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[John West]

More capacitance in a DC power supply circuit more fully stabilizes the circuit voltage. It won't affect the actual voltage of the circuit. An electrolytic capacitor acts like a small reservoir of charge, but it won't charge up to more voltage than it is supplied. NOTE: Electrolytics must be placed with correct polarity or they will most likely fail in a possibly loud and messy way.

The rule of thumb for the small value capacitors (100n) is to place one across the power rails very close to each IC it filters, not use a bulk one for all the IC's. They need to be close to each IC to do their job well.

The bulk electrolytic capacitor (200 to 330 mfd or so for three chips) can simply be one cap of three times the nominal value used with one chip. Place it near the regulator. If the actual power supply is more than a couple of feet from the bd, then an additional couple hundred mfd cap and another 100nF cap should be placed on the input to the 7805 as well. Note that the capacitors on the high voltage side of the regulator should have voltage ratings at least 30% above the voltage they will operate at.

Aluminum electrolytic capacitors work well, but tantalum electrolytic caps work especially well. I use either, but if I have one of each handy I will install the tant..

Those are rough rules of thumb used for reliable capacitor filtration of circuit bd power rails and IC's throughout the electronics industry.

If you've already fabricated the bd, then just add the capacitors on the bottom to test the result. If the bds haven't been fabricated yet, run the test, then add the caps to the design. Either way, a solid design needs those caps.

 

[alistairsam]

ok
will add the capacitors (one 100n for each picaxe close to its ps pins), and 220mfd or more near 7805.

reason i mentioned increase in voltage is based on experience with audio amplifiers where we use 4700mfd to 10000mfd caps across the power supply and that raises a 24v supply to well over 38v even under load and when in use (it does drop to 26v or so depending on instantaneous power draw)

 

[John West]

That's due to the fact that you're using unregulated power supplies, and the larger capacitors raise the average voltage reading of the raw rectified pulsing DC voltage, bringing it closer to the peak voltage of the incoming unregulated pulses. The voltage read is a function of your meter's ability to accurately read a pulsing DC voltage (high ripple content) as a fixed voltage.

Typical Hi-Fi amp power supplies just step down the mains AC, then rectify it for positive and negative outputs, then add big capacitors to try to even out the raw rectified voltage pulses. Their power supplies are merely "filtered," not regulated. Not fancy by any means. Quite crude in fact, but cheap, and good enough for most consumer use.

The very best Hi-Fi amps now use fully regulated power supplies with constant voltage regulated outputs. Their sound can be exquisite, noticeably superior to a similar amp with an unregulated supply.

In the 5 Volt circuit you're using, both the peak and minimum voltage the picaxe circuit sees remains at 5 volts all of the time thanks to the 7805 (and filter capacitors.) There are no incoming DC pulses with higher voltage peaks coming from the 5V power supply, so the electrolytic capacitor just provides a little local reservoir of 5V energy in case the circuit demands some significant amount of instantaneous current.

The small capacitors, likely ceramic or mylar film installed close to the IC's, provide a path to ground for any very high frequency noise that may find its way into the circuit, such as RF from radio transmitters or harmonics from nearby light dimmers and such, and will keep it from getting to the IC's.

That's the basic idea behind the use of the two sizes and types of capacitors used on such circuit's power leads.

 

[alistairsam]

thanks. that explains a lot.
I will test and post results.