Fuse Question

[Jon1967us]

I have a portable Dumont series string set I'm working on that has a blown fuse after the Damper. Aside from figuring out the cause of said fuse failure, I'm trying to source an appropriate replacement.

The part numbers indicate, and the removed original part are rated .3A / 125v.
The confusion arises when I look at the schematic as see the fuse in the path of 255v.

A 125v rating doesn't look sufficient, but maybe I'm interpreting wrong and perhaps the fuse only needs to be rated for RMS being that it's a slo-blo and the schematic voltage is Peak (of a sawtooth wave)

Part Numbers
https://ibb.co/nwrm7jk

Circuit
https://ibb.co/DR1xMkn

Fuse and Holder Pic
https://ibb.co/8PrpSzG




Cat
https://ibb.co/cXFN1md

[Electronic M]

Originally Posted by Jon1967us

I have a portable Dumont series string set I'm working on that has a blown fuse after the Damper. Aside from figuring out the cause of said fuse failure, I'm trying to source an appropriate replacement.

The part numbers indicate, and the removed original part are rated .3A / 125v.
The confusion arises when I look at the schematic as see the fuse in the path of 255v.

A 125v rating doesn't look sufficient, but maybe I'm interpreting wrong and perhaps the fuse only needs to be rated for RMS being that it's a slo-blo and the schematic voltage is Peak (of a sawtooth wave)

Part Numbers
https://ibb.co/nwrm7jk

Circuit
https://ibb.co/DR1xMkn

Fuse and Holder Pic
https://ibb.co/8PrpSzG




Cat
https://ibb.co/cXFN1md

Fuses critical rating is current. Voltage is the voltage at which it fails at that current. Fuses have a very small resistance (usually under an ohm) and the by ohms law the fail current may subtly vary with voltage....in practice in most TVs that subtle variance is small enough to be unimportant.

Nice cat.

[old_coot88]

A fuse's voltage rating has more to do with its 'dialectric' strength against arc-over. It's the highest 'safe' voltage allowable across the popped fuse. In the application discussed here, it looks like a 125V fuse would be under-rated, since there'd be 255V across it if it popped.

[dieseljeep]

Originally Posted by old_coot88

A fuse's voltage rating has more to do with its 'dialectric' strength against arc-over. It's the highest 'safe' voltage allowable across the popped fuse. In the application discussed here, it looks like a 125V fuse would be under-rated, since there'd be 255V across it if it popped.

The information is very dated!
Fuses have changed drastically, per U/L and the National Electrical Code.
Fuses made years ago are no longer suitable. Sand filled and porcelain tube fuses are common now in the higher current ranges.
I'm trying to remember the various fuse types that I encountered when I was working as an Industrial Electrician.

[jr_tech]

If the 125 volt rating is for AC, wouldn’t that imply that the blown fuse would be able to stand off the peak to peak voltage of a 125 volt sine wave (about 350 V), or am I thinking wrongly?

jr

[Jon1967us]

@Electronic M, Let me see if I understand that, at 255V we're actually seeing a lower current, normally. At a higher potential, but a lower current, and the fuse cares about current. Once the current reaches .3A and if the voltage is at a potential of 125v or higher, then it will blow.

For what it's worth, I did put a 1/3A 125v replacement in the set for testing. The test did not blow the fuse, and this was running the set at full power.

[old_tv_nut]

Originally Posted by Jon1967us

@Electronic M, Let me see if I understand that, at 255V we're actually seeing a lower current, normally. At a higher potential, but a lower current, and the fuse cares about current. Once the current reaches .3A

*and if the voltage is at a potential of 125v or higher,*

then it will blow.

No, the voltage across the fuse will never reach hundreds of volts before it blows. The fuse resistance is a few ohms at most. It blows when the current reaches the rated value, heating the element to the melting point. When the fuse has opened, then it will have across it whatever voltage the circuit produces. The 125 volt rating is only the safe level that the open fuse is guaranteed to block without arcing. (If the fuse is rated for 125 volts AC, then the actual DC or peak AC safe blocking voltage is 1.4x125.)

[Jon1967us]

@old tv nut

Roger that.