Surface Mount Resistor Popped

[Watson A.Name \Watt Sun - the Dark Remover\]

I found that the surface mount resistor burned a black spot on the PC
board, and flew off and melted itself into the inside of the plastic
case. So I measured the size of the resistor, and found that it's 3mm
long by 2.5mm wide, and it has to dissipate about .45 watt. I don't
have any reference for what size SM resistor does what wattage, so I'm
looking for some advice. I figure on replacing the burned one with a
half watt thru hole resistor, but there's another one of the same size
on the board and I'd like to know if it's able to handle that much power
(apparently the burned one wasn't!). What's the dissipation rating for
these sized resistors, and should I be worried about the remaining one??
Thanks.

 

[James T. White]

Sounds like a Size 1210 which would be 3.2mm X 2.5mm. The dissipation rating
can vary by manufacturer. For Vishay's CRCW resistors, the 1210's are rated at
..33W at 70C regardless of value. For KOA/Speer's RK73B resistors, the 1210's
are rated at .50W for 1K and under and .33W for values over 1K, again at 70C.
Depending on the resistance value it sounds like your resistors are nearly at or
just above their rating although the failure mode you described sounded more
like some sort of surge event rather than just simple overheating. Hope this
helped.

 

[Watson A.Name \Watt Sun - the Dark Remover\]

Sounds like a Size 1210 which would be 3.2mm X 2.5mm. The dissipation rating
can vary by manufacturer. For Vishay's CRCW resistors, the 1210's are rated at
.33W at 70C regardless of value. For KOA/Speer's RK73B resistors, the 1210's
are rated at .50W for 1K and under and .33W for values over 1K, again at 70C.
Depending on the resistance value it sounds like your resistors are nearly at or
just above their rating although the failure mode you described sounded more
like some sort of surge event rather than just simple overheating. Hope this
helped.

Thanks for the info. The value was 6.8k, so it's probably .33W.

What's funny is that it was working ok for awhile, then it went bad. I
don't think there was a surge, I think what happened was what often
happens when regular carbon composition resistors overheat, they start
to go down in value, and as they start to go down, the dissipation
increases, which gets into a runaway condition and then the resistor
just goes pop. Or fizzle, or whatever. The thing self destructs.

I think what I'm gonna do is get a 1W or 2W flameproof resistor from
Fry's NTE resistor mess, and try to squeeze it in there. I don't think
it's gonna be difficult to make it fit. The higher wattage should hold
up against having the 120VAC line across it(!)

 

[Boris Mohar]

Thanks for the info. The value was 6.8k, so it's probably .33W.

What's funny is that it was working ok for awhile, then it went bad. I
don't think there was a surge, I think what happened was what often
happens when regular carbon composition resistors overheat, they start
to go down in value, and as they start to go down, the dissipation
increases, which gets into a runaway condition and then the resistor
just goes pop. Or fizzle, or whatever. The thing self destructs.

I think what I'm gonna do is get a 1W or 2W flameproof resistor from
Fry's NTE resistor mess, and try to squeeze it in there. I don't think
it's gonna be difficult to make it fit. The higher wattage should hold
up against having the 120VAC line across it(!)

120V, 6.8K will dissipate 2,11W If this is continuous duty you might want
to try something bigger. Are you sure that it was 6.8K?

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

120V, 6.8K will dissipate 2,11W If this is continuous duty you might want
to try something bigger. Are you sure that it was 6.8K?

It has to handle 56VDC, and possibly ringing current, which can be up to
120VAC, 2S on 4S off. So I don't think it'll ever have to dissipate 2W.
But then never say never. ;-)

I'm fairly sure, it's the same as the other circuit on the board, and
the resistor says 682 on it. I went by Fry's last night, and they were
out of 6.8k 1W and 2W resistors, so I'll have to check my junque box.
Or else put two 3.3ks in there, if they'll fit.

