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NTC inrush current limiter in mono block amplifiers

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#21
Diabolical Artificer said:
On a pair of monoblocks I made I used a thermistor for a few reasons 1) to reduce the fuse value and 2) as protection not for the amp but to stop any tripping of RCD's etc. The amps had big toroids, high capacitance (1100u) and series OP valve heaters.


I used the "bung in the biggest round black thing wot I've got" approach.

Andy.
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I guess when someone designs an amp or modifies an amp to be just under the edge of failure during normal operating conditions then, ya gotta do what ya do to keep the wheels on. I dislike being afraid of flipping the ON switch. But then others like to jump off of buildings with a parachute and hope it opens just above the ground. They... hope... a downdraft doesn't come along during the descent. Perhaps the OP's amps were designed on the edge with limited PT windings load handling on the cold starts..? Too small primary capacity or heater winding capacity... But if you want to go to the moon, ya gotta do what ya gotta do, right?
 
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#22
20to20 said:
I guess when someone designs an amp or modifies an amp to be just under the edge of failure during normal operating conditions then, ya gotta do what ya do to keep the wheels on. I dislike being afraid of flipping the ON switch. But then others like to jump off of buildings with a parachute and hope it opens just above the ground. They... hope... a downdraft doesn't come along during the descent. Perhaps the OP's amps were designed on the edge with limited PT windings load handling on the cold starts..? Too small primary capacity or heater winding capacity... But if you want to go to the moon, ya gotta do what ya gotta do, right?
Click to expand...
I see, no answer to any question at all...
 
#23
voltwide said:
I see, no answer to any question at all...
Click to expand...

Going back to the OP's post? I think the answer for him was ...neither one. They are likely both too small. If the primary fuse is 10A then the load was probably 7-8A constant and maybe 20A during heater starts. So he really needs a 1.5x operating load rated thermistor for reliability long term. And full ventillation for it.
 
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