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Thread: 1F caps on 12V rail

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    Default 1F caps on 12V rail

    I have a question based on my electronics background, not my IT one (which is years old, and applicable to mainframes and big mini's).

    Todays Gaming PC's power supplies often derive a single 12V rail at a huge number of amps - my PSU delivers 62 amps (continuous), which is a huge current. Hell, that is almost what the starter motor of a small car uses! And it is far from being the biggest PSU available.

    Now, from this 12V rail, other voltages are derived - 5V and 3.3V for instance - so you really want your 12V rail to stay at 12V even when a sudden load hits it. Older styles of supply would derive all the required voltages separately and this newer concept is superior by miles imho. Evidence of a sag affecting the system would probably manifest itself by hanging or unexpectedly rebooting - that's an educated guess based on PSU's in large computers, and their behaviour when this happens.

    Also, car audio enthusiasts use 1F caps for the identical purpose. If their car sound systems drag a sudden load to power every speaker when at full volume, a small car battery may not deliver enough current, and the voltage will drop momentarily. Or so it is claimed - and it is theoretically correct (I'm not going to argue whether it happens or not). They remedy this by drawing power from one or two 1 Farad capacitors. The power supplied by them does not need to last long, only for the duration of the drop, and the cap(s) are recharged immediately upon a brief removal of the excess load.

    My question here is whether I will benefit by using a 1F (@24V) capacitor across the 12V rail of a gaming PSU. Would it help - ever? If the 12V rail sags, the cap will hold it up for a short time; and short times are all we are talking about. Should something drag it to (say) 9.2V for 6 seconds, you will probably be stuffed with or without a large capacitor. But if that sag to 9.2V lasts 640mS, it is a whole different story - the 12V rail will remain at 12V. And no, I have not sat and calculated this yet, these numbers are off the top of my head but are in the right territory.

    Also, you do NOT place a capacitor like this directly across the 12V rail. For a short period at turn-on it is seen as a short circuit by the PSU. Naughty!! Instead you use a low value series resistor which limits that initial inrush of current - use a low value resistor to limit that initial current - I have not worked out the ideal value but a 1.2 ohm resistor will limit the inrush current to the cap to 10A. You might even consider 1.5 or 1.8 ohms. 12W will be dissipated by a 1.2 ohm resistor so a large wattage device is needed - say 20W. 1.2 ohms is also low enough to allow significant current flow-back to the 12V rail if it droops.

    In addition, my design would have a fast diode in parallel with the resistor. It is forward biased when the 12V rail droops so current flows OUT (of the cap) via the diode, to the 12V rail; but otherwise reverse biased (so you do not overload the PSU at turn-on). A drawn circuit diagram would show the arrow part of the diode symbol facing the +12V rail, NOT the cap, and (resistor and diode in parallel) would be in series with the positive terminal of the cap - going to the +12V rail. You will sacrifice 0.7V to the diode but this will be lessened by the resistor to a much lower amount. In fact, the diode will be a seriously heavy duty one, and you may lose more than 0.7V when forward biased. No worries - the resistor will overcome much of this loss.

    So, there is the thinking, theory, and some rough calculations.

    Is there ever any evidence that a sudden unexpected current draw compromises the power supply? This is certainly NOT INTENDED as a solution to overcoming under-rated PSU's. Wrong answer for the wrong reasons. This question is aimed at correctly rated PSU's that may not have a lot of reserve. In my rig, video cards dissipate 100W (ea) and a 2600K chip takes another 95W. As to what else is used, and where, isn't added up, but a 750W supply runs it fine. This is a question that could help when a very brief drop-out or sag occurs. It is not to overcome inherent design faults or under-rated PSU's.

    If in any doubt about how important manufacturers are about the output capacitance, look at the CoolerMaster GX range of PSU's where the wording in their 'Special Features' list includes <quote> "Huge bulk capacitor for hold up time >17ms at full loading". Now, 17mS is not terribly long, and a 1F capacitor will extend this greatly, although I am unsure by what exactly - at least 20 times I imagine. I have yet to calculate it, and everyones individual rig would be different. But note how CoolerMaster make an big deal of this exact issue (and rightly so). My suggestion is merely a bigger version of what they are already doing.

