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RTC = Fuse?

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ftransform:
So I am troubleshooting a PCB and it has input protection I do not understand. On the live wire it has 3 amp fast blow glass fuse. Easy enough. However on the return it has a component (which is now disintegrated) which looks like a little ceramic disk capacitor.

The PCB has the word RTC written and the picture shows a resistor with a oval drawn around it. The connection from the return to the circuit is sent through this component, so its natural state is short circuit.

What is this thing? A thermal fuse?
Why would you fuse the return as well as the live?

I can't find any explanation on google.

PA0PBZ:
RTC is real time clock, everyone knows that!  :P
From your description it smells like a NTC, but why it is in the return... no idea.

ftransform:
Nmv, it is on hot not on return, the slow blow fuse is on return.

Anyway, should the NTC have the same value as the fast blow fuse? How should I select a replacement?
Why is it even there?

Is the NTC shielding the fast blow fuse?

PA0PBZ:

--- Quote ---Anyway, should the NTC have the same value as the fast blow fuse? How should I select a replacement?

--- End quote ---

Since a fuse has its value in Amps and how fast it blows, and NTC's have a resistive value at 25C you can't say that!


--- Quote ---Why is it even there?
Is the NTC shielding the fast blow fuse?

--- End quote ---

It is probably there to limit the inrush current, it has a (relative) high value when at room temperature, then it heats up from the current and the value goes low.
Think of what happens when you put AC on a diode bridge with a big capacitor at the output. At first the cap is empty so there will be a lot of current flowing into it.
So yes, in a way it protects the fuse, but that is not the main reason why it is there I guess.

For the replacement... I don't know. No schematics I guess, and not another one to have a peek inside?

SeanB:
Grab one off a PC power supply and use it in the position. Normally they are a 5R resistor when cold, and drop to around 1R when hot. Probably you will find the diodes in the bridge have gone short circuit ( one or all 4) and on the other side of the bridge the semiconductors that are there are probably short circuit as well, possibly having blown the plastic off the top. Check capacitors then on primary and secondary for failure or high ESR, and replace as needed.

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