Softstart for variac

[Mr. Moscode]

Buy several CL100 surge suppressor from Mouser. It's a few bucks. If one doesn't do it put 2 in series. .5 ohms cold and next a short hot. If you want I think I have some. I would have to look for them. PM me. You could have them for the price of the postage.

Since it's a variac, I assume you will monitor the output. I have rig that has a cheap meters in it for my test bench. Right now it's a light dimmer. Ha.

The analogue meter gives me an instant feedback of a short or hi inrush. The digital display is $20 power monitor I found on amazon or fleabay. Volts, amps, watts, power factor. It doesn't come on at very low voltage. It's a very useful tool. Switchable ground lift, bypass variac switch. But never a problem with inrush.

 

[trobbins]

Using an NTC likely introduces a performance issue. The NTC needs to cope with the full rated current of the variac - with some margin - so an 8A variac likely needs a NTC rated for 10A continuous. The issue is that the NTC won't be 'hot' unless mains current is above about 30% of the max rated NTC current (eg. above 3A), so below 3A the NTC may be presenting substantial resistance and hence sag or other regulation type issues may arise. Whether that effects the practical use of the variac is up to the user and the NTC used etc...

 

[Mr. Moscode]

Using an NTC likely introduces a performance issue. The NTC needs to cope with the full rated current of the variac - with some margin - so an 8A variac likely needs a NTC rated for 10A continuous. The issue is that the NTC won't be 'hot' unless mains current is above about 30% of the max rated NTC current (eg. above 3A), so below 3A the NTC may be presenting substantial resistance and hence sag or other regulation type issues may arise. Whether that effects the practical use of the variac is up to the user and the NTC used etc...

FWIW
. 5 ohm falls to .09 @ 4 amps and lower as the current goes up.See the spec sheet, attached.
The new number is a CL101 made/carried? by Amphenol. Good for 16 amps

Either it works or it doesn't but for the price $3.90 at Mouser or less if bought surplus or you PM me . . . even cheaper, if I can find it.
Simple. But hey, it's only a suggestion with a track record success.

Here's the spec sheet for these. Note the resistance v % of full current.
I assume the variac is for testing and repair use.

 

[nsalis]

"Nice but OWCH!!! £140 from RS!!! That's twice what the variac cost!!"

If you are confident in dealing with life-threatening voltages, you can also wire an inrush current limiter yourself. A proven product is the Camtec ESB101.16.

https://www.camtec-powersupplies.com/data-12/2/camtec-esb101-032022-00-0.pdf
or

https://tinyurl.com/3z49aezw

ns

 

[Brian Roth]

Bah Humbug (lol). A incandescent bulb in series is the time trusted solution. Yes, you can still find them if you look. Look for "incandescent yellow bug lamps"...although all I recently found were 60 Watt rated. I've also seen that clear 150W bulbs are still available....I have several in my basement (they glare like crazy, but I only need them when looking for a long-lost piece of luggage <g>).



Bri

 

[ccaudle]

A incandescent bulb in series is the time trusted solution. Yes, you can still find them if you look.

Maybe look a few aisles over at the hardware store. I am pretty sure that lights inside ovens will continue to be incandescent for quite a while longer, the temperature I use for browning my turkey at Thanksgiving is right at the peak reflow temperature I have used for lead free solder, so unlikely to be sticking LED designs inside my oven.

 

[Tubetec]

A simple incandesent bulb , its a current limiter/indicator in one ,
its a no-brainer ,off the shelf .
Peavey used standard over the counter car indicator lamps as tweeter protection ,
it acts as an indicator lamp via the box porting and a limiter to transient HF spikes very effectively ,
Id typically double or triple the rated input power of the speaker on the amps and drive away till the cows come home ,

Punters at the gigs often asked is there a ghost in the speaker ,

 

[mikeyB]

Yeah, I can get my hands on 60W but not 100W. Would 60W do the job??

 

[JohnRoberts]

Peavey used standard over the counter car indicator lamps as tweeter protection ,
it acts as an indicator lamp via the box porting and a limiter to transient HF spikes very effectively ,
Id typically double or triple the rated input power of the speaker on the amps and drive away till the cows come home ,

Punters at the gigs often asked is there a ghost in the speaker ,

Not to quibble about Peavey lore but I recall one improvement to the classic light bulb tweeter limiter that Peavey invented back several decades ago while I was still kicking around there. The improvement was to combine a PTC fuse device shorting across the typical light bulb in a series limiter topology. This approach reduced the distortion typical from light bulb limiters at modest levels when only partially turned on. Once power levels reach high enough to open up the PTC device, this lets the light bulb do its full limiting function. This topology delivered lower distortion in use, not a feature typically expected by the market coming from Peavey.

Sorry I don't remember what they called it, I'm sure they had a cute name for it.

JR

 

[Mr. Moscode]

A. If it ain't life threatening voltage where's the fun?

B. CL101, no timer, relay, or switch and is the size of a quarter, or 2 for 50 cents.

Aren't we overthinking this? I guess that's what we do to be worth our salt.

 

[Tubetec]

I never saw a passive Peavey with a dead tweeter ,
the bulbs can wear our or die from vibration after a while ,
The handy thing is any motorist worth their salt should have a set of spare bulbs in the glove compartment or emergency tool kit .
The PTC sounds like an interesting addition ,
There is noticable compression on the peaks and despite the rise in distortion its actually not a bad sounding compression in its own right on a single source , tweaking the bulb turn on curve with the PTC adds another possible sonic dimension , more a peak limiter effect ,
The Grampian reverb uses light bulb limiting ,

Could you get away with a passive line level light bulb limiter ?
a network of bulbs and PTC's that gives a few options to the comp/lim curve , obviously added make up gain as required,

I have a 6V bulb with a bypass switch on the speaker output of a small 5w tube amp I modified ,
I use it to drive reverb tanks , it stops the reverb getting swamped on the peaks , a kind of gating effect on the reverb tails ,
You end up getting a higher average output from the spring which is always good thing .

