[NJARC] Current limiting Devices

[David Sica]

Well, for the anal-retentively cautious among us, this might be one of the
few good uses for a Variac.  Not sure it would ever be worth the trouble,
but I do know a couple of people who are compulsive about 'soft-starting'
stuff by bringing up the voltage slowly.  Not to re-form old filter caps,
but to avoid the inrush current situation being described here.

There's got to be an easier way...

-- Dave Sica

New Jersey Antique Radio Club



On Tue, Nov 29, 2016 at 11:23 AM, TrainBee via NJARC <njarc at mailman.qth.net>
wrote:

> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
>
> Hi Jim,
>
>     This opened up quite the conversation yesterday.  Al Klase made what
> seemed to be a big issue rather simple.  Sure I could go and get the
> varistor (mail order for me) or I could just drop in a 50 ohm 5W resistor.
> The 50 Ohm resistor is the value for the radio under normal operation.  The
> schematic called for 880 ohms for the start-up, then to drop to 50 ohms.
> If I had the room, I could have put a monster (for this size radio) 10W
> resistor in there.  It didn't take long to make the choice.  Then I got to
> thinking, all I needed to do was to bleed off some of the unused voltage
> and have something to absorb the inrush current.  This little tike uses 4
> tubes, 50C5, 35W4, 12AU6 and 12AV6.  It's no wonder the original part blew
> up.  It is about the diameter of a pencil eraser and maybe 3/8" thick.  It
> couldn't take years of heat.
>
>     Here in Maine, AM during the day is a pipe dream.  What I couldn't
> believe was that I was picking up a station, in the basement, at 10:30 AM,
> from not too far from here on it.  You can't do too much to tweak it to
> hear better.  This was made for local station use at best.  I'll be curious
> to see what I can hear with it tonight.
>
>     So, I hope some of us learned a little bit about varistors and
> alternate solutions.  Just in case you missed what this radio is, look for
> a Philco C-579 of the 1954 vintage.  Curiously, someone actually sold one
> of these a few years ago for $95.00.  Who would have guessed?  I picked
> this up at a local flea market for $10 and it's in pretty good cosmetic
> condition.  I attached the Beitmans page if you care to take a look.
>
> Regards and thanks.
>
> Joe Devonshire
>
>
>
> -----Original Message-----
> From: JIM HEARNEY via NJARC <njarc at mailman.qth.net>
> To: njarc <njarc at mailman.qth.net>
> Sent: Tue, Nov 29, 2016 6:37 am
> Subject: [NJARC] Current limiting Devices
>
> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
> I believe the varistor should work.  This reminds me of a problem that I
> have not been able to solve:
>
> "how to you use voltage measuring equipment, aka analog oscilloscope, to
> measure a very short transient such as a current surge? "
>
> The first part I think is solved using a current probe, bench made or
> store bought, but how do you record the high speed transient.....the old
> fashioned way....poloroid camera on scope.. and single triggered trace?
> What do you use for the trigger?  A scope with memory option makes second
> part a lot easier...I imagine!
>
> Jim
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