RTC = Fuse?

[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! 
From your description it smells like a NTC, but why it is in the return... no idea.

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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]

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

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!

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

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.

[Neilm]

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.

Are you sure is says RTC not PTC? PTCs are often used as re-setable fuses. They heat up when a small current flows and reach a steady state, but if a large current flows it heats up a lot. This heat increases the internal resistance causing it to get hotter. Eventually the resistance will be so high it goes open circuit and cuts off the current.

The main problem with them is ensuring that they go open circuit before they blow - in this case it sounds like they didn't get the sums right.

NTCs are usually used for limiting the start up current and they work in a similar but opposite way. Generally, I would expect to find them in the phase rather than the return.

Neil

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I'm not sure. I have a working unit but that one also suffers from the same exact problem! I shorted the leads out on that one but now since I have two I would like to buy parts. It is for a SMPSU that controls a peltier cooler.

I was not going to trouble shoot the circuit and just buy parts before I do anything with it but I suppose I may as well test some capacitors and diodes. The other one worked great when it was shorted. I suspect that these units failure was due to the environment they were in, which has particularly bad electrical wiring. These units are made with the bottom dollar let me tell you.

The last unit had its RTC/PTC/NTC disintegrate (but the fuse remained unburnt, so shorting the RTC/NTC/PTC leads out was enough).
This unit had the fuse fry and the RTC/PTC/NTC got severely charred and damaged (where the shell fell off when I poked it with a feather) but the electrical connection remained. This device must have malfunctioned (i.e. its natural state is short) because there is no other path for input current to take other then this device. So this triggered but failed to short so the regular fuse was popped.

Any ideas about other stuff I could crudely solder onto the PCB in order to offer better input protection?

[bitwelder]

BTW, what is the function of that PCB? It would help in order to make an informed guess whether the "RTC" is a PTC, NTC or something else, and perhaps also at why that component may have blown up.

[ftransform]

BTW, what is the function of that PCB? It would help in order to make an informed guess whether the "RTC" is a PTC, NTC or something else, and perhaps also at why that component may have blown up.

As mentioned the power supply for a peltier cooler.

[Wytnucls]

A PTC in series on the supply side, followed by a MOV or better still a GDT (gas discharge tube) across to the output ought to do the job, protecting the circuit against overvoltage. (Both of the same voltage, 270V for instance in Europe)
The fuse (250V) is there to protect for overcurrent (usually due to a short in the circuit), anything over 3A.

[ftransform]

A PTC in series on the supply side, followed by a MOV or better still a GDT (gas discharge tube) across to the output ought to do the job, protecting the circuit against overvoltage. (Both of the same voltage, 270V for instance in Europe)
The fuse (250V) is there to protect for overcurrent (usually due to a short in the circuit), anything over 3A.

I think I will bodge in a gas discharge tube I took out of something else in addition to the other repairs. 
Are PTC's known to be particularly flakey components? As I mentioned on the other supply the fuse remained shorted but the PTC was damaged to a point where it completely disintegrated when I touched it with my pinky.

[Neilm]

PTCs can overheat and be destroyed if they have more current than the pass current but less than the stop current. They end up over heating and breaking. This happens if the designer has either miscalculated the fault current or the circuit has endured a prolonged overload.

Neil

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PTCs can overheat and be destroyed if they have more current than the pass current but less than the stop current. They end up over heating and breaking. This happens if the designer has either miscalculated the fault current or the circuit has endured a prolonged overload.

Neil

Ugh, I'm having trouble selecting a replacement.
Do you think a 4R one will work as per the suggestion of SeanB?
There are so many choices.

Do you think I could maybe hook up the device to a logging meter, get a graph of the current vs time and make a selection based on that information?
Documentation for this device is unavailable.

[Neilm]

The only way to properly specify this would be reverse engineer the circuit and figure out the expected fault currents, accounting for the ambient temperature in the unit.

If you try hunting on the web for specifying these. They also referred to a "resettable fuses".

Neil

[ftransform]

The only way to properly specify this would be reverse engineer the circuit and figure out the expected fault currents, accounting for the ambient temperature in the unit.

If you try hunting on the web for specifying these. They also referred to a "resettable fuses".

Neil