[Elecraft] K2 Muting

[W3FPR - Don Wilhelm]

Jack,

It is not uncommon to 'wire-OR' bipolar devices.  We commonly do that with
other CMOS family devices which also spec a source/sink capability of 25 ma.
or more.

Yes, that is equivalent to shorting to ground with one of the devices at a
logic high level.

The sourcing spec for a bi-polar device tells me that is the max it can
drive.  I have not seen a limit on the 18C452 cumulative drive limit (number
of outputs sourcing current simultaneously), so I don't believe that is any
different than one output driving several loads for which the total current
sourced would be a total of 25 ma. in the logic high state.  The current
comes from the equivalent of a collector resistor within the device itself.

OTOH, Where overload usually occurs in bipolar devices is at Vol (when the
device itself is holding a logic low level), when all attached devices (in
sum) could cause greater than the I-sink rating to be conducted through the
saturated output transistor - this usually results in a quick death.  In a
'wire-or' application where only one of the driving devices contains the
collector resistor, the sink current is not increased by adding another open
collector driver, and for that reason, I did state that it should be an open
collector driver.

I admit I could not find detailed drive/sink information specific to the
18C452, and I am basing my conclusions on the specs for the remainder of the
family.  If you have information that differs, please let me know where to
find it.

73,
Don W3FPR

----- Original Message ----- 

> Ack! You sure you want to do this, Don? Pin 17 is bipolar drive, so you
> would be shorting a relatively high-current output (25 mA per spec)  to
> ground. That has a good potential for letting smoke out of the MCU.
>
> IF you do this, I HIGHLY recommend a current limiting protection resistor
> in the circuit. A few hundred ohms should do, but might also affect muting
> operation...
>
> On Dec 9, 2004, at 10:41 AM, W3FPR - Don Wilhelm wrote:
>
>> For something more temporary, an open collector transistor connected to
>> pin 17 of the MCU (Control Board U6) would do the job for you - drive the
>> base of the transistor with 3 to 5 volts to turn it on when you want the
>> K2 muted.  A 2N2222 or similar should work just fine.
>