Fusible resistor size?

Thread Starter

Majnoon

Joined May 22, 2020
4
I have a small 6W uv water sterilizer that ceased working after a small leak. 120V unit with 350V output. I opened it up and saw a blown component on one leg of the input ac connector. Not much of it was left to identify other than it was small and looked glassy. I assumed iniitially that it was a simple fuse but on closer exam saw it was labeled "fr" which I presume means fusible resistor. My question is what was its likely resistance? DC impedence of input side without fr is 1.2 ohms. I presume it's feeding a small step up transformer but hard to follow the circuit details on the small pcb. I don't know there wasn't any downstream damage as well but thought I'd try to resurrect. I assume it was about 2k ohms but any help with my limited info would be appreciated.
 

Thread Starter

Majnoon

Joined May 22, 2020
4
PCB photo is attached. The component location in question FR1 is in the lower left corner. The component itself left only a few glassy fragments a few mm's in scale with no identifiable features. You can see a small fragment still connected to the base on the right side of the location. Apologies for the vagueness but it's all i have. The ballast rating is 6 watts with a nominal current max draw of .1 amps at 120 volts 60 hz along with DC impedence between the two AC input pointsUV ballast.jpg of 1.2 ohms.
 

dendad

Joined Feb 20, 2016
3,591
It is a switch mode power supply. Directly connected to the mains. If you are new to electronics, I reckon you'd be best served by leaving it alone. I do not work on these things unless theybare connected to an isolation transformer. Also, add a mains incandecent lamp, try 40 Watts, in series to prevent the smoke getting out again. This will work ok as long as the supply does not try to run its load. The lamp will flash when you turn the supply on (after the fuse has been replaced, if that is what it is), then gomout or have a dim light. If it stays bright, there is a short somewhere.
Check the diodes, D1to D4 for shorts. Also, the two power FETs ot transistors.
But be careful as the caps can hold a charge that bites!
Only work on it when unplugged.
If the input resistance is 1.2 ohms, there is a short somewhere. I would expect it to measure open if that is a fuse blown.
Unless it is being fed from an external transformer and that is what you are measuring.
Maybe a good photo of the botton of the board will help to trace the circuit a bit.
 

rsjsouza

Joined Apr 21, 2014
253
I don't recall seeing a fusible resistor in glass - mostly they resemble a resistor but with very glossy enamel or a simple resistor with color bands, etc.

It doesn't mean this is not one, but I would think this resembles a glass fuse with leads.
glass_fuse.png

Unfortunately, to be absolutely sure you would need a photograph or another identical equipment to know the value.
 

Thread Starter

Majnoon

Joined May 22, 2020
4
It is a switch mode power supply. Directly connected to the mains. If you are new to electronics, I reckon you'd be best served by leaving it alone. I do not work on these things unless theybare connected to an isolation transformer. Also, add a mains incandecent lamp, try 40 Watts, in series to prevent the smoke getting out again. This will work ok as long as the supply does not try to run its load. The lamp will flash when you turn the supply on (after the fuse has been replaced, if that is what it is), then gomout or have a dim light. If it stays bright, there is a short somewhere.
Check the diodes, D1to D4 for shorts. Also, the two power FETs ot transistors.
But be careful as the caps can hold a charge that bites!
Only work on it when unplugged.
If the input resistance is 1.2 ohms, there is a short somewhere. I would expect it to measure open if that is a fuse blown.
Unless it is being fed from an external transformer and that is what you are measuring.
Maybe a good photo of the botton of the board will help to trace the circuit a bit.
the PCB sits in a box and appears to be glue mounted so i can't remove without breaking the board itself. I'll try the lamp idea. The ballast can be replaced at a reasonable cost with a non-OEM unit so there's a limit to the heroics i was willing to try but the lamp in series sounds reasonable rather than doing an actual smoke test. The 1.2-1.5 ohms I noted as the input impedence was between the one AC leg that's still connected and the other leg on the board with the fragment still tied to the board so it does represent the circuit impedence. The mains connection as you note is open.
 

Thread Starter

Majnoon

Joined May 22, 2020
4
I don't recall seeing a fusible resistor in glass - mostly they resemble a resistor but with very glossy enamel or a simple resistor with color bands, etc.

It doesn't mean this is not one, but I would think this resembles a glass fuse with leads.
View attachment 207839

Unfortunately, to be absolutely sure you would need a photograph or another identical equipment to know the value.
Many thanks, yes this would be my best guess as to what the component initially looked like and "fuse" is what i had originally assumed. However, the "FR" designation on the pcb suggested otherwise to me at least. These ballasts are typically rated for 0.1 amps max draw so that would provide guidance on replacementselection but I presume these have low resistance values like most fuses. If so then hooking up mains to fused circuit with impedence of <2 ohms doesn't seem wise to me and per dendad this suggests that the system is already blown.
 
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