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A capacitor, or not

 
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A capacitor, or not

Post by crackle » Tue Oct 13, 2015 4:09 pm

Hi
Does anybody recognise this small component from the markings on it.
IMG_4618s.jpg
IMG_4618s.jpg (55.26 KiB) Viewed 1137 times

I thought it was a capacitor, but it measures 2.5 ohms.

I have drawn out the circuit of the Peavey ST15 cross-over unit the item is marked x050 in circuit.
If it is a capacitor, what is its value and what function does it perform across the 10 ohm resistor.
Is it just a bypass for very high frequencies to pass to the speaker.
Image1.jpg



Thanks
Mike

 
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Re: A capacitor, or not

Post by Amrad » Tue Oct 13, 2015 4:32 pm

Hello Mike,

It's a self restoring, resettable fuse!

Regards,

Dave.

 
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Re: A capacitor, or not

Post by crackle » Tue Oct 13, 2015 4:42 pm

Oh, I would never have thought of that, how does that work in the circuit, what is its function.

Is it normally short (it measures 2.5ohms) but if the current rises to a predetermined value the PPTC will open and the full current will flow only through the 10 ohm resistor ????

I have found these specs. from http://www.fujutec.com/pdf/20130101.pdf
resettable fuses.jpg

I guess that answers my question. :bba


Thanks
Mike

 
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Re: A capacitor, or not

Post by Cathovisor » Tue Oct 13, 2015 5:36 pm

It's to protect the tweeter under high power drive conditions.

 
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Re: A capacitor, or not

Post by crackle » Tue Oct 13, 2015 5:59 pm

It didn't do its job very well!! Sam had a bout of uncontrolled feedback and the tweeter blew..
I wonder, the PTC was glued to the circuit board with hot melt glue, would it not be better if it was in free air. The copper circuit board could have been acting like a heat sink.

The winding of the tweeter speech coil is a flat conductor, it measures 0.02mm x 0.5mm, which if I am correct is equivalent to .02mm which as a fuse, is at a pure guess, probably about .1 amp, times 25 as it is 25 times thicker in the other dimension. This makes the speech coil wire able to carry 2.5 amps....
Have I worked it out right??

Mike

 
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Location: Basildon

Re: A capacitor, or not

Post by crackle » Mon Oct 19, 2015 6:55 pm

Hi
I have another question about the PPTC.
The PPTC is rated at 0.5A and 60 volts. The amplifier is in theory capable of giving in excess of 40V RMS on the output into 8 ohms. What would happen if the peak volts across the PPTC when it was open were, say, 70 volts or more? What would the PPTC do, fail short circuit or open circuit, or something else.

Thanks
Mike


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