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I need to implement a TTL to RS485 converter into an home made project and searching by Google I found this module with related schematics:

enter image description here

Could you help me by indicating the correct voltage for the two capacitors C1 and C2 shown? For example, if you would take capacitors with a voltage of 16v or 50v DC would it change anything or would everything work correctly?

enter image description here

Thank you very much for your time and support.

Best regards

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    \$\begingroup\$ That would depend on capacitor type and size, which you don't reveal. Also, that image is put here without proper copyright attribution, you did not draw it yourself. Please add the missing info. \$\endgroup\$
    – Justme
    Feb 18 at 16:56
  • \$\begingroup\$ Post edited adding all informations. It is asked for a personal home-made purpose and to learn more about capacitors in a DC circuit. \$\endgroup\$
    – Simone
    Feb 18 at 17:17
  • \$\begingroup\$ It still does not have all the information requested (source of the copyrighted image is missing, and the physical sizes of the capacitors). And assuming it is a DC circuit is slightly misleading, because if it was a DC circuit, the capacitors would do nothing. The data transmission makes AC currents, which is why the capacitors are needed to provide stable DC voltages even if there are pulsed AC currents running in the circuit. \$\endgroup\$
    – Justme
    Feb 18 at 20:13
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    \$\begingroup\$ Please don't expand the question to be a "what should I buy" type of question, as that is off-topic and may get closed. Your original question was already answered and you should mark the best answer, or ask further clarifications if you feel you did not get an answer you accept. \$\endgroup\$
    – Justme
    Feb 22 at 18:21
  • \$\begingroup\$ @Simone - Hi, As commented, it is not allowed to change a question after answers have been provided and you have now asked the new question here. Therefore I have reversed (rolled-back) the last edit. Thanks. \$\endgroup\$
    – SamGibson ♦
    Feb 22 at 18:55

2 Answers 2

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The circuit runs on 5V.

The capacitors are ceramic, which do have less capacitance at higher voltages, and this is related to the capacitor physical size as well.

Based on this, the capacitor should have higher rating than 5V, and 16V already has enough margin. But for example, a 6.3V capacitor would not be recommended.

For hobbyist purposes, the said capacitors of either 16V or 50V should be fine for bypassing the MAX485, regardless of their physical size, which is not given in the question.

The absolute maximum rating of the MAX485 is irrelevant, as even if supply voltage would exceed 12V, the MAX485 will already be damaged, and even a 16V capacitor will handle some overvoltage (for example they might be tested to handle 250% of the rated voltage).

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  • \$\begingroup\$ So higher the voltage is better? A 100v capacitor would be better? \$\endgroup\$
    – Simone
    Feb 19 at 9:41
  • \$\begingroup\$ @Simone That is a good question. In theory, yes, 100V it will be better and so will 1000V too, but in practice, you have a limited budget for price and PCB area, so you really don't want to pay for a 100V capacitor if a 16V capacitor is good enough. Usually, if a circuit like this has a 100nF capacitor, the value is likely rounded up already, so in practice, even if you used a 6V 100nF cap with only 33nF of effective capacitance in a 5V circuit, it should still work. The exact absolute value of effective capacitance is rarely specified. \$\endgroup\$
    – Justme
    Feb 19 at 10:04
  • \$\begingroup\$ If the absolute maximum rating of the MAX485 is irrelevant, why not 6.3v? 6.3 > 5 by 26%. \$\endgroup\$
    – Rodo
    Feb 19 at 18:04
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    \$\begingroup\$ @Rodo I thought I explained it already. You can if you know what you need and what you are buying. If you don't know, just use higher voltage. If you buy a 100nF 6.3V capacitor in a 0201 package, the capacitor has a capacitance of about 10nF at 5V. That's a factor of ten less you get than what you think you bought. So it will not be OK, if you need 100nF. \$\endgroup\$
    – Justme
    Feb 19 at 19:00
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    \$\begingroup\$ @Rodo this article is very interesting: edn.com/… \$\endgroup\$
    – Simone
    Feb 20 at 20:48
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Either 16v or 50v will work. Although, 16v is a bit too close to the "ABSOLUTE MAXIMUM RATINGS Supply Voltage (VCC) ...12V" of the MAX485 specs. for my personal comfort. I would go with the either 25v or 50v.

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  • \$\begingroup\$ Thanks for your support. Can I ask you to explain why this requirements, so we can take a lesson, please? The circuit VCC is 5V. \$\endgroup\$
    – Simone
    Feb 18 at 18:52
  • \$\begingroup\$ Rating the capacitors based on absolute maximum rating is irrelevant. The chip is intended to use at 5V. \$\endgroup\$
    – Justme
    Feb 18 at 19:41
  • \$\begingroup\$ @Simone: I rather stock a supply of 25v or 50v so I can use them in other projects that may have a higher voltage supply than just 5vdc. \$\endgroup\$
    – Rodo
    Feb 19 at 17:51

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