Icom IC-746 Faq page 26

Everyone would agree that transmitting for 3 minutes at 20 amps till the repeater time-out
cuts you off is not a peak. How short a time constitutes a peak and at what duty cycle?
Without some sort of specification or standard, "peak" and "intermittent" could mean just
about anything. Most of us feel more comfortable with a "better safe than sorry" approach.
This dual rating system for power supplies is a great source of confusion. Specifically, you
could buy a power supply rated for 20A and have it fail under full load. The Astron RS-20
and Pyramid PS-21 are examples of such supplies. While both are excellent supplies, they
just can't keep up with an IC-746 under full power.
The exact definition of intermittent, peak, or surge as used by the power supply
manufacturers has not been established.
(Some of us are trying to talk to the supply manufacturers about this to get more details)
To quote a tech from Samlex:
Maximum rating is surge current requirement for starting reactive loads or short term
dynamic loading and is limited to a few seconds.
This supply is not designed for pulsing loads.
Currents beyond the maximum rating will force the unit into current limit and the output
voltage will drop and it will no longer be regulated.
Meanwhile, MFJ said (of the MFJ-4322):
Our units will shut down at surge. At 25 amps surge, the power supply will shut down at
about ten seconds.
So while it is theoretically possible, most people do not use supplies rated less than 20 amps
continuous. In fact, most use substantially bigger supplies.
Bigger Supplies
Why use a bigger supply? There are several reasons.
As noted earlier, the closer you operate a supply to its rated capacity, the greater the electrical
noise, mechanical noise, heat, etc, ...
Strain leads to wear-and-tear. So it can be argued that a bigger supply should last longer.
The IC-746 is rated at 20 amps max, but loads as high as 21-22 amps have been measured.
"Your mileage may vary" definitely applies.
The continuous rating is generally based upon an ambient air temperature of 20 C. The
rating degrades as the temperature rises, but by how much, we're not sure. Filed Day
conditions will be stressful to your supply.
13.8 V times 20 A yields 276 watts. There's a tendency for some supplies to drop voltage a
little when they get hot or overloaded. There's also a tendency for some radios to increase
their current pull when the voltage drops. At 12 Vdc, that same 276 watts would imply 23
amps. You push the supply capacity, which drops voltage a little, which drives the rig
current up, which pushes the supply further ... then BANG. You blow a fuse. (AK6R has
actually seen this effect with his 756pro)
There are plenty of other devices that need DC power. You need to account for them as well.
IC-746 FAQ
Page 26 of 113
2003-02-08