[R-390] Current Inrush

[Bernie Doran]

 I had no idea that the people on this site were so ignorant that they can 
not read a simple graph!!! I have used these critters for years and find 
nothing works better or simplier, and yes the CL90 does fit most of the old 
receivers quite well.   I have not seen any old receivers that draw 100 ma.
----- Original Message ----- 
From: "Robert Nickels" <ranickel at comcast.net>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, December 04, 2012 12:39 PM
Subject: Re: [R-390] Current Inrush


> 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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