RF generally has measurable things: power, noise figure, impedances. Audio tends to be more arbitrary, as in "I like how that sounds."
I think sine waves are boring, at both ends of the spectrum.
Just about all film resistors 10 ohms and below are nickel film.
These fuse rather predictively to an energy value in excess of their ratings. Like fuses, they can provide protection from single-fault abnormals, reducing ancilliary component and trace damage. There need be no dynamic rupturing, as in other film types, but in some cases (eg HV fet gate drive) unavoidable.
Combined with low voltage, low power zeners, reliable surge protection can also be developed.
It would have served better use in the line to the regulator and off-board contacts, in your schematic, but that's designer/bean counter prerogative.
RL
Check out Diodes' AP22652 current limiter chip. Very slick.
We tried a TI E-fuse thing and it worked like a real fuse, just once, except that it failed shorted.
A wire bond might be a good fuse. Imagine an IC that had a couple of input pins and a bunch of outputs and just wire bonds inside.
Then it's not a fuse.
Sometimes i use very thin wire as fuse.
Why does that not surprise me at all?
A fuse is just a thin wire in a safe place. As long as I know the wire is safe from burning other thing, it's as good as a fuse.
Phil nailed it. At power on, the 2200 uf is a dead short circuit to DC. At that instant it will draw (or try to draw) infinite current from the 13.8 V supply. Of course the supply has internal resistance, and connectors and circuit traces/wiring adds more, but you still get a huge draw. Exactly as Phil said, that resistor limits the peak current. As the 2200uf charges, the current draw reduces and limiting peak current in that circuit is no longer important - until the next time the transceiver it powered off long enough for the 2200uf to fully discharge. After that, at the next power on, the resistor once again is important as a peak limiter.
That peak limiting action stresses the resistor every time the radio is turned on (unless it was turned off for only a brief time). Over the course of ~35 years it is not remarkable that it failed. That is exactly what Phil indicated. And he also mentioned the fact that it (in conjunction with the caps) provides a measure of noise filtering.
The thread may be a source of confusion. It may be best if you re-read, and stick with, Phil's reply.
Ed
Filtering for the amp from engine noise. It says 13.8V so assuming a vehicle powering it.
The resistor probably blew from inrush power too many times to charge that 2200uF cap.
boB
If the 1 ohm is the main inrush limiter, it’ll absorb the same amount of energy as it would in the case of an input short, i. e. 1/2 CV**2.
That’s 14V **2 * .0022F ~= 0.4 J, which is a lot for a film resistor of that size—like 100x the usual rating.
Cheers
Phil Hobbs
It's a 4W amp. I expect that 1R is only a fraction of the source impedance, and that the cap ESR is also a good fraction of that
1R figure.Anyways, until a complete repair is achieved, the single point fault is all just speculation.
RL
Excellent point. I overlooked how big that cap and how it must 'be seen' by the in-rush current at switch-on until you mentioned that. Many thanks.
And you continue to prove me right!
Which is the point exactly. The 1 ohm 1/4 watt film resistor doesn’t do anything much about inrush, because it can’t.
Cheers
Phil Hobbs
I did a quick LTspice simulation of this and for that first millisecond, the 1 Ohm resistor dissipates and average of about 75 watts which is certainly possible for a 1/4 watt SMT resistor but probably right on the edge of specifications for a pulse rated part at
1 msIt is very possible this is why it opened up if it did that a lot and was maybe already hot.
boB
All you can see is the paint job on the top. If you could strip off the paint, you could get information that could give you a clue. Carbon film is black. metal film is shiny, and metal oxide tends to have coarser spiral cut than either.
Which Phil? THere are 2 on this thread and they each say totally differnt things. Phil A says the resistor *does* limiit inrush current and Phil H says it does essentially *nothing* to limit inrush. I know who *my* money's on (hint: not the guy who thinks RF and audio are the same thing) . :)
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** Huh ????? Attempted humour ? ...... PhilOn a sunny day (Mon, 14 Aug 2023 09:35:06 +0100) it happened Dan Green snipped-for-privacy@hotmail.se wrote in snipped-for-privacy@4ax.com:
If RF and audio are the same, in defense of Phil A: they are closely interwoven in commie-nukation even communication :-)
Bats for example use frequencies that, according to wikipedia "Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air. Many bat detectors are limited to around 15 kHz to 125 kHz at best. Bat detectors are available commercially and also can be self-built."
So it is hard to draw a border line around what 'audio' is. If all you ever worked with is a LF audio amp..
I have build some nice ultrasonic systems too, 44 kHz
One uses the air, the other EM waves, circuits are the same, so do the laws of fishsicks. Apart from Albert E, but Jeroen knows that.
I remember my first FM transmitter, it was a dynamic microphone element in series with the 9 V battery supply to a single Ge transistor oscillator causing Vce changes that caused Cce changes that caused frequency changes so much you could hear a clock tick over the radio. So all electrickety after the sound was transferred to motion by the voice coil moving in a magnet... Audio over radio is cool.
The fuses in British plugs are slow blow and frequently take up to 6 minutes to blow in an over-current event (excepting short-circuits and the like). Sorry, Bill, R30 won't protect the plug fuse and if I may say so, it's a very strange claim to make coming from such an expert as yourself.
Actually the guy who work in audio, and know how often RF issues come up in studio work, versus the guy who is a physicist.
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