470 Ohms instead of a 470 Ohm resistor?

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autodafe
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470 Ohms instead of a 470 Ohm resistor?

Post by autodafe » Fri Aug 10, 2018 1:55 am

I didn't have a properly sized SMD 470 Ohm resistor and placed a 470 Ohm ferrite pearl instead.
It looks like this has burnt the module (antumbra 6MIX, it was working before with the 470 Ohm resistor mounted "almost" vertically...)
is it possible? what's the difference between a resistor and a ferrite of the same value?
Or maybe I made some other

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Post by flts » Fri Aug 10, 2018 2:08 am

The ohms value of a ferrite bead is not the DC resistance, but the peak impedance / AC resistance, often specified at 100 MHz or similar high frequency. Which means that Ω value will matter very, very little in most cases where you'd use a resistor in audio electronics.

The DC resistance of a SMT ferrite bead is probably well under 1 ohm. So for all practical purposes, you've replaced a 470R resistor with <1R resistor. It's highly probable you've destroyed something else in the module by that part swap.

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Post by autodafe » Fri Aug 10, 2018 2:49 am

thanks. I am an idiot then. I read 470 Ohms and went straight away...

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Post by flts » Fri Aug 10, 2018 4:04 am

Yeah, happens to all of us! For once that's something I _didn't_ learn the hard way unlike a lot of other stuff in the hobby... :hihi:

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Post by cnicht » Fri Aug 10, 2018 4:39 am

Hopefully it’s only the ferrite bead that has burnt out acting like a fuse.

I think manufacturers of these components should be a bit more explicit in terms of the reactance of these devices emphasising the inductance first as this is fixed (mostly) whereas the reactance (expressed as the headline ‘resistance’) is not.

Ah the joys of AC analysis! :hmm:

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Post by autodafe » Fri Aug 10, 2018 5:05 am

cnicht wrote:Hopefully it’s only the ferrite bead that has burnt out acting like a fuse.
sadly not, I removed it and nothing works anymore...

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Post by autodafe » Fri Aug 10, 2018 5:13 am

these are the schematics:

http://antumbra.eu/content/bom/6mix/v1- ... matics.pdf

in the event R25 gets shorted, what should I check first to verify what could have been burnt ? (loks like I shorted +12 to GND, so it can be anything...I suppose???)

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Post by m.o » Fri Aug 10, 2018 5:45 am

D1 and/or R3 would have seen the full short circuit current, so check those.

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Post by autodafe » Fri Aug 10, 2018 8:45 am

m.o wrote:D1 and/or R3 would have seen the full short circuit current, so check those.
thanks a lot for the hint. Will check them first :-)

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Post by autodafe » Fri Aug 10, 2018 9:28 am

Thanks m.o :yay:

Following your suggestion I replaced Diode D1....no luck. Replaced R1 and Bingo!
The module now works, all channels work EXCEPT the LEFT button doesn't light up and doesn't seem to mute the channel...I can live without the mute, I will order a new pushbutton in my next Mouser cart...or maybe something else blew that prevents the button from working???


Image

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Post by mskala » Fri Aug 10, 2018 9:40 am

cnicht wrote:I think manufacturers of these components should be a bit more explicit in terms of the reactance of these devices emphasising the inductance first as this is fixed (mostly) whereas the reactance (expressed as the headline ‘resistance’) is not.

Ah the joys of AC analysis! :hmm:
Ferrite beads are better described as frequency-dependent resistors with some inductance on the side, not primarily inductors. At the frequencies where they're typically intended to be used, the resistive component, not the reactive component, dominates the impedance. Here's a typical plot out of a data sheet (from https://www.mouser.ca/ProductDetail/Fai ... %2f7yqw%3d):

Image

This bead is specified for 100MHz. The inductance is certainly not "fixed" - unlike that of an ideal inductor, the inductive reactance of the bead drops off at higher frequencies (kind of like a capacitor undergoing self-resonance) and the typical operating frequency is past that peak, in the range where the capacitance is starting to take over. Telling you the inductance it looks like at lower frequencies where it behaves as an inductor at all, would not accurately describe that. The resistive component is also poorly behaved, but is more relevant to the device in actual use. So the bottom line is that there's no trivial simplification that works and one has to actually pay attention to more details.

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Post by cnicht » Fri Aug 10, 2018 10:37 am

Thank you for the tutorial.

It’s really interesting to see how these devices behave and to know I’m getting an LCR network for my money! :sb:

I wonder how effective this component is in this application?

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Post by Synthiq » Fri Aug 10, 2018 11:15 am

autodafe wrote:The module now works, all channels work EXCEPT the LEFT button doesn't light up
If you blew up the 10ohm resistor due to high current, I bet you blew up the LED in the button as well and maybe also the switch contact controlling it but I must admit I didn't expect the other switch for the mute function to be affected.

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Post by synthetek » Fri Aug 10, 2018 11:39 pm

I suggested the same thing in your other thread :doh: checking the diodes and the 10R resistors. The 470R look like they are current limiting resistors for the LED in the switches so the led is probably blown now but your original problem with the led being on all the time and it not muting sounds like you might have installed the switch backwards.

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