Why the diode in a wall wart hum buster?

The diode stops any existing load on the wart from discharging the 470uF electro between charging pulses - so only the following device is involved in creating DC supply ripple.

Plus prevents reverse polarity errors.

( Apologies to Penny )

.... Phil

Protection against reverse polarity

FWIW I've also used diodes as nonlinear resistors in CRC filters before. But the use of such things is limited. And fwiw 470uF is a bit on the small side.

NT

AFAIK my existence is for real, but you never know with all of the make believe in this world. You actually may want to hold off asking that question until after you see my Altoid jelly bean mixer, which ought to debut shortly. :0)

OIC. Indeed.

Thank you,

My first impulse is to also use the biggest cap on hand. But the 3300 uF

Phil nailed it. The diode prevents backfeed from the 470 uF cap into the wall wart. Polarity protection's just icing on the cake.

Thank you,

On 04/12/2017 11:06 AM, snipped-for-privacy@gmail.com wrote: ,

I'd imagine; what are the situations where you'd want a current-dependent low value of resistance in a CRC filter that also has a negative tempco?

The biggest cap is going to be unsuitable. The larger the C the more the peak diode current, and they don't have infinite abilities.

A wallwart that produces hum is an unregulated wallwart, which couldn't care less about that.

Actually it's what determines if your circuit works or dies.

NT

To filter an unregulated supply mainly. I have also used them elsewhere less often. The tempco has always been a trivial thing.

NT

It's for use with multiple FX pedals and the like, running from a *common supply* that has significant ripple. It then acts as a post filter for individual devices.

The name alone tells you that.

.... Phil

Phil, I know little about what the circuit is doing. But it seems to me you could stick a cap. multiplier, in there instead of the diode and do a better job. (Sell that to the audio market.) I've made nice cap mult's out of TIP31, 32's, but there's likely a lot of power transistors that will work.

George H.

I've never understood why anyone would use a cap multiplier instead of a voltage regulator.

You've obviously never made a circuit where you really care about the noise. (no offense)

George H.

Great answer. No info, but otherwise great.

Sorry, have you ever measured the noise from a voltage regulator? (Some are OK...) George H.

Well, that depends on what sort of "noise" you care about. Often PSRR is more important than circuit noise. ...and then the frequency of interest can change the entire solution again. One size doesn't fit all.

Sure, all sorts of noise and interference, layers of the onion, down the bottom there's thermal and shot noise...

George H.

The only time I've seen a cap multiplier recommended was to remove hum from the power supply for the audio circuit in a ham receiver. Like I said, I don't understand why a regulator wouldn't be used. I don't think ham radio audio circuits need to worry about thermal or shot noise.

The point is that there isn't one solution. It depends on what's important. In this case, I'd expect PSRR to be more important than supply noise.

You still see them used in SMPS to clean up DC output rails for sensitive circuitry.

.... Phil

To create smoke,because it is rather under-rated to say the least.