Capacitors in Grove-RS232

Products & Technology Others
1

Why in the Grove-RS232 module capacitor C1=100nF, but capacitor C2=470nF? Should these capacitors have the same capacity?

RS232
RS232551×370 42.9 KB

2

Hi there,

these are greta chips , replaced the old 1488 & 1489 only old timers with RS-232 will know those. The Max chip uses a charge pump to make it work. those are the charge-pump capacitors that the MAX232 (and its variants) use to generate ± voltage rails from a single 5 V (or 3.3 V) supply.

  • C1, C3, C4 are the flying capacitors that alternately charge/discharge and move charge around in the internal pump.
  • C2 (and C5 in some variants) are the reservoir/output capacitors that stabilize the generated rails (e.g. V+ ≈ +10 V, V− ≈ −10 V).

Why 100 nF vs 470 nF?

  • The flying caps (like C1) only need to transfer charge during each clock cycle. As long as they can handle the switching frequency, 100 nF is plenty.
  • The reservoir cap (C2) needs to hold up the rail under load — RS-232 drivers can draw more instantaneous current when toggling outputs into a 3–5 kΩ load. A larger value like 470 nF (or even 1 µF in some datasheets) keeps the rail stable and reduces ripple.

So: they don’t need to be the same, because they do different jobs in the charge pump. We used to use big ass 1uF caps back in the day at Black Box corp. :grin:
The newer caps allow way smaller and more stable and allow the pump to be very, very efficient. RS-232 has never been better IMO. :+1:
Look at the MAX datasheet for more.

HTH
GL :slight_smile: PJ :v:

3

The ADM232 datasheet (it’s an analog of MAX232) says that both capacitors serve as storage elements. In the vast majority of schematics, their capacities are the same, but SeeedStudio for some reason made them different. That’s where my question came from.

…The –10 V level is also generated from the input 5 V supply using C1
and C2 as the storage elements…

ADM232
ADM232309×182 22.5 KB

ADM232_2
ADM232_2439×377 83.4 KB

4

Hi there,

Yes, two factors is what’s at play. You already know what it is :grin:
:money_mouth_face:
I don’t know if it’s AI or some Bean counter in Engineering, Looking at the results on the “typical performance Char’s” The sweet spot for what they anticipate for a MAX external power and the cheapest or should I say less expensive way out meeting in the middle Maybe?

I doubt you get a concise answer, but let’s see if someone takes the bait. :+1:

Inquiring minds want to know too.
Good Stuff man

GL :slight_smile: PJ :v: