When I was young(er) my brother, a former NASA aerospace engineer who went into audio equipment repair, showed me how to build a current-limiting light bulb gadget. It’s worked for me for many decades (arithmetic is pretty simple,, VA=W so a 100W light bulb will pull a steady-state 0.85A on a 120 VAC circuit and blow if inrush current exceeds… I forget it’s been so long) but it’s only been the past ten or so years I’ve started using a variac upon advice of many in the field. Seems like a reasonable precaution given the time/money invested, I just don’t know how necessary it is, really.

This setup was all new to me but seemed to be recommended by various YouTube channels. As I was starting to get paranoid after changing all my electrolytics I thought I’d give it a go. Great fun making it but an extra cost for more or less a one off. The information I had was if the bulb glowed you had a short circuit. Being in the UK I got up to half voltage and the bulb glowed. Now I was worried. I went onto forums and now I was told that was normal and I might need a second 100W bulb-confused or what. I juggled the resistance figures in my head not knowing how much current my power amplifier needed. I tried again and the bulbs started to light before my voltage reference tube would strike-which I was used to seeing immediately on normal power up. Looking at the voltages I was getting nothing seemed untoward and I then checked the current. It seemed that the 200W of bulb seemed to cut in too early and choke the supply of current. Stephe kindly reassured me that she tends to use this method for initial testing. As I knew I’d wired everything correctly and there was no smoke smells or bangs I then went for a normal power up and rely on the fuse to blow. All went OK and sound emerged from the speakers-phew! After finally receiving my power plug it showed the amplifier takes about 1.4A at 240V so unfortunately it seems that when the bulbs illuminate they’re restricting the current to well under an amp max. Maybe if I’d have been brave enough and pumped the variac up to 240V I might have got a signal through the amp but I was too scared of the Big Bang theory! 🙂

For over 100 years, USA, European electronic industry powerhouses then Japan, Inc., and now Chicom, Inc. (also encompassing all of greater Asiatic prosperity sphere countries like Taiwan, Malaysia, etc.) have been cranking out radios, audio gear like amps/preamps/crossovers and (but not limited to) medical equipment jam-packed full of electrolytic capacitors.

I can’t, for the life of me, believe that every single factory takes such great care exercising due diligence in ensuring each and every capacitor is exposed SLOWLY to increasing amounts of current until full power is reached.

Transformers are chokes. Chokes are chokes and, while I don’t know the actual math (any more, calculus and physics is 40 years behind me now), the moment the electrical connection is completed, so too is the magnetic field which “energizes” the circuit.

I find it difficult to believe out of the hundreds of millions of pieces of future landfill electronics that, each and every piece was first turned on the way you show OR as I do too and have had recommended to me by IEEE technogeeks and hobbyists since I was doing this stuff in Jr. High School circa 1970.

I too use a variac both a 20 amp unit about fifty years old for powering up newly assembled (or discharged for reworking/recapping) gear and another 15A model to limit our wall voltage to 118V (this goes into a sine wave regenerator/UPS unit for my HiFi stack) for older gear built for 115V which was standard US outlet voltage way back when as our local power company - BGE - very frequently cranks up the voltage to 125 and higher. In years past it’s been as high as 127 peaking at 132 (I ran a monitor with memory for over a week one time) which burned out several pieces of audio and video equipment including several high-value UPS units and we had a bear of a time going through our homeowner’s insurance to recoup the costs.

I think of this practice as good insurance but, is it really necessary?

BTW, how’d it turn out?