Inrush current limiting

[GainFreak]

Hi,

Usually NTCs are used for this purpose but after some reading I found out that there are some controversial opinions on that subject. According to some NTCs are useless in amps that don't have constant current draw and most guitar amps fall into this category. Some even claim that under certain circumstances they may explode before the fuse blows. In the same time most big companies are using NTCs in their amps. Some of them a lot:

http://s1028.photobucket.com/albums/...t=DSC02314.jpg

So is it a good idea to use them or not and what to look for when choosing one ?

[Steve Conner]

If you're not restricted by budget, a proper soft-start with a switched-out resistor is better. NTCs are a cost cutting measure.

One good compromise is a NTC that gets shorted by a relay once the startup sequence is complete. If there's something wrong with the amp, the NTC will blow the mains fuse, whereas a soft-start resistor would just burn out.

[GainFreak]

Thanks for your quick reply.
I'm not restricted by budget and adding a delayed relay line is not a problem but from user's point of view it's easier and maybe safer to replace a blown fuse than a burn out resistor which can do additional damage on the PCB?
Any quick rule how many Ohms the NTC should be for a particular amp/power supply?

[Enzo]

Well, it is always prudent to be cautious when folks that slinging words around like "most."

"Most" guitar amps are little low power solid state ones. True enough. Your is a tube amp, and will be drawing considerable power just at idle. Watch the solder on those Bugera thermistors.

My opinion? Not worth the effort.

[Enzo]

Aha, simulpost.

If you are fixing this one, all the more reason to look at the solder. I have yet to see a bad one, but I have to resolder them on just about every Bugera I see.

[GainFreak]

Well, it is always prudent to be cautious when folks that slinging words around like "most."

True.

If you are fixing this one, all the more reason to look at the solder.

I'm not fixing the amp on the pictures. They are from another forum.
I'm more interested if it is worth for new builds just to be on the safe side? I was thinking ff the big companies are implementing it there must be a reason for that.

[defaced]

I'm not restricted by budget and adding a delayed relay line is not a problem but from user's point of view it's easier and maybe safer to replace a blown fuse than a burn out resistor which can do additional damage on the PCB?

A delayed relay isn't needed. Simply putting a 100k, 3w to 5w in series with the primary power supply caps that gets switched out when the amp goes off standby work quite well - check the archives, I've posted a schematic of how I have it in my amp and there's been some discussion on this before. If the resistor is sized properly, the likely hood of you ever cleaning it off of a board is small.

[GainFreak]

Simply putting a 100k, 3w to 5w in series with the primary power supply caps that gets switched out when the amp goes off standby work quite well

I didn't quite get that. Did you mean the HV secondary?

[defaced]

That's correct. I meant the first filter caps when I said "primary", not as in the primary of the transformer. I put mine between the rectifier and the first filter cap.

[Enzo]

Keep in mind that big companies face a lot of pressure we little people do not. The fact that Bugera stuffs all those inrush limiters in that amp may have a lot more to do with meeting diverse international regulations than anything real. Connor can speak to it better than I, but these days power supply makers have to worry about power factor correction. It is not enough that your new thing work right, it must also not make undue starting surges. Leo Fender wouldn't have cared about it. So point is that when you see it in an amp, chances are it is more about outside the amp than inside it.

[GainFreak]

The fact that Bugera stuffs all those inrush limiters in that amp may have a lot more to do with meeting diverse international regulations than anything real.

I'm pretty much sure this is the reason for that. I've seen NTCs in other amps just not so many. Usually one or two should be enough.

[kg]

the only time i've had to include inrush current limiting is in the main B+ of the baga. i had to because i kept blowing 15a circuit breakers!

then again that's 620vdc into 1,000uF through a ~70r winding.

[Enzo]

Yeah, well this Bugera thing is just a 100 watt tube amp, might as well be a Marshall.