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DCIS, SMART, HART (followup)

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3 - The mA signal is passed through a 250 Ohm resistor to produce a 1-5 V signal for the op amps. Thus there is a minimum of 1 watt power consumption. This is sufficient to exceed most sources of interference.

Walter.

sorry walter for this ghost query, but i fail to understand why a 4 to

20 mA current flowing through 250 ohm, would waste 1000mW 'minimum'. if power consumption is I.R and not I*I.R, i need your tution. i figure the power wasted across resistor as 4mW(4*4*millie*millie*250) to 100mW(20*20*millie*millie*250) please correct wrongness in my understanding.

D'jay

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dhananjay
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D'jay,

Sorry, I obviously made a mistake. What time of night was that posted?

The minimum power across the range resistor is 4 mA * 1 V = 4 mW. The maximum power in the whole loop is typically 24 VDC * 20 mA = 480 mW.

4 mW is still much greater than the amount of power involved in a thermocouple signal which might be milli volt^2 * meg Ohm.

What is "ghost query"? I'm not familiar with the term./

Walter.

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[edited]

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Walter Driedger

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