Resistor Fuse for AC 220V Circuit

[aj_augus]

I want to use SMD resistor as a fuse in 220V ac to DC circuit . I need the fuse before rectifier. I saw in one ballast design, they used 0.5ohm 1/2W THD fusible resistor.
Can I use same value SMD?what are the things to consider?

 

[wwfeldman]

i would not expect the resistor choice for one application to work in another application
but is seems to me better to use a fuse in a fuse holder

it is easier to replace than a soldered component
it doesn't waste power under normal conditions
you have to examine the fusing characteristics of a fuse or a fusible resistor in any case

if you choose to use a fusible resistor, you need to look at the fuse characteristic
in addition to the usual resistance, power, tolerance, temperature coefficient, etc

 

[schmitt trigger]

There are fusible resistors specifically designed for the task.

Using a regular resistor would give unpredictable results, but it can be used in a hurry.

 

[BradtheRad]

You might make your own fusible link simply by soldering in a short thin wire.

I had a glass fuse which blew. I saw a thin wire inside, so I thought I'd homebrew a replacement. I detached the metal caps and soldered in a wire taken from stranded copper cable. It was a chore to make. I forget whether I tested it to see what current made it burn or whether I put it off.

 

[KlausST]

Hi,

I don't recommend to playing around with parts that are not meant to use this way.

A fuse is a safety device. It is meant to give more safety than without.
A simple resistor is not meant to burn. One does not know "when" it burns, not in temperature, not at which current, not at which time.
All this is specified or regulated with a fuse or a fusable resistor.
A standard resistor when slightly overpowered will overheat, how much is unknown, maybe the solder melts, maybe not, maybe your PCB material burns, maybe not, maybe the resistor catches fire, maybe not.
When the resistor eventually burnt open, then there is high voltage across the resistor...how does the resistor react? Still burn, arcing?

With a fuse you know (it is specified) that it will not overheat, it won't hurt the PCB, it won't catch fire, it will get open to withstand a certain voltage. But you need a relatively big resistor for a "Maximum Applicable Voltage after Fusing" of 220V.

Be sure your "fuse" increases safety, not decreases safety...or creates new risks.
Feel responsible for others lives and others belongings.

Klaus

 

[aj_augus]

Thanks, for the valuable feedback.
We try to use resistor which is fusible type. (MELF)
Basically, in my design, We use normal glass fuse (for the entire board). But this is to give extra protection to the rectifier. (Which goes to DC part).

This is the specification for a Fusible MELF resistor... I expected 5Ohm also can work with 200 V... But as per (P * R)^1/2
Vrated = = 1.58V
Irated = V/R = 0.316A
It is not capable.

 

[aj_augus]

It was a fusible MELF resistor. It fused without and change in the physical appearance... and became 2M Ohm range.. So 230V voltage drop was completely on this resistor.

 

[KlausST]

Hi,

you confuse values and specifications...
Please give a link to the datasheet.

Klaus

 

[aj_augus]

Fusible MELF Resistor - FM | Surge Resistant MELF Resistor Manufacturer | FIRSTOHM

Manufacturer of SMD enabled structure, 0207 MELF style. Fusible MELF Resistor (FM) is an SMD-enabled structure with MELF 0207 size, featuring a power rating from 0.33W to 1W and a resistance range from 2.2 ohms to 10K ohms, which is better in heat dissipation than a chip resistor and stronger in...

www.firstohm.com.tw

 

[KlausST]

Hi,

O.K. since the datasheet does not specifiy a "Maximum Applicable Voltage after Fusing" I agree with using the "working voltage".

Klaus

 

[wwfeldman]

how fast do you want the fusible resistor to respond to an over-current?
as i read the fusing characteristics of the FM26 (1/3 W),
under constant voltage conditions, it should fuse at almost 7.5 W in 1 second
the smallest standard resistance is 2.2 ohm, which, at 7.5 W is 1.8 A
but the graph appears to show a range, which tops out at about 60 (70) seconds
(the log scale is hard to read)

at 14W load, the fusing time range seems to be about 0.08 second to 1 second
is that fast enough?

 

[KlausST]

Hi,

If you use a higher value resistor at the same current, the dissipated power is higher and the trip is faster.
But did you notice: with 1,8A and 2.2 Ohms the voltage drop is about 4V.

Klaus

 

[wwfeldman]

time for fuse action is power dependent
choice of small resistor is to reduce losses

OP's placement of the fuse/fusible link is on the input side,
so the assumption of constant voltage for the fuse times is incorrect

best choice is a real fuse

Thin Film Chip Fuses - Littelfuse

www.littelfuse.com

 

[FvM]

A fusible resistor comes into play if your application needs a resistor, e.g. for inrush current limiting of a rectifier with filter capacitor, and you want to enforce safe behavior in case of shorted rectifier or downstream circuit failure. Obviously the resistor must be capable of absorbing the inrush energy in normal operation. I doubt that the 50 ohm MELF resistor does. Apart from energy rating, 300 V working voltage isn't sufficient for 230 VAC.

Using a fusible resistor as fuse replacement makes no sense to me.

 

[aj_augus]

A fusible resistor comes into play if your application needs a resistor, e.g. for inrush current limiting of a rectifier with filter capacitor, and you want to enforce safe behavior in case of shorted rectifier or downstream circuit failure. Obviously the resistor must be capable of absorbing the inrush energy in normal operation. I doubt that the 50 ohm MELF resistor does. Apart from energy rating, 300 V working voltage isn't sufficient for 230 VAC.

Using a fusible resistor as fuse replacement makes no sense to me.

Replacement is needed if the cost is the factor...

 

[mhmdghfr]

Hi
I have sort of a similar problem: As described in another website I need to replace a fusible resistor in front of an AC input (in a Samsung mobile charger (EP-TA800)), but it is unlikely that I would find the exact component labeled 2w1.1Ohm, seemingly a fusible resistor.
So what kind of fuse (probably a tiny SMD) and with what specifications can I use (perhaps in series with a 2W1Ohm resistor) before the rectifier circuit for the intended purpose (that is the board and mobile protection)?