Toroidal mains Tx inrush limiter - retrofit

dunno but talking to a philosopher last week for his take on occam's razor , he put me right in that it is a mistranslation of Occam's raison ... I run science talks locally and he will be talking on the philosophy of science, Nicomachean Ethics, Occam's whatever etc series details

In article , Stroonz writes

Given that he seems to be obsessed with ass, I suspect he probably hangs out in his local gay leather bar.

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Mike Tomlinson

In article , William Sommerwerck writes

That's a resettable *thermal* fuse, NOT a fuse designed for overcurrent protection.

--
Mike Tomlinson

In article , Gareth Magennis writes

Occam's beard?

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Mike Tomlinson

"Mike Tomlinson"

** What a BORING know nothing, f****it bullshit artist.

The LESS you know about a topic - the MORE you feel compelled to talk bollocks about it.

I call that sort of rampant narcissism: " pommy disease ".

Wot a shame it ain't fatal.

.... Phil

I had always thought that at least some polyfuses /were/ resettable. Regardless, thanks for the answer.

Nigel,

Also keep in mind as a possibility that the main filter caps are getting tired, and a bit leaky, so that if the amp has not been switched on for a week or 2, it might have abnormally high inrush current as the caps charge/form back up.

Lastly, the older 'spring wire' fuses seemed more tolerant of the inrush current compared to the modern chemically treated wire ones. Maybe use an old style fuse, if you can find one.

It would be helpful if you would mention the make and model of the product in your posts.

Regards, Tim Schwartz Bristol Electronics

Nice self-description.

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Live Fast Die Young, Leave A Pretty Corpse

Polyfuses (in the sense of the term I've always heard used) are self-resetting, unless you hit 'em so hard that you damage them.

They aren't designed for use at mains voltages/currents, though... at least, I haven't seen any that are.

The self-resetting thermal "fuses" that you'd find in a bulk eraser or similar "not intended for a 100% duty cycle - let it cool down!" device are (I think) of the bimetallic type, somewhat akin to what you would find in a traditional wall thermostat.

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Dave Platt                                    AE6EO
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"Tim Schwartz"

** This is rather fanciful thinking.

An inrush surge ( on a 240V supply) that will take out a T3.15 fuse has to be in the order of 40 to 50 amps. Toroidal transformers in the range of 220 to 300 VA with rectifiers and filter cap banks will provide such surges almost every time they are switched on.

** Correct, but hard to find them in other than 3AG size and they are much more expensive. ** Where did you hear this one ??

The ones I use are said to be made from "plated wire" - meaning a thick tin plating over copper. Some are coil shaped and some are not.

** Bloody oath it would.

What a colossal ASS Kook is for regularly leaving that crucial info out.

.... Phil

Me neither, other thing with polyfuses is that there's approx 2:1 current difference between trigger point and hold point. I have 16V and 30V poly- fuses here, generally for 12V or 24V SLA battery powered gear.

Some are the same thing as used in clothes dryer for thermostat, 100% duty cycle for years on end ;) Lower temperature for thermostat and a higher temp one for overheat safety cutout.

Grant.

"Grant"

** That is utterly misleading !!

There are no separate trigger and hold points with PolySwitches.

The makers specs allow for some device to device variation in the actual current level that will trigger a RANGE of similar devices to change state and go high resistance.

The max current that fails to trip all examples of a given type is called: " I Hold ".

The minimum current that will trip all examples of a given type is called: " I Trip".

" I Trip " is double " I Hold".

For any given device, the trip current is one number.

Once a device has tripped, the current needed to HOLD it in the new state is a function of the applied voltage and is generally 5 to 20 times less than the trip current.

.... Phil

This green thermistor limiter 10 deg C over ambient with no throughput and

20 deg C over with 0.4 amp mains with amp driving load.

In article , N_Cook writes

Bit warmer than I would be comfortable with.

--
Mike Tomlinson

"Mike Tomlinson" Nutcase Kook"

** So neither of you has a clue how NTC surge limiters are meant to work ?

..... Phil

and

Odd you say that. The next amp I looked at, this time mixer amp but also

300W and toroid Tx. Owner bought knowing 2 of the 8 channels were duff and used as such for some years. He now needs all 8 ch . Before he owned it , it must have been dropped and broke the awkward size/shape mains switch. So someone spade jumpered the leads and covered with heatshrink, leaving un-blanked off hole in chassis. Under his heatshrink and touching the ordinary mains wire sleeving is an inrush thermistor. Black , with 0.8mm leads, marked SG 130, 3.2R cold, 0.8R 20 sec of soldering iron barrel. Presumably originally fixed to the sw and unsupported other lead other than cable tie further down the cable loom. Hopefully originally there was some woven glass sleeving over the thermistor

data for SG130 RTI Surge Guard, 0.07 ohms at 7 amps max , 2.5R at 25 deg C

27 joules rating for a 15 x 6.3 mm black (body radiator?) lump and .8mm leads for anyone who can convert to temperature (not on the RTI datasheet)

It looks as though you use the de-rating curve

So for 7 amp NTC running outside of short duration peaks say an average 1/3 of 7 amps of mains 240V for driven amp and less than rated mains fuse then

125 degrees would be the operational maximum of the thermistor. Would be nice to know a rule of thumb formula for real life "black body" radiator/convector temperature and power consumption whether disc thermistors or rod resistors

Testing this SG 130 on a 12V SLA and headlamp, stabilised at 2.8 amp, .91V over it and 102 deg C (ambient 18 deg C), no woven glass sleeve covering