The AM Forum

ALthough this is an ebay item, this post is more properly put here in Technical, IMHO.

A seller on eBay is selling a kit of 35 different values/5 pieces each NTC thermister current inrush limiters for $15!  Item # 7559921503  There are 18 kits left as I write this, get 'em now!

These things are about $2-3 each at DigiKey!  The kit has a very nice assortment of values that operate from 1 amp to 11 amps steady state current, and 5 ohms to 120 ohms initial (cold) resistance.

For those who don't know what they are, they are thermisters with an initial resistance of 5-100 ohms (depending on type) and when current flows thru them, they heat up, thereby decreasing resistance to .05 -.5 ohm (again depending on the thermister).  They therefore absorb the initial turn-on surge creating a very nice soft-start.  Just the thing for saving wear and tear on the irreplaceable transformer in that old boatanchor.  You use them by just putting one in series with the AC line and make sure it's not up against anything as they get hot in use.

The data sheets are available here:  www.avxcorp.com/docs/masterpubs/tempNTC.pdf

Hola John,

I still have the inrush limter "kit" you gave me about a year ago.  I put a few in parallel,  in series with the 3KV HV transformer's 240AC  primary. They do heat up for a moment and do the job. Since it's a capacitor input supply with no choke, this is a very desirable item to use to protect all components, including the transformer for inductive surge flashovers, the diodes from surge currents, and reducing the long term effects of capacitor damage thru large charging current surge..  The only problem is that the lights don't blink much when I turn on the breaker anymore...  :-[  [Not as manly]

TNX for the donation. I also recommend John's suggestion of grabbing an assortment of these for that price.

73,
T

Glad they are getting some use, T.  I still have a box of about 500 or a thousand (I think I paid $10 for the box!).  But the ones I have are too heavy duty to do much good for stuff like receivers, etc - the ones I gave you are 1 ohm cold and good for 20 amps IIRC.  The small current draw from a rx wouldn't even begin to heat them up, aand 1 ohm is probably too little to do a lot of good on a low current device.

73 John

John,

That sounds like something I should look into. My 20V3 transmitter was never meant to be cycled on and off as it is here. The plate transformer and others in this rig would probably live much longer with inrush currant limiting. I'll have to look at the manual and schematic to figure out how many and of what current I'll need. I think the parts list gives the transformer ratings.

Since the transformers are all 240 volts I'll need one limiter on each ac leg to each transformer? Thanks for bringing this up.

Mike

Mike, if you want some 20 amp/1 ohm cold jobbers, lemme know.

I think the highest current ones in that kit are 11 amps, you could probably parallel them.  The kit would do your receivers and just about anything else with an unobtanium xformer.

73 John

I have been using these thermistors in receivers (R-390A) etc, and they do a great job of taming that initial inrush surge. Stops that thumping, "cheewang" sound. They do get hot..VERY hot, but they are designed to work at high temps. I agree with John in that they should be installed in ALL OB receivers.

Now, I recently have been refurbing and modifying a DX-100 - SS supplies, etc. I put a 16 ohm cold at 4 amp Thermatronics thermistor in the HV plate xfrmr primary and it works FB. When I key up the rig, the plate current rises fairly quickly to full value without a huge inrush surge, the thermistor quickly becomes very hot and all is well. The problem I forsee with this method of inrush taming is when the QSO is not an old buzzard type of roundtable. If I allow 30 seconds to a minute or so, the thermistor cools off and is ready for another round of suppression. If I key up for a few seconds, unkey for a few seconds, then key up again, (break-in ops), the thermistor is still hot and presenting very low resistance so it is useless for it's intended purpose.

When I did up my Gates BC-1G a few years back. I used SS time delay relays that short out 10 ohm 25 watt resistors (one in each leg) after about 1/2 to 1 second. They work FB for break-in with the big Gates that was certainly not intended for this type of operation. I even added one to limit the initial surge of the filament inrush. I have since added this system to my T-368. These SS TD relays can be had fairly cheaply on Ebay. I try and get the ones that have a fairly low time range (.1 to 5 seconds). You can get the relay, the socket and 2 resistors for $10-15.

Here is a pic from my webpage of the one for the plate step-start in the Gates: 

http://webpages.atlanticbb.net/~w3np/images/bc1g/Gatescontrol906.jpg

I wished there was a way to quickly cool the little thermistors for more effective use in smaller transmitters as they are cheap, easy to install and don't take up much room.

Any thoughts???

just bought a kit

John

thank you, very much
would never have seen
the listing

Mac