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
Last edited:
This happens here too !
Use a self excited relay so that power is not reapplied before YOU decide
Yves.
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
Last edited:
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!)
You're welcome . . . this is very common in factories . . .
Figure a lathe who stops and restarts with the cutting tool inside . . .
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
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
Attachments
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
Last edited:
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
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
Last edited:
Take precautions with these inrush current limiters, especially if you're experimenting. They explode suddenly like firecrackers throwing pieces around, and let out a terrible stink and smoke...
I had a CL-80 blow for no known reason. I replaced it and it's been fine for a year. Lesson is that I use goggles when I'm working with an open chassis that has an inrush current limiter that's exposed.
I had a CL-80 blow for no known reason. I replaced it and it's been fine for a year. Lesson is that I use goggles when I'm working with an open chassis that has an inrush current limiter that's exposed.
A few years ago many people were recommending the same part # for various applications - CL-90 I think it was - from Thermometrics. - but specifying a NTC Thermistor should be done by calculation. Otherwise it's like saying "I wear size nine shoes, you should get some."
These guys make good ones and have a good website. Their engineer has helped me on the phone a few times and taught me some good stuff in the process.
These guys make good ones and have a good website. Their engineer has helped me on the phone a few times and taught me some good stuff in the process.
Bravo Hearingspace - First sensible post I've ever seen on Inrush Current Limiters on any of the forums I follow.
NTC Inrush Current Limiting Thermistors are NOT a case of one size fits all but must be selected on average steady state current draw.
When cold they have high resistance which suppresses the inrush current. When hot they have negligable resistance. If not specifed correctly they either don't get hot enough to reduce their resistance enough or they fail from getting way too hot.
There will always be one particular part which suits the particular application.
Here is another application note:
http://www.epcos.com/web/generator/...operty=Data__en.pdf;/PDF_Applicationnotes.pdf
I've used them a lot in the day job to supress peak currents at switch on in aircraft.
If (when the correct device is chosen) they still don't give enough inrush current suppressiion then I've used 2 (of the same rating) in series.
You can NOT use them in parallel. One device will heat up faster than the other and then hog all the current (till it expires and then the other will take over and expire in sympathy).
Cheers,
Ian
NTC Inrush Current Limiting Thermistors are NOT a case of one size fits all but must be selected on average steady state current draw.
When cold they have high resistance which suppresses the inrush current. When hot they have negligable resistance. If not specifed correctly they either don't get hot enough to reduce their resistance enough or they fail from getting way too hot.
There will always be one particular part which suits the particular application.
Here is another application note:
http://www.epcos.com/web/generator/...operty=Data__en.pdf;/PDF_Applicationnotes.pdf
I've used them a lot in the day job to supress peak currents at switch on in aircraft.
If (when the correct device is chosen) they still don't give enough inrush current suppressiion then I've used 2 (of the same rating) in series.
You can NOT use them in parallel. One device will heat up faster than the other and then hog all the current (till it expires and then the other will take over and expire in sympathy).
Cheers,
Ian
That's the reason to use equipment with a metal chassis in a wooden house like mine. MOV's may be UL registered but a wild gash can set alight to the dust and this is exactly what happened in another case where I had to waste a costly CO2 extinguisher putting out the fire.. I avoid these devices like the plague.
richy
This happens here too !
Use a self excited relay so that power is not reapplied before YOU decide
Yves.
Hi,
You got a point there, but what do you think about this circuit:
https://picasaweb.google.com/vbkolsen/HiFiAlbumForTVA1TVA100#5668827848470641250
...would like to try this on my TVA-1 on: TUBEAMP The original inrush limiter is burned.
regards
I've been debating this for my tube bass guitar amp too, which already has a 'standby' switch.
I'd like to use a make-then-break relay. One set of normally-open contacts would short across the NTC thermistors on the B+ when the relay is activated. That would make it cool in normal use, and always ready to limit inrush and have decent protection as long as the drain resistors drain down the filter caps at a slower rate than the cooling of the thermistors. Another contact set would connect the B+ supply to the amplifier load, like the standby switch origianlly did. The make-then-break relay would be wired to self-latch then disconnect the closed toggle switch so I would not have to use a momentary switch or pushbutton so that I would not change the appearance or basic operation of the amp and could keep the original stock toggle. But a dangerous power interrution and restoration would break open the self-latch and cause it to revert to standby mode with the thermistor back in-circuit; the user would have to turn the standby toggle switch off then on again to re-latch and play.
I'd like to use a make-then-break relay. One set of normally-open contacts would short across the NTC thermistors on the B+ when the relay is activated. That would make it cool in normal use, and always ready to limit inrush and have decent protection as long as the drain resistors drain down the filter caps at a slower rate than the cooling of the thermistors. Another contact set would connect the B+ supply to the amplifier load, like the standby switch origianlly did. The make-then-break relay would be wired to self-latch then disconnect the closed toggle switch so I would not have to use a momentary switch or pushbutton so that I would not change the appearance or basic operation of the amp and could keep the original stock toggle. But a dangerous power interrution and restoration would break open the self-latch and cause it to revert to standby mode with the thermistor back in-circuit; the user would have to turn the standby toggle switch off then on again to re-latch and play.
Last edited:
Yes ..understand...to avoid the constant heat of the inrush limiter by use of a high voltage relay across the limiter.
If you can accept a small change of the bass amp function?:
If we look at the relay as automatical B+ protection guard, what should deactivate the relay, and let the inrush limiter take over.....too much voltage (mA) across the cathode resistor of the output tubes? ....say ...200mA before break?
Do you have the limiter partnumber/name?
This should be possible to calculate.
In the circuit I use for transistor DC amplifiers the threshold value is +-8Vdc. I shall ask my genius friend tonight of a simple circuit that takes care of the issue....then you only need one on-off contact switch for your bass guitar amp and one red/green double led to tell you go or no-go.
I have thought about this circuit for the TVA-1 KT88 amp, which at the same time tells you something about the remaining lifetime of your tubes...older tubes "runs" with the bias.
.....just a thought