How to calculate power ripple, and then to add the sutable Capacitor?

Noise? Does this mean hum, or does it mean motorboating, or does it mean broadband hash, or ?

Nothing wrong with using a scope, but this is pretty much chapter 1 of any power supply design tutorial.

Tim.

Boki,

If you're using a switching supply, your best source of equations would be the app notes for the regulator chip that you're using. For a simple full wave rectifier-capacitor input filter type design, here is a rule of thumb that I have found useful: ~ For ripple < 10%, the following rule applies: . Let R = the load resistance (Ohms) . Let C = the filter capacitance (Farads) . Let f = Frequency (Hz) . Pi = 3.14159...... (2PifRC) = 10 yields (roughly) 10% ripple and 10% load regulation. This rule scales (roughly) linearly, so, for example: . (2PifRC) = 20 would yield roughly 5% ripple and 5% load . regulation Remember this is just a rough rule of thumb, but it gets you close. Regards, Jon

ya, I have to review them recently.

basically, I think I can measure the ripple first, and then try to calculate how larger RC time do I need, am I right?

but it seems that only a capacitor between VCC and GND, so what I have to consider about R is total circuit?

Best regards, Boki.

Given the context of prior quotes, I think you're using an LM386.

They are prone to motorboating (that is low-frequency instability) without good supply isolation. If the noise you're hearing sounds like a pulse train (tick-tick-tick at 5 to 50 ticks/second), that's probably what's happening. If it sounds like hum, then it's more likely a problem of supply filtering, that is, ripple. If it sounds like hiss, it's either noise on the supply lines, or high-frequency instability, the latter again an isolation problem. Ripple reduction is a matter of reading an intro text on power supply design (it's probably covered in AoE, I don't remember offhand). Isolation is harder to solve theoretically, because you don't have enough information about the supply impedance or about the internals of the LM386; trial-and-error is the best approach there.

In either case, looking at the output and at the supply line (near the chip) with an oscilloscope can tell you a lot. Especially if the problem goes away when you touch it with the scope probe :-)

This site may help:

Ed

You're asking the wrong question.

Your 'noise problem' is unlikely to be related to any ripple current requirement. The reservoir capacitor(s) in the power supply do this.

It would help if you explained what the noise sounds like though !

Graham

Or your finger !

That kind of instability was very popular in the 70s / 80s.

If Boki used a more suitable amplifier chip the problem would likely vanish.

Graham

The rule is that an 8300 microfarad capacitor has its volts of ripple equal to its amps of current at full wave 60 Hertz.

Everything simply scales from there.

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It's not RC. It's I=cdV/dt. I is the max supply current, dV is the ripple Voltage you are willing to accept, and dt is the period of the waveform on VCC.

For example if you build a power supply from fully rectified 50 Hz, then your waveform is 100 Hz. So you want to use 10ms as your dt. Now you can solve for c:

c=Idt/dV

HTH

For high-current power supplies you then you have to calculate the ripple current for the capacitor and select a part (or parts) which can handle the ripple current.

--Mac

I read in sci.electronics.design that Don Lancaster wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Sat, 1 Oct 2005:

10 000 uF for full-wave 50 Hz. Even simpler.

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Actually about 8000 uF @ 50Hz and 6700uF @ 60 Hz

Don incorrectly assumed a discharge time = 1/2 cycle.

Graham

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Sun, 2 Oct 2005:

Unfortunately for the harmonic levels on the mains, the charge time for high-efficiency SMPS is so short now that the half-cycle assumption is almost true.

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Uh ?

Peak loading results in flat-topping which actually extends the charging time ! The ac supply's way of 'compenating'. ;-)

Graham

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Sun, 2 Oct 2005:

But I've done the measurements.

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

What were you measuring ?

Just doesn't sound right to me.

I'll take a look at an smps I'm working on right now. It certainly isn't the case for classic line frequency transformer based psus where the charging time is as much as 2.5ms at lower powers ( lots of DC R from the transformer ) and typically around 2 ms at medium / high power.

Graham

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Sun, 2 Oct 2005:

SMPS between 30 W and 75 W. Short conduction angles, like 18 degrees, or

1 ms in your terms. And it wasn't only me: all the members of the IEC Task Force saw it done. Some, but not all, eyebrows were raised.

More; you can get to the magic 65 degree conduction angle (3.6 ms) that meets the IEC/EN 61000-3-2 Class D limits. Not so easy with a toroidal transformer, but a few ohms in series helps.

Transformer/rectifier supplies do have lower harmonic emissions.

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Ok, I have a 33W smps I can compare with.

I'll bet the relatively small reservoir cap has something to do with that. I can't recall offhand if we use 68uF or 100 uF.

Was there any R in the way at all btw ?

as

I'll also take a look at an R-core based supply we have. The DC R in those is very high indeed. The R-cores have the lowest stray flux I've ever met btw. Haven't got my hands on an O-core yet but they should be even better.

Graham

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Sun, 2 Oct 2005:

Probably not enough if you have to meet the immunity requirements against voltage dips and interruptions. People are using much the same capacitors as for 200 W supplies. They must be, otherwise we wouldn't get such small conduction angles.

The only impedance in series with the rectifier is to limit the inrush current to 50 to 80 A or so, to avoid things going BANG!

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

When you say 'my amplifier', is this a new design of yours or an existing unit that you are repairing? Calculating the ripple is OK for an initial design, taking regulator and amplifier app notes into account.

The scope would be the best approach for a repair. The design may have been OK, but components may have drifted over time.

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