Inrush question

[stven]

I originally asked this question in the power supply forum but got no response, so asking again here. I obtained two GE Sensing SM CL-60 inrush limiting thermistors for a LM3886 chipamp project but am wondering whether it makes sense to use them and if so, exactly how.

Would you add them anywhere, and if so before the PS capacitors or after, or both?

thanks in advance,

Steve

[lgreen]

In my view there is no reason to use them in a LM3886 amp; even with big toroid and big caps. I've got one with 500VA and >40,000 uF and no fuse ever blew over the last few years.

So there is no reason to soft start these amps, and thus no reason to put in the CL-60s.

But there is still another reason not to use them- the chipamps will draw very little current until a dynamic passage, and then draw a lot of current. Your CL-60s will not have time to heat up and will inhibit the dynamics of these large passages because their in-line resistances will be too high.

So save them for a class A amp and feel good about using no protection on your chipamp.

[Ted205]

what rating is your power supply ? transformer VA and the the ammount of capacitance used?

i've used them on my bpa300 before the transformers with a bypass relay.

[AndrewT]

use one in the mains primary feed and bypass it with a relay to ensure it cools down for next start up and for the reasons Lgreen mentioned. The relay should have a 100ms to 300ms delay.
This soft start will allow you to use a lower rated fuse to start your amp without nuisance fuse blowing.

[tomchr]

A toroid - especially a big one followed by some hefty caps - will draw quite an in-rush current. It is actually common for the toroids to fail after 15-20 years of service because of issues stemming from the mechanical stresses caused by the in-rush current. Basically, the magnetic field set up by the current will try to straighten out the windings of the transformer. So every time you turn on your amp, the windings will basically rub against the corners of the core until the in-rush current drops. So after 15-20 years of service, it's common for the toroid transformers to short out and start blowing fuses.

As others have said, I suggest using an in-rush limiter on the primary side of the transformer. Short out the in-rush limiter with a relay once the power supply voltage reaches, say, 2/3 of the final voltage. Or trigger the relay after a fixed time delay (1 sec is probably reasonable).

~Tom

[pacificblue]

So if you don't use an in-rush limiter aka soft-start circuit, you have an excuse to build a new amplifier at least every 15-20 years. With soft-start you can forget about that hobby, because your amp will last forever.

[wwenze]

...only if you switch off your equipment though.

But I worry for my capacitors on the other hand.

[tinitus]

If the relatively small fuse doesnt blow, I dont see why there would be anything to worry about

[tomchr]

Depends on what you mean by "relatively small". There isn't really any reason the fuse should be more than about VA/Vmains. So approx a 2 A fuse for a 500 VA transformer on 230 V. I wouldn't mind using a 3 A fuse. But in my experience, it takes slow-blow fuses in the range of 6.3~10A to survive the in-rush.

If the transformer emits a loud growl and the lights in the room go dim when you turn on your amp, I would insert an in-rush limiter. It's better on the supply caps as well as on the transformer. And I always use an in-rush limiter on the filament transformer in a tube amp to protect the filaments. Remember the cold resistance of a filament is a good 10x lower than that of a hot filament. So on a cold start, the filament sees quite the in-rush current.

For the small transformers (say <100 VA), I wouldn't worry about in-rush limiters as primaries of smaller transformers tend to have higher DC resistance.

[tinitus]

Originally Posted by stven

I obtained two GE Sensing SM CL-60 inrush limiting thermistors for a LM3886 chipamp project but am wondering whether it makes sense to use them and if so, exactly how.

Tom, if the supply is big, ofcourse your right
I dont see any sense in using a big supply in connection with a small chip amp
A smaller 250VA supply might even sound better, and less in need of inrush limiter

Steve, regarding your thermistors, maybe this helps
http://www.firstwatt.com/downloads/F5-om_sm-080527.pdf
Looks funny, but its 2x 120V in paralel