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'[SX] A question for Gunther'
2005\08\20@215616 by Electronegativityn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Electronegativity wrote:

Hello Gunther!

I am looking at your suggested circuit for a 7805 voltage regulator, and wondering why you chose a 100uF capacitor on the input-side, and a 33uF capacitor on the output-side.

Also, if you would be so kind as to elucidate the differences in functionality between ceramic, electrolytic, and tantalum capacitors I would be truly grateful.

-Alexander
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2005\08\20@235434 by g_daubachn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Hello Alexander,
in choosing the capacitors, I followed an application note I once found. As the regulator actually performs two tasks - stabilizing the output voltage, and filtering out ripple at the same time, the output capacitor can be smaller. This also makes sure that on power-off, the output capacitor does not force a reverse current through the 7805 (as the input capacitor is larger). In addition, I have added the diode for bypassing this current, in case some device with a larger capacitor installed between plus and minus is connected to the regulator.

Each capacitor has a so-called Equivalent Series Resitance (ESR). The lower this resistance is, the better can it filter, i.e. short ripple. "Regular" aluminum electrolytic capacitors have relatively high ESRs, varying with the ambient temperature. Tantalum capacitors (mre expensive) have a much lower ESR. Ceramic capacitors have much less capacitance compared to electrolytic or tantalum capacitors. To combine the advantages of the low-priced electrolytic capacitors with the low ESR of tantalum and ceramic capacitors, you can put in parallel an electrolytic and a tantalum, or ceramic capacitors.

The 7805 datasheet recommends to attach two 100nF ceramic capacitors as close as possible between the input and output pins and ground to avoid that the regulators starts oscillating.

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2005\08\21@114822 by Electronegativityn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Electronegativity wrote:

Thanks Gunther, it seems like your circuit is designed for maximum fail-safe stability.
I was thinking of doing away with the diodes to reduce part count though; unless you think this would be extremely unwise.

-Alexander
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2005\08\21@140957 by g_daubachn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Alexander,
you are absolutely right - when knowing the 7805's environment, just leave away the diode protecting against wrong input polarity. You also can leave away diode to bypass reverse currents when the total capacitante on the output side is smaller than the ones on the input side. You should not leave away the two ceramic capacitors in order to avoid unwanted oscillation of the 7805 that even might kill it. BTW, another datasheet I have here, recommends a 330nF capacitor on the input side, and a 100nF on the output side.

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