 

[Ben Bradley]

In
alt.binaries.schematics.electronic,sci.electronics.repair,sci.electronics.misc,
"Watson A.Name - \"Watt Sun, the Dark Remover\""

It has to handle 56VDC, and possibly ringing current, which can be up to
120VAC, 2S on 4S off.

That sounds like a phone line (though 56V sounds higher than the
USA standard 48VDC onhook voltage), and the 120VAC ringing voltage is
superimposed on the 56V. That means it's an offset sine wave with
peaks of 56+120*1.414=225.68V and 56-120*1.414=-113.68V. If you do the
calculus, you can calculate the RMS equivalent. The power is MORE than
just 120VAC across the resistor, and if I envision the RMS equivalent
voltage correctly, the average power is MORE THAN the sum of the
powers of the separate AC (120VAC^2/6800) and DC (56^2/6800)
components, or MORE THAN 2.12+0.46=2.58 watts.

So I don't think it'll ever have to dissipate 2W.
But then never say never. ;-)
Woops...

I'm fairly sure, it's the same as the other circuit on the board, and
the resistor says 682 on it. I went by Fry's last night, and they were
out of 6.8k 1W and 2W resistors, so I'll have to check my junque box.
Or else put two 3.3ks in there, if they'll fit.

 

[Roger Gt]

"Ben Bradley" wrote
: "Watson A.Name wrote
: >"Boris Mohar" wrote
: >> "Watson A.Name wrote
:
: >> >Thanks for the info. The value was 6.8k, so it's probably
..33W.
: >> >
: >> >What's funny is that it was working ok for awhile, then it
went bad.
: >I
: >> >don't think there was a surge, I think what happened was
what often
: >> >happens when regular carbon composition resistors overheat,
they
: >start
: >> >to go down in value, and as they start to go down, the
dissipation
: >> >increases, which gets into a runaway condition and then the
resistor
: >> >just goes pop. Or fizzle, or whatever. The thing self
destructs.
: >> >
: >> >I think what I'm gonna do is get a 1W or 2W flameproof
resistor from
: >> >Fry's NTE resistor mess, and try to squeeze it in there. I
don't
: >think
: >> >it's gonna be difficult to make it fit. The higher wattage
should
: >hold
: >> >up against having the 120VAC line across it(!)
: >>
: >> 120V, 6.8K will dissipate 2,11W If this is continuous
duty you
: >might want
: >> to try something bigger. Are you sure that it was 6.8K?
: >
: >It has to handle 56VDC, and possibly ringing current, which can
be up to
: >120VAC, 2S on 4S off.

Max telephone is usually -56VDC But the Ringer voltage is normally
86VRMS +/- 5% max, down to 45VRMS min.

Normally the Ringer AC is coupled via a capacitor in modern
phones. (There is no real bell coil.)

: That sounds like a phone line (though 56V sounds higher than
the
: USA standard 48VDC onhook voltage), and the 120VAC ringing
voltage is
: superimposed on the 56V. That means it's an offset sine wave
with
: peaks of 56+120*1.414=225.68V and 56-120*1.414=-113.68V. If you
do the
: calculus, you can calculate the RMS equivalent. The power is
MORE than
: just 120VAC across the resistor, and if I envision the RMS
equivalent
: voltage correctly, the average power is MORE THAN the sum of the
: powers of the separate AC (120VAC^2/6800) and DC (56^2/6800)
: components, or MORE THAN 2.12+0.46=2.58 watts.
:
: >So I don't think it'll ever have to dissipate 2W.
: >But then never say never. ;-)
:
: Woops...
: >I'm fairly sure, it's the same as the other circuit on the
board, and
: >the resistor says 682 on it. I went by Fry's last night, and
they were
: >out of 6.8k 1W and 2W resistors, so I'll have to check my
junque box.
: >Or else put two 3.3ks in there, if they'll fit.
: >> Boris Mohar
: >
:

 

[John Woodgate]

I read in alt.binaries.schematics.electronic that Ben Bradley <ben_nospa
[email protected]> wrote (in <i43ue09ob88bc4bltbppb38fo3io8943vb@
4ax.com>) about 'Surface Mount Resistor Popped', on Fri, 9 Jul 2004:

if I envision the RMS equivalent
voltage correctly, the average power is MORE THAN the sum of the
powers of the separate AC (120VAC^2/6800) and DC (56^2/6800)
components, or MORE THAN 2.12+0.46=2.58 watts.