    So, in summary, is there a possibility that once in a while such a cap would aid the system, or is it completely unnecessary? CoolerMaster certainly makes a big thing about the size of the output capacitor. So, the need for a big output cap isn't in doubt, simply the added benefit. I have a spare 1F 24V cap, so is it worth using this way, or a waste of $90?

    Cheers, Brett

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    Default

    Depending on the cap's ESR, it could be more trouble than it's worth, since it would contribute to the ripple voltage levels. Also, investing $90 in a "power cache" system seems a bit extreme, especially for your system... But as far as the general idea is concerned, this has some potential, but there seems to be no tangible benefit in your particular case.
    Careful what you wish for... You just might get it.

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    Completely unnecessary. All the filtering a good unit needs to stay in ATX spec is in the PSU already.

    Caps in car audio are a band-aid over a problem better solved with an upgraded alternator. If your lights are dimming, it means you don't have the current capacity to power both the audio system and the rest of the car at the same time. Caps will only help with momentary transients... they'll do nothing for your dubstep cranking session

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    The hold up time of a psu is the time it will continue to run when the mains power is disconnected at full load. This is mainly dependant on the input capacitance, not the output. 1F is A LOT of power. Like Wolf said, 2x 3300uf output capacitance for the 12v rail is plenty for something like a 750W to keep it in spec. The output capacitance only really effects the transient response, which is what you are concerned about here. Adding another 2200uf or so output capacitor would not only greatly increase the transient response but may also reduce the ripple.
    Quote Originally Posted by JFK
    The American, by nature, is optimistic. He is experimental, an inventor and a builder who builds best when called upon to build greatly

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    Quote Originally Posted by Oklahoma Wolf View Post
    Completely unnecessary. All the filtering a good unit needs to stay in ATX spec is in the PSU already.

    Caps in car audio are a band-aid over a problem better solved with an upgraded alternator. If your lights are dimming, it means you don't have the current capacity to power both the audio system and the rest of the car at the same time. Caps will only help with momentary transients... they'll do nothing for your dubstep cranking session
    Lights dimming during "thuds" or not, hook your Oscope up to your car battery while the car is running. It ain't pretty.

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    That's why a good car amp design pays attention to input filtering

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    SMPS (switching-mode power supply) is a closed loop system which means the source will adjust itself corresponding to the load current. A sudden load increase will cause an instant droop in the output for a very short period (less than 1ms) since the control loop hasn't responded to the load change and the output capacitors discharge to supply the load. Once the source responds, it increases the output power/current to supply the load and compensate the voltage droop on the capacitor, and voltage on the capacitor returns to normal value after a new steady state is established. The PSU is designed to handle some amount of transient load so you need not worry about it.

    Too large capacitance on output load may become a burden to the power supply, because it causes a big load transient during startup and takes a long time for PSU to fully charge it. It may also cause a loop stability problem.
    It's been a hard day's night and I've been working like a dog.

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    12W will be dissipated by a 1.2 ohm resistor so a large wattage device is needed - say 20W. 1.2 ohms is also low enough to allow significant current flow-back to the 12V rail if it droops.

    P=I2 x R = 10 x 10 x 1.2 = 120W and not 12W

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    The car guys are trying after the fact fixes, also for reasons unknown efficient sound reproduction has been replaced by MY POWER bragging rights.
    It's like associating power handling (heat dissipation) with speaker sound capability.
    Hi jack I know, but the craziness involved ...
    Near as I can tell whats saying is if you have a psu problem as stated the fix is an in spec power supply.

    The car analogy is equally GIGO and match components. I doubt todays electronic functions on automobiles appreciate the surges.
    Klipsh Kornerhorns forever!!

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    Quote Originally Posted by walterm View Post
    Klipsh Kornerhorns forever!!
    Those are fine if you want to go to 40Hz or so. Want to go lower? Think bigger:

    http://www.youtube.com/watch?v=moVwMmdVOnA

    300W into that thing rattles the concrete floor around it. If it fit in a car, the windshield would be in trouble

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