shortcomings of its limited internal smoothing. Noise maybe implies high instantaneous currents, I'd guess the cause of the shutdowns.

You might find you can get better performance if you can arrange some phase rotation in your multiple PWMs?

 

[Krafter]

On PCs using proprietary PSU & motherboard, like Dells, the mb is often packed with bulk smoothing capacitors. Running such a PSU on its own is likely to show up the shortcomings of its limited internal smoothing. Noise maybe implies high instantaneous currents, I'd guess the cause of the shutdowns.

You might find you can get better performance if you can arrange some phase rotation in your multiple PWMs?

Maybe that's worth a shot... I don't know.... This is for a party where there is going to be hundreds of people. I have big plans for these LEDs and a MDFLY soundboard being powered for this power supply. I'd hate for it to fail during the party.

 

[bluejets]

#8 would be my bet.

 

[neiltechspec]

I have used a few old original Microsoft Xbox (one of the most unreliable products ever made) power supplies.

While the Xbox was rubbish, the PSU's were very good. 12v @ 16Amps or 12 Amps depending on version.

They are fan cooled (almost silent) and very easy to modify, just connect the red (5v stby) & blue (power on) wires together (via a switch if you want).
12v outputs are on black & yellow.

The LED shows Green when on, Amber in standby & Red in Fault condition (overload / short etc.).

Neil.

 

[Haku]

Seconded on the old Xbox360 PSUs. They're cheap on eBay because the consoles themselves often fail long before the PSU ever will, and whilst the 5v output is always on the 12v output can be switched on/off by putting a 5v signal back down the blue wire with a picaxe pin. The current draw on the blue wire is almost non existant, it's just looking for a 5v signal.

I haven't chopped off the plugs on my PSUs, I bought some Xbox360-to-slim adaptors and just cut off the slim plug, so if the PSU does die I can easily swap it out for another.

 

[Krafter]

Sweet! I just happen to have a couple of extras laying around. One of them is in the current range that I need. I also have a couple of slim adapters. I used to work on consoles so I have all kinds of crap laying around.

I'll give it a go and see how well it works.

 

[Hemi345]

If the Xbox PS doesn't work out, I can recommend the Mean Well brand of power supplies. I have a 12V 8.5A version for my kitchen under cabinet lighting that is working well. I bought it at Mouser for $20