If only that were true! You could generate power by adding AC and DC!

 

[[email protected]]

120V, 6.8K will dissipate 2,11W If this is continuous duty you might want
to try something bigger. Are you sure that it was 6.8K?

Trouble is, you don't know the whole circuit. I suspect the 120 v is
current or duty cycle limited - there could be another resistance or
reactance in the circuit that we don't know about. Has to be, if it was
really a .33 watt unit.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

Boris Mohar wrote: [snip]

120V, 6.8K will dissipate 2,11W If this is continuous duty you might want
to try something bigger. Are you sure that it was 6.8K?

Trouble is, you don't know the whole circuit. I suspect the 120 v is
current or duty cycle limited - there could be another resistance or
reactance in the circuit that we don't know about. Has to be, if it was
really a .33 watt unit.

It's a Progressive 77HP toner, and the circuit is a bidirectional
red/green LED, and the resistor. As far as I know, there is no other DC
circuit, only an AC signal coupled to the output thru a cap.

My problem is that it's used in a digital PBX environment where the
telephone line can supply up to 5W of power(!) That's why the
_constant_ 56VDC. There is no ring current on the digital PBX, only on
a regular POTS line.

 

[[email protected]]

Boris Mohar wrote:

[snip]

120V, 6.8K will dissipate 2,11W If this is continuous duty you

might want

Trouble is, you don't know the whole circuit. I suspect the 120 v is
current or duty cycle limited - there could be another resistance or
reactance in the circuit that we don't know about. Has to be, if it

was


really a .33 watt unit.

It's a Progressive 77HP toner, and the circuit is a bidirectional
red/green LED, and the resistor. As far as I know, there is no other DC
circuit, only an AC signal coupled to the output thru a cap.

My problem is that it's used in a digital PBX environment where the
telephone line can supply up to 5W of power(!) That's why the
_constant_ 56VDC. There is no ring current on the digital PBX, only on
a regular POTS line.

OK! That makes sense. At 54 volts (~2 v for the led) and with no
ring, that's about .43 watts. You could probably go to a 10K
resistor and still see the LED easily, and y7ou could use a .33 watt
unit. Maybe it's worth trying, at least temporarily, to see if the
LED is bright enough?

 

[Watson A.Name \Watt Sun - the Dark Remover\]

Watson A.Name - "Watt Sun, the Dark Remover" wrote:

Boris Mohar wrote:

[snip]

120V, 6.8K will dissipate 2,11W If this is continuous duty you

might want to try something bigger. Are you sure that it was 6.8K?

was really a .33 watt unit.
It's a Progressive 77HP toner, and the circuit is a bidirectional
red/green LED, and the resistor. As far as I know, there is no other DC
circuit, only an AC signal coupled to the output thru a cap.
My problem is that it's used in a digital PBX environment where the
telephone line can supply up to 5W of power(!) That's why the
_constant_ 56VDC. There is no ring current on the digital PBX, only on
a regular POTS line.

OK! That makes sense. At 54 volts (~2 v for the led) and with no
ring, that's about .43 watts. You could probably go to a 10K
resistor and still see the LED easily, and y7ou could use a .33 watt
unit. Maybe it's worth trying, at least temporarily, to see if the
LED is bright enough?

Thanks for the idea. I may just try a higher value, since I can't seem
to find a 6.8k 1w flameproof resistor at my usual haunts. I found two
15k 1Watters, and if I can squeeze them in, I may parallel them. That's
about 7.5k.