[R-390] Current Inrush
Drew P.
drewrailleur807 at yahoo.com
Thu Dec 6 01:06:47 EST 2012
Robert wrote:
[snipped]
"Instead, many boatanchor applications would be better served with a
soft-start circuit. Again, these are widely used in industry and not
rocket-science, and are easily home-constructed. A simple approach
uses an NTC to limit inrush current (or just a power resistor) for a
short period of time after AC power is applied, but this device is then
bypassed by contacts of a relay to provide full line voltage to the
protected device. All that's required is a slow-rising voltage
(usually just an R-C time delay) to provide a delay before the relay
contacts pull in. The soft-start may be harder to fit inside the
radio, but building it in an external enclosure makes it easy to use
with different radios. Since the NTC is only in the circuit for a
short period of time (from 1/4 second to several seconds, typically),
sizing of the NTC is less critical and there is no heating issue because
the device is bypassed nearly all the time. Both the voltage and time
can easily be determined by the builder."
And, IIRC, Dave Wise implicated an NTC thermistor in an R-390A in frequency instability in one or more of the crystal oscillators, due to degraded power supply regulation.
Both concerns, inrush and undesirable series resistance may be addressed with a simple, classic circuit. The NTC thermistor (or fixed resistor) is inserted in series with the transformer primary supply (hot lead). Then, relay's coil (appropriately voltage rated) is connected across the primary. The relay's normally open contacts are strapped across the resistor, that's it. Upon application of power, the resistor limits the surge, and the primary voltage stays low until the surge has largely passed. With reduced current comes less voltage drop across the resistor, the primary (and relay coil) voltage rises, and the relay pulls in, bypassing the resistor and applying full voltage. Some experimentation with the resistor and with relay pull-in characteristics is in order. A resistor can be inserted in series with the relay's coil and/or the armature spring tension can be varied.
Drew
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