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Maximum voltage into stamp pin. — Parallax Forums

Maximum voltage into stamp pin.

FORDFORD Posts: 221
edited 2006-06-28 22:28 in BASIC Stamp
What is the maximum voltage allowed into a stamp pin (bs2p24) ?

I know its possible to put rs232 voltage into a stamp pin through a resistor, but I would like to know the maximum practical voltage, and also is there a simple formula to set the resistor value for the voltage in.

(its not as simple as ohms law is it ?)

Reason is that I would like to detect 12 and 24 volts with a stamp, and if I can safely just do it with a resistor I will, otherwise I'll·put in an optocoupler, but its for a pcb which has already been made.

Cheers,
Chris

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-06-27 22:52
    Chris,

    ·· The optocoupler is the way to go.· The voltage input information can be found in the datasheet for the SX, which is at the link below.· I hope this helps.

    http://www.parallax.com/dl/docs/prod/datast/SX48BD-Data-v1.2.pdf

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-06-28 07:37
    See page 52 and beyond. It may be too much for some Stamp users.

    The Absolute Max is 7Volts difference between Vdd & Vss, but a nominal +5volts is considered ideal for the BasicStamp.

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    "If you want more fiber, eat the package.· Not enough?· Eat the manual."········
    ···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-06-28 13:59
    The reason you can put a signal in to the stamp, that is over 5 volts (IF you use a current limiting resistor) is that the stamp has "protection diodes" on each pin. These can't conduct much current (1 mA max, I believe) but their purpose is to 'clip' any voltage over 5 volts, and any voltage under 0 volts. So, if you put in a current limiting resistor of the proper value (22 Kohm is used for RS232 +- 12 volts) you can read these signals with a BS2.

    Using a plain resistor doesn't give you much safety, though. If somebody inadvertently raises the line too high, it can still blow the BS2 I/O pin. Thus the recommendation for an opto-isolator -- those are much cheaper to replace.
  • FORDFORD Posts: 221
    edited 2006-06-28 22:28
    Thanks Allan, Kramer, Chris.
    Thats cleared that up for me. Thanks Allan for the explanation on the 1ma max on the protection diodes, thats cleared it up for me. Obvioulsy not much tolerance to just go selecting a resistor. Will go the opto way.

    Thanks again,
    Chris,
    West Oz.
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