[BULK] - [Amps] IC-718 and AL-811: Icom manual misleading, in terface nec...

[Steve Katz]

SOA is surely something to consider, except the IC-718 uses a keying relay
for the amplifier "SEND" function.  Check the schematic.  They rate it "16V,
2A," without any consideration for simultaneous or not; however, that still
begs the question, "Why only a 16V rating for relay contacts?"  It's an
amazingly low rating, even for tiny TTL relays....-WB2WIK/6

-----Original Message-----
From: Japerlick at aol.com [mailto:Japerlick at aol.com]
Sent: Friday, February 04, 2005 11:22 AM
To: stevek at jmr.com; W7MJM at arrl.net; amps at contesting.com
Subject: Re: [BULK] - [Amps] IC-718 and AL-811: Icom manual misleading, in
terface nec...


Actually, this is quite common in transistor switching applications.
Transistors (whether FETs, Bipolars, IGBT's whatever) have have "Safe
Operating Areas" which is a technical term for the range of simultaneous
voltage and current levels it can safely sustain.  

For example, the venerable 2N2222A is rated at 75 Volts VCBO, 40 Volts VCEO
and 0.6A max Collector Current...but the max power dissipation is 0.5 watts.
Obviously you cannot sustain 75V at 0.6A SIMULTANEOUSLY.   Manufacturers
specify a safe operating area--usually in chart form--showing how much
current the transistor can pass for given voltage conditions--both in
forward and reverse bias conditions.  It's important to note that these
charts do not necessarily correspond to a simple curve of the maximum power
dissipation.  

Combined with safe operating areas is the consideration of junction
temperature.  Almost all of these specs are given for device junctions at 25
degrees C (about room temperature).  Of course, you can imagine, the ratings
degrade at higher temperatures.  Overtemperature and overvoltage are the two
most common transistor-killers.

So, yeah, the Alpha guy is probably right...you can't get both max voltage
and max current simultaneously....and it is not that simple.