Its getting hard to find the old style incandesants here , halogen is the modern equivalent with better effeciency than the old style tungsten .

 

[mikeyB]

Found a 100W and a 60W bulb, tried each in series with the variac. Success!! - no longer trips the breaker. Waiting on bits to rebuild the variac with a few added features, will post details later for anyone interested.

 

[audioteslallc]

Precharging a variac or power transformer to reduce / eliminate excessive inrush current

The following might help if you are experiencing these issues.

Here's what I believe causes the inrush current and how I like to target the problem. The circuit being switched on has it's static magnetic "particles", if you will, scattered randomly instead of perfectly lined-up as in normal operation. If the particles are sitting the "wrong way" during start-up, they would first need to be turned around so they can "lock-in " with the rest of the gang. During this transition from random to lined-up the current could easily exceed 8 times the full load current of the winding. So if you have an 8 amp variac for example, you might see over 60 amps of inrush current. This large current would likely exceed the 20 amp breaker you're plugged into. Huge third harmonic content created during this transition doesn't help maters either. So bammm, your breaker trips or the fuse blows.

Here are three possible solutions, some practical, others not so much:

Zero Crossing Detector. As the term suggests, closing the switch when the voltage applied is at precisely zero crossing would be ideal. This concept is used in some high powered, high voltage circuit breakers. It's expensive to apply, requires reference voltage and knowledge of "how fast" the contacts need to close to be properly implemented. Too much for the DIY guy.

Input Current Limiting Resistance. Great for small inductive loads. Examples would include large wattage power resistors, incandescent light bulbs, etc, Downfall is if there was instantly full load applied and the resistors are not removed from the circuit in a "reasonable" amount of time, the resistor might catch on fire due to excessive power dissipation. This method is quite popular and often used---In my opinion it works, but it's not the best option.

Magnetic Precharging. As the name implies, the "scattered magnetic particles" are first lined-up at a low level of current before the main contact is closed. I do this with large variacs and plate transformers that are troublesome contact burning, breaker tripping SOB's. I use a run capacitor (the kind used in refrigeration A/C units) to effectively "precharge" the iron core. Use the same circuit as in example above, but change the resistor out in favor of a run capacitor. This series capacitance is usually dropped out with a bypass contact within a second or so. The advantage of this compared to using a resistor is the capacitor doesn't get hot yet produces the desired result of energizing the variac without nuisance breaker tripping.

Conclusion: Replace the resistor in the second example with a run capacitor and you're all set.

 

[trobbins]

Fourth possible solution: identify the cb that is tripping, and determine if there is a swapout opportunity with a cb that has the same thermal rating (eg. 10A), but a higher magnetic rating (eg. 10-15x the thermal rating D curve, rather than standard C curve).

 

[mikeyB]

Thank you for your reply guys. The 60W/100W bulb does the trick. I'm just rebuilding the variac - just waiting on some parts. Got a "traffic light" switch on sequence to ensure correct operation. Red neon indicates power into unit, then 1st switch energises the variac through the lamp, this also illuminates a yellow neon. A 2nd switch then shorts across the bulb and another set of contacts connect the load, also illuminating a Green neon. Thus the load is only connected when the bulb is shorted across. Just working out if i can use a 2 pole or if i need a 3 pole for the 2nd switch.

 

[mikeyB]

Finally got around to re-bulding my variac!! Bit of a beast, but I do also test/repair PA power amps as well. Cheap current meter not accurate at low readings, I'll probably get a couple of different ranges (1A and 300mA) and mount in external boxes, for the more sensitive testing!!
Used it a few times now and providing I switch on in the right order, I haven't tripped the breaker at all.
(Video shows the soak bulb doing it's job!!)

Apologies to moderators if the video is too large - delete if necessary!!

View attachment Variac.mp4

Schematic.

 

[trobbins]

Are we assuming the variac has a 10A output rating?

I locate my cheap digital mains power meter before the variac and use it to identify current used by the variac (typically about 50mA for my 240V 300VA variac) and the test equipment. Compared to placing the mains power meter after the variac, the meter operates at a nominal AC voltage and hence provides acceptable current measurement accuracy. To me the variac dial indication of AC voltage is sufficient for benchwork, and final testing would not include the variac, so actual mains voltage to the equipment under test can be measured and noted.

 

[mikeyB]

It's rated at 8A, just using the kettle as a to hand load. I wouldn't push it above 8 amps for any length of time (I'd be roasting in the workspace if i did!!) The dial is "0 to 100" (% effectively), so rather than doing the conversion I'd sooner have a direct voltage reading, bearing in mind the fluctuation in mains voltage, it's usually a constant 246Vac but I have seen it dip below 230Vac on a few occasions. With the 300V meter I can set the output voltage to within 2V of the reading, this has been checked against my Fluke DMM.
I want current reading drawn by the load only. As you can see, I like having the little 20W pygmy orange (or red) bulb as a work area indicator that there's something connected to the variac, particularly as the lamp gets brighter with higher voltage and I'm aware of it without a direct line of sight, I know to be cautious! I have the option of connecting a "remote" bulb as well for better sighting, but I don't want this "indicator" current drawn to be read in the variac output measurement.
The power up "traffic light" procedure works for me as a reminder to power up in the right order. Believe me, it's a right royal PITA to have to go to the main consumer unit if the variac trips the mains and as I've said, with the variac as it is now, I have not tripped the mains once and I bet I've switched it about 40-50 times now!! Happy man with variac now!