78xx + FAT capacitors = ripple elimination?? - diyAudio
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Old 4th September 2009, 12:50 AM   #1
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Default 78xx + FAT capacitors = ripple elimination??

I was just thinking. The ripple performance of a 78xx (after bridge rectifier) is just so-so using the small datasheet-recommended input and output caps.

What if we just put GIANT caps on the input AND output sides of the 78xx? Would that effectively give ripple performance on par with the expensive fancy voltage regulator circuits? Are there any downsides to this? If ESR is a concern, what if multiple low-ESR caps in parallel were used?

Thanks!
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Old 4th September 2009, 01:31 AM   #2
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One problem with using a huge cap after the reg is that on power-on,the cap looks like a short,and may trigger the regulators overload protection. Having said that,I've put some large caps (up to 10,000uf or so!) after the 78xx regulators before,and not had any real problems that I can remember. You will want to make sure that the regulator has a diode connected across the IN and OUT pins,reverse biased (cathode to input side),so that on power-off,the output side can can discharge into the input side. Most regulators don't like being reverse biased,which can happen if the input side cap discharges faster than the output side cap.
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Old 4th September 2009, 02:09 AM   #3
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Why not try putting a shunt-type capacitance multiplier after the 78xx?

IIRC, the 78xx will become a 1A CCS if shorted, so there shouldn't be a problem with using large caps.

Beyond that, better performance can be had if you build a discrete regulator (I'll bet you can get better than the 78xx simply by building around a TL431, with no other ICs).

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Old 4th September 2009, 09:55 AM   #4
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Quote:
Originally Posted by smellygas View Post
I was just thinking. The ripple performance of a 78xx (after bridge rectifier) is just so-so using the small datasheet-recommended input and output caps.

What if we just put GIANT caps on the input AND output sides of the 78xx? Would that effectively give ripple performance on par with the expensive fancy voltage regulator circuits? Are there any downsides to this? If ESR is a concern, what if multiple low-ESR caps in parallel were used?

Thanks!
One thing to remember is that ripple rejection for mains hum isn't an issue for amplifiers, as they generally have hugh ripple rejection ratios at lf. The (load induced) hf ripple is more a concern for amps, but that is less easy to get rid of with just more capacitance. Caps thend to become less effective at hf due to series ESL for instance.
BTW Increasing cap after the rectifier increases hf components in the ripple so that may actually work backwards.

So if your concern is not per se with mains hum, you'd better going to some hi performance regulator, with, the ultimate, at-the-load remote sensing.

jd
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Old 4th September 2009, 05:03 PM   #5
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Thanks for the help so far. The 78xx's are for a DAC. So it's okay to put a 10,000uF output cap on without worrying about a short-circuit on powerup causing the thermal protection to kick in? I guess it would be okay if it was current-limited to 1A, I just wouldn't want it to shut off completely. Also, is the diode absolutely necessary in practice?
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Old 4th September 2009, 10:06 PM   #6
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If your output cap is much larger than your input cap, it is a good idea to have a diode there. Probably don't need a huge one, just one that can withstand some surge current depending on your output cap.

It shouldn't turn off completely if it goes to 1A. These regs are used as CCS sources in tube amps and such; I don't think you have anything to worry about.

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Old 10th September 2009, 06:13 PM   #7
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Quote:
Originally Posted by smellygas View Post
I was just thinking. The ripple performance of a 78xx (after bridge rectifier) is just so-so using the small datasheet-recommended input and output caps.

What if we just put GIANT caps on the input AND output sides of the 78xx? Would that effectively give ripple performance on par with the expensive fancy voltage regulator circuits? Are there any downsides to this? If ESR is a concern, what if multiple low-ESR caps in parallel were used?

Thanks!
In general you do better with mulit-stage filters then just making one stage bigger. The effect of the stages multiply. But making a big stage is additive.

You will see better ripple rejection if you split the large cap in two and connect them with a resistor. This makes a "pi" filter. It is very much better then the same amount of capacitance all in one cap. Of course the resistor drops the voltage. The is the price you pay but if the regulator is after the pi filter it does not matter much.

If you don't like the resisor's voltage drop you can get better performance with an inductor but at higher cost. If you need very clean DC you can use multi-section pi filters.

More info:
http://en.wikipedia.org/wiki/Capacitor-input_filter
http://www.engineeringpost.com/modul...=article&sid=5
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Old 10th September 2009, 09:47 PM   #8
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Putting a big cap after the reg isn't a good idea.

Besides the "short on power up" issue, it will make ripple rejection worse, beacuse the error amp input is effectively shorted to ground.

Big cap on input, then let the regulator do its job

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Old 10th September 2009, 10:23 PM   #9
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Be far better off finding a way to get an LM317T in there.
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Old 11th September 2009, 12:40 AM   #10
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Quote:
Originally Posted by richie00boy View Post
Be far better off finding a way to get an LM317T in there.
True dat! But as he says:
Quote:
The 78xx's are for a DAC.
The DAC and digital ground will make far more system rubbish than the 78xx.


Oh hey, here's an idea... 78xx for the DAC, use seperate grounds and a 317T for the analog side

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