[R-390] Current Inrush

Robert Nickels ranickel at comcast.net
Tue Dec 4 12:39:23 EST 2012


The NTC thermistor method of limiting inrush current was used in 
millions of TV sets but these were well-engineered applications where 
the part was properly sized.    As is common in the internet age, 
semi-truths and incomplete solutions become propagated and archived for 
the naive to find and assume to be correct.

NTC thermistors that are inserted full-time in series with the load need 
to be properly sized - the manufacturers provide datasheets and 
calculators for this purpose, there's no "one size fits all" solution.   
For example, a CL-90 is designed to run at a stable temperature with 
approx. 1A, with a minimum of .5 and a maximum of 2A current.   If you 
use one on a load drawing say, 100ma, its resistance will vary quite a 
lot with ambient temperature  (after all, thermistors are widely used as 
temperature sensors).  You don't want the voltages in your radio varying 
with the season ;-)

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.

I'm not familiar with what's in the box sold through ER but I'd guess 
it's just a full-time NTC with a cute little meter to show you the 
rising voltage.   I think it makes more sense to put full-time NTC 
limiters inside small devices that present consistent loads, and provide 
a true soft-start to help pamper the big iron.

73, Bob W9RAN


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