NTC THERMISTOR as SOFT START for TUBES

[M Gregg]

Originally Posted by richwalters

You'll have not much fun on a "brown out"......or serious intermittant interruption of the AC with a low thermistor hot resistance. Where I live there are alot of bad interrruptions and a thermistor wouldn't survive long.

richy

LOL,
yes I agree,

If you have power going off-on with a hot thermistor then it would probably take the mains fuse. However the MOV may save the circuit.

Best perhaps I should re-phrase to worst power interupt I have seen took out an 11KV substation killed 4 people-had been in there a few hours before.

Regards
M. Gregg

[M Gregg]

richy,

(OFF topic)

What causes the brown outs in your area, dirty insulators, lightning, overcurrent transient?

Just for fun, when are you going to tell us how to modify a computer PSU to power a tube circuit?

Regards
M. Gregg

[Yvesm]

Originally Posted by richwalters

You'll have not much fun on a "brown out"......or serious intermittant interruption of the AC with a low thermistor hot resistance. Where I live there are alot of bad interrruptions and a thermistor wouldn't survive long.

This happens here too !
Use a self excited relay so that power is not reapplied before YOU decide

Yves.

[M Gregg]

Originally Posted by Yvesm

This happens here too !
Use a self excited relay so that power is not reapplied before YOU decide

Yves.

Yves,

Great idea--the retentive relay on- off push button.

There you go with Yves idea make a box with a power relay and on/ off push button to plug in the system!

Yves <<<gold star award!

Regards
M. Gregg

[simon7000]

Originally Posted by trobbins

Using a MOV across the primary winding is probably a better way to manage the turn-off voltage stress on primary switchgear. I doubt the NTC bypass relay contacts will see a stress (as it is delayed) - the main amp ac mains switch contact will take the stress.

A CL60 is not much series resistance (10 ohm cold) - I would have thought a CL-70 or even CL-80 would be more appropriate, as your average max input current is not going to get above an amp or so. Even then, you may still see heater 'flash', as the heater turn-on characteristic is relatively slow peak decay compared to capacitor charging and transformer in-rush.

Ciao, Tim

A MOV across the transformer will limit a voltage rise, but that is not the case here. There is still a load on the secondary so there will not be an inductive voltage spike on turn off. There will still be a voltage from the current across the now high resistance mode inrush current limiter. If you are using a small relay it is good practice to place a small RATED capacitor across the relay. If the capacitor fails open, the relay life will be shortened. If it fails closed the current limiter will be bypassed and a fuse should blow. There is little danger of fire or other downsides. Sometimes adding a part improves things such as safety other times it can reduce reliability.

A MOV if it fails can catch fire. When used with an inrush current limiter it may not blow the fuse. The intent of using a voltage clamp is to protect the circuitry, particularly the insulation on the primary of the transformer. As a properly sized MOV does not provide voltage regulation only surge protection, they are often not useful in power amplifiers as the first filter capacitor will absorb much of the spike energy.

In industrial locations spikes of several thousand volts are seen on unprotected AC lines.

Around here the AC distribution to the pole mounted transformer runs around 13kv. The Hot wire is on top of the pole (some places use the neutral on top) that way if lighting hits the main distribution line it only affects one of the three Hot lines and is reduced before it gets inside. That is why when some lighting strikes cause damage it is often only to half of the stuff plugged in.

The idea behind the neutral on top is that the neutral is the grounded wire so the lightning is safely shunted. When that fails due to corrosion or over-current then everything gets blown up. Also a spike of only 100 volts on a neutral will fry much audio equipment's inputs. (Fixed quite a few of these!)

[Yvesm]

Originally Posted by M Gregg

Yves <<<gold star award!

You're welcome . . . this is very common in factories . . .
Figure a lathe who stops and restarts with the cutting tool inside . . .

[M Gregg]

Originally Posted by Yvesm

You're welcome . . . this is very common in factories . . .
Figure a lathe who stops and restarts with the cutting tool inside . . .

The direct on line starter -- as we know it. I must admit I had not thought of it applied to HIFI power out.
You have a way of thinking around the problems! LOL

Where is richy's computer / tube PSU? lol

Regards
M. Gregg

[richwalters]

Originally Posted by M Gregg

richy,

(OFF topic)

What causes the brown outs in your area, dirty insulators, lightning, overcurrent transient?

Just for fun, when are you going to tell us how to modify a computer PSU to power a tube circuit?

Regards
M. Gregg

see pic.
In bad conditions..one was a luck shot, the other long exposure just behind the trees 200m away but both made me jump and my hair stand on end, like butter down the spine..the streaks between the chalets is particulary nasty for sat TV's and anything connected to wall outlets ! There is no protection for this type intensity storm we get in the Alps and the highest point i.e a conductor doesn't offer protection. Mysterious it may seem, a uS change in magnetism on flash seems to determine where it strikes. Most cloud down.

richy

[M Gregg]

Originally Posted by richwalters

see pic.
In bad conditions..one was a luck shot, the other long exposure just behind the trees 200m away but both made me jump and my hair stand on end, like butter down the spine..the streaks between the chalets is particulary nasty for sat TV's and anything connected to wall outlets ! There is no protection for this type intensity storm we get in the Alps and the highest point i.e a conductor doesn't offer protection. Mysterious it may seem, a uS change in magnetism on flash seems to determine where it strikes. Most cloud down.

richy

That looks scary!

I would switch to battery power and inverter and keep away from the sockets if only for damage limitation. Maybe even a generator so i could disconnect from the mains supply.

After the storm switch back to mains supply.

Whoooo makes me shiver to look at it...how many lines men get zapped from a strike 100 miles away: OK its dead you can go up now....

Best wishes
M. Gregg

[trobbins]

The voltage across a warm/hot NTC is likely to be at most a few volts - and the relay contact shorts this after turnon. At turn-off - if using the incoming mains switch - the NTC shorting relay contact would open well after the event. As such, I don't see much stress on the relay contact at all (certainly nowhere near approaching its rated AC3 level), or need to add any form of RC contact snubber.

The primary-side stray inductance of larger PT's, added to the worst-case time in the cycle, can cause a primary side over-voltage transient - especially for loosly coupled secondaries such as HT windings, and in the off-time of all rectifier diodes where loading is only from heaters. Modern switchgear and transformers would typically not have a problem - however older gear may well notice it - I've tripped the earth leakage protection on an old 1950's power switch where the active has arced over to the metal case of the switch.

MOVs of small stature typically fail by cracking/blowing themselves open-circuit, or take out the fuse - probably with less impact than a failed NTC, but both devices require caution in placement as their cases are not to be taken as an insulator, and the NTC is designed to operate hot, and both devices could plausibly fail with high power dissipation where temperature causes burning.

Ciao, Tim