• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Thermistor protection during brownouts

I understand closing the power switch on a toroid power source may destroy the switch if the switch happens to be closed at the zero crossing of the line voltage. A simple solution is adding an NTC thermistor in series with the switch. Its high resistance when cold will limit the inrush current and spare the switch (and provide a crude soft start for the rest of the circuit). This resistance drops to near zero within minutes. Perfect ..unless the amp is turned off and back on before the thermistor has a chance to cool. Its fine if the user is aware of this, but suppose a brownout occurs. I like the thermistor idea and its soft startup. I know a relay could be incorporated, but i like the soft startup of the thermistor. I suppose a Uninterruptible Power Supply (UPS) would solve this. Thoughts on anyone who has had experience with this?
 
The problem is not so much burning the contacts of (properly designed...) switches, but to make the circuit breaker trip with this high current pulse. Usual 230V household breakers of 10 or 16A start tripping more or less often from about 400VA transformer power on, as far as they don't have a specially high pulse current tolerance ("C-characteristic"). I always take thermistors or pre-resistors bridged by relays as soft-start for my transformers.
 
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I had a problem with my non-vacuum-tube power amplifier's inrush current at startup. After roughly thirty switch-on events, the mains fuse would pop. A slow-blow / time-delay mains fuse. So I decided to add an inrush current limiter disc in series with the transformer primary. (Here's a Forum thread about my experiences).

Many months later, on a different non-vacuum-tube power amp project, I fell in love with on/off switches that are NOT rated to handle 115VAC or 230VAC or more than 0.2 amperes of current through the switch contacts. So I whipped together a PCB design that uses the low-voltage, low-current front panel switch, to activate and deactivate the coil of a high power relay. The relay's contacts switch mains power on and off to the power amp. And I figured, as long as I have a relay and a PCB anyway, I might as well add a big honking inrush current limiter disc too, to protect the mains fuse and to implement a gradual soft start. The (Forum thread about this H9KPXG board), includes scope photos of mains current during startup.

From time to time, various Forum members occasionally sell finished, assembled boards, for example (link 3), but those boards seem to disappear quickly. PCB manufacturing files ("Gerbers") are freely downloadable, so anyone can DIY one or more pcbs if they are so inclined.

_
 

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The problem is not so much burning the contacts of (properly designed...) switches, but to make the circuit breaker trip with this high current pulse. Usual 230V household breakers of 10 or 16A start tripping more or less often from about 400VA transformer power on, as far as they don't have a specially high pulse current tolerance ("C-characteristic"). I always take thermistors or pre-resistors bridged by relays as soft-start for my transformers.
The typical MCB in our residential installations are 16 A of the B tripping characteristic (B16). The C16 that you mention is more tolerant of current pulses than the B16 (I/In of 5-10 x instead of 3-5 x).
I have a single phase 230 V transformer type (as opposed to inverter) MIG welder that can output 150 A and will trip a B16 easily at the start of a weld but not a C16.

For a Marantz PM-7200KI that my brother owns I made a soft start as the Talema (~250-300 VA) toroidal transformer would trip B16 MCBs on power on. A thing I don't think it should do, but it does. The soft start cured the problem. The switch never got damaged in the years he used the amp without the soft start (in a house with fuses instead of MCBs).

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The zero-crossing of the AC is a red herring, Doesn't matter. Thermistors are good but if you have a timed relay bypass the thermistor after the first 10 seconds when the rush event has past the thermistor can cool off and be ready for the next event. See my thread of Yet Another Softstart. You will see I also like to have the switch only turn on a low voltage to the softstart circuit which handles the big load.
 
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I understand closing the power switch on a toroid power source may destroy the switch at the caps
As always, depends on the circuit & what it is meant to do.
So I will bore everyone again with one of my craziest experiences.

In this case I'd built a variable DC PS of 4.5 KV, One Ampere to power up a Phillips 2.5 KWatt Magnetron.
Most all of it was vacuum tube technology including the error amp & output driver.

The PT was about 2 cu ft of Hammond iron & copper, the rectifier a full bridge of 7136's.
It was choke input to 8 micro F, so ~6KV with no load at the caps.

I had run it up several times, all seemed OK. But at shut down after some success I got a very large bang.
My Glorious Leader, Prof Anderson had found a very effective SYA (Save Your A$$) setup that we had included.
That was a short across the caps powered by gravity. lifted up by a solenoid while the equipment was running.
The contacts on the DP relay I'd used to switch the 230V supply had welded shut. So fuses blew.

Later I measured the primary inrush on that circuit, it was 450A. The fix was a simple stepped circuit,
an extra relay to start running thru four of One R, 25W WW resisters, followed six cycles later by the full
230VAC source.

All circa 1963, on the way to a PEng. And No more explosion! At that phase of my life:D!
 
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I understand closing the power switch on a toroid power source may destroy the switch
A properly sized switch should have no such problem.
Yes, there are switches designed specifically for this issue - called high in-rush current switches. See attached data sheet for some from Digi-Key.
The issue is not unique to audio. It exists with lots of equipment that use motors as well.
As for the NTC in an audio amp, it should have no problem with a brown-out since a big part of the in-rush is from charging the filter caps and not just the torroid.
During a brown-out the filter caps will remain mostly charged so the recovery surge will be much lower than the cold start in-rush current.
Keep in mind that the NTC must be properly sized as well. There are many to choose from.
 

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Carlyboy, you refer to a 'brownout', which is a drop or sag in voltage, so not a significant contributor to in-rush current. Did you mean 'blackout' ?

What experience have you had with 'brownouts' such as fluctuations in incandescent bulb intensity. A deep sag may cause a switchmode power supply in say a TV or PC to reboot if the mains voltage gets suitably low and the holdup time of the equipment is not very good.

Different electricity distribution schemes can cause short blackouts for a few seconds, such as with auto-reclose breakers that trip from lightning, and then try to reconnect to see if the short circuit has cleared. The lights certainly go out in that scenario.
 
I use non-latching GFCI for much of my equipment. This requires me to actually get out of my chair and decide when to re-energize the circuit or piece of equipment plugged into it. They trip at the slightest flicker and don't re=energize without action on my part.
Here's one example: https://www.legrand.us/wiring-devic...-with-1-foot-cord-set-manual-reset/p/1597cs1m
A GFCI, or RCD as I know it as, should not trip because of voltage dips. It should only trip if a current imbalance occurs between L and N of more than the specified value.
 
That seems overly sensitive to me.

The typical RCD over here is rated to trip before a 30 mA current imbalance between L and N is reached. They are installed in the distribution board protecting up to 4 circuits per RCD. There are also MCB+RCD combinations in a single device called RCBO. But they too are rated at 30 mA tripping current for the RCD part of the RCBO. Basically the whole house is RCD protected.

Your GFCI seems to trip at just 4-5 mA of imbalance, but am I correct in assuming you only use them in/on sockets?
 
I plug my power strips or the individual pieces of equipment into them. I have 5 total. They work great at protecting my equipment from hot-restarts which is what I want. Not overly sensitive in my situation. Never have they tripped for a fault in my equipment. Well, the one on my bench main power strip has plenty of times during new builds or experimentation. How do you know that mine trip at 4-5mA imbalance?
Just like THIS