Mesa cards (7i76) field input pull-up resistor

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9 years 11 months ago #83486 by cts1085
I know that Peter has recommended PNP over NPN due to no extra parts needed but i find myself with NPN switches.

I also know that Peter has recommended a 2K 1W pullup resistor in these cases.

Is it feasible to use a higher resistance pull-up to lower the current draw when grounded?

In my case I am using 24v field Vcc

for example, a 15k 1/4watt resister still leaves 15v present (after voltage drop) - and the resistor stays cool.

Would a 10K work?

Just trying to reduce heat and current consumption i guess.

Thanks for chiming in...

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9 years 11 months ago #83487 by tommylight
It should work.

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9 years 11 months ago #83493 by PCW
4.7K would be about the highest i would trust for a 7I76 pullup
This would give ~20V in the high state and 24V field voltage.
A 4.7K resistor would dissipate ~ 1/8W at 24V so that should not be an issue

At 24 V the 7I76 input thresholds would be 16.8V (going high)
and 9.6V going low (60% and 40% of field voltage)

A 10K resistor will only pull the input up to ~16.5V so will not provide a valid high level

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9 years 11 months ago #83499 by cts1085
Thank you very much!

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