[Johnson] Power on surge

[Rodger Singley]

Like Larry pretty much everything that comes across my bench gets inrush protection.  I get almost everything from Mouser however when I was restoring my Desk Kilowatt Mouser wasn't stocking the high current units I needed for it and I got them from Digikey.

The advice to be conscious of heat generated when mounting is spot on; for the same reason leave some lead length and don't use the leads to put the device under tension because that will destroy it from stress generated during the heat/cool cycles with resultant metal expansion and contraction. 

I select them based upon having them run at about 80% of rated current as a ballpark figure, it isn't critical however don't overdo it (like using an 18 amp rated unit for a receiver).  For the greatest inrush reduction select the highest initial cold resistance but this level may not be desirable in all cases and the plate supply for AM/CW gear is one example.  There will be some thermal lag, for example operating AM my Desk KW takes a bit under a second to come from about 90% to 100% of full power.  It isn't noticeable on the air but it can be seen via the output meter.  This same effect will occur on the first CW character giving a bit more shaping but after this the inrush limiter stays hot between characters so it only impacts the first character and isn't an issue.  However if one went overboard in this case with a very high initial resistance then the effect of reduced initial transmit power might become a problem.

For more modern linear amplifiers using a "step start" system with a series resistor shorted by a relay whose coil is powered via a R/C time delay circuit works very well to reduce the surge from charging the filter caps.  I used this system instead of inrush limiters for the plate supply for my homebrew contest grade amplifier which is built into a floor rack using three 4CX800 tubes and a large P. Dahl transformer.  This avoids the need for finding high current inrush limiters for this type of supply however the tube filaments and separate screen supply do have inrush limiters.  If you use the step start relay system don't make the error committed by a lot of commercial producers and allow the amp to go into operation if the relay fails to close.  This results in at minimum a badly burnt resistor and in many cases a destroyed circuit board.  I used a relay with an extra set of contacts and that set of contacts is in series with the PTT line so until the step start relay closes to short the resistor the amplifier cannot be put into transmit mode.

Inrush limiters CAN be used in series for higher initial resistance but I have never found a situation requiring this to be done.  They may NOT be used in parallel for increased current capacity because no two units will behave identically and it is likely one will "hog" most of the current and then will burn up.

For complex pieces of gear you may need multiple inrush limiters.  For most vintage transmitters generally two are needed, one for filament and lower voltage circuits and a second just for the plate supply.  The protection for the lower power circuits should be installed after the tap point for the plate transformer primary switch or relay so that it can be chosen for optimal operation at the lower power circuit current level.  More may be needed for more complex gear.  My Desk KW has multiple inrush limiters protecting the filaments for the high power tubes, the low voltage and bias circuits, and the plate supply.

As Jim Garland noted in his Electric Radio article you can often fuse a circuit at closer to actual run ratings simultaneously providing better protection while still reducing "nuisance" fuse loss from surges.  In many circuits fuses are chosen to withstand the high starting surge and are far beyond optimal for normal operation thus providing slower/less protection in case of fault or severe mis-tuning.

Rodger WQ9E


-------- Original Message --------
> From: Van Lincoln <vlincoln at frontiernet.net>
> Sent: Tuesday, June 02, 2015 8:48 PM
> To: Larry Oneets <oneets123 at gmail.com>
> Subject: Re: [Johnson] Power on surge
> 
> Where is the best deal on inrush current limiters please, Thank You  WD8AAM
> Van
> 
> 
> 
>