Re: Idle Curiosity -- 4-20mA Signalling

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Bruce Varley's Avatar (by Gravatar)Bruce Varley posted on
October 7, 2009, 12:06 am
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Tim, I may be corrected but I'm not aware of anything. I've always consulted
the system specs, which generally specify in terms of maximum allowable loop
resistance. It's pretty common for equipment these days to be powered from
24V, so that gives one an indication, there's some voltage loss across the
current source electronics, plus the loop resistance voltage, plus whatever
you use for current sensing. AFAIAA, 250 ohms, or 1-5 volts, for input
circuits is still typical (but in practice it can be far lower).



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Spehro Pefhany's Avatar (by Gravatar)Spehro Pefhany posted on
October 7, 2009, 12:48 am
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Re: Idle Curiosity -- 4-20mA Signalling



On Wed, 7 Oct 2009 12:06:28 +0800, the renowned "Bruce Varley"


Yes, the 24V (or 32V or 40V or whatever) gives you an idea of
worst-case power dissipation, but there can be long wires on these
things and they need protection against relatively high transient
voltages (think lightning strike at the old refinery), which usually
means discrete components, not ICs, will take the brunt it.


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Spehro Pefhany
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Bruce Varley's Avatar (by Gravatar)Bruce Varley posted on
October 7, 2009, 8:28 am
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Re: Idle Curiosity -- 4-20mA Signalling





Thanks for reminding me, if your loop is category Ex-I (intrinsically safe)
then the loop has to support the voltage drop of the safety barrier if one
is used. Barriers can have significant voltage drop.



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