Re: [SI-LIST] : Schottky diode termination

Mike Monett (add@csolve.net)
Thu, 29 Oct 1998 03:17:06 -0800

Robert Muir wrote:
>
> Some years ago a colleague of used the input protection diodes of
> logic devices as part of the normal operation of a circuit and
> experienced an interesting problem.
>
> The circuit was an 8-channel digital input module of an industrial
> controller taking input voltages from 0 to 60V. If I recall
> correctly the input was taken through a 200K current limiting
> resistor to the input of a 74HC244 buffer, with a 2M resistor also
> on the 74HC244 buffer to ground. Even with a 60V input the maximum
> current through the protection diode is only 0.3mA, well below the
> limit of 20mA. The circuit worked nicely and was put into
> production.

[...]

> Regards,
> Robert Muir

Sounds like the 0-60V signals were logic levels.

Why not change the 2Meg resistor to 18k? This would give 0-5V at the
input to the 74HC244, which should allow it to detect if the signal level
was high or low.

This is a simple change to the bill of materials. Then, any 74HC244
should work. No need for special part selection and procurement.

Best Regards,

Michael R. Monett,
Automated Production Test
mailto:add@csolve.net

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