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Surge Capable Chip Resistors

D

D from BC

Jan 1, 1970
0
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Research effort:
I looked at a few datasheets without surge data..
Then I found this:
http://www.ohmite.com/catalog/pdf/tfs.pdf
This is smd ?? It's got leads..
This will work, but what else is there.?

Any suggestions...
D from BC
 
E

Eeyore

Jan 1, 1970
0
D said:
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Research effort:
I looked at a few datasheets without surge data..
Then I found this:
http://www.ohmite.com/catalog/pdf/tfs.pdf
This is smd ?? It's got leads..
This will work, but what else is there.?

For pulse operation you really need to consult the manufacturer.

Graham
 
W

Winfield Hill

Jan 1, 1970
0
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Research effort:
I looked at a few datasheets without surge data..
Then I found this:http://www.ohmite.com/catalog/pdf/tfs.pdf
This is smd ?? It's got leads..
This will work, but what else is there.?

170V and 22k is 1.3W, which is far over the power rating for
small SMD resistors. You could use Panasonic's thick-film ERJ
2010 (0.5W, risky) or 2512 (1W) types - they allow 2.5x over-
power. Or better (and smaller if mounted standing up), use a
through-hole with appropriate construction (not a film type).

The Ohmite TFS part you found will be expensive and painfully
hard to get (Mouser). It has substantial excess capability
(3kV and 6J, vs your 170V and 26mJ). I wouldn't bother.
 
K

Ken Smith

Jan 1, 1970
0
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Research effort:
I looked at a few datasheets without surge data..
Then I found this:
http://www.ohmite.com/catalog/pdf/tfs.pdf
This is smd ?? It's got leads..
This will work, but what else is there.?

www.irctt.com/pdf_files/PWCR.pdf

There are two problems with these:
1) Actually getting them
2) Paying for them
 
B

Ben Jackson

Jan 1, 1970
0
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Do you need it to properly resist for that pulse, or just survive?
Something like a ESD protection diode or TransZorb could probably
save the resistor. EDS diode data probably doesn't cover over the
us-range, but picking a random TransZorb datasheet on Vishay's site
shows graphs into the 10s of ms.
 
D

D from BC

Jan 1, 1970
0
Do you need it to properly resist for that pulse, or just survive?
Something like a ESD protection diode or TransZorb could probably
save the resistor. EDS diode data probably doesn't cover over the
us-range, but picking a random TransZorb datasheet on Vishay's site
shows graphs into the 10s of ms.

It's needs to somewhat resist properly..
R=22k+/-5k and the resistor can be nonlinear. (Oops left out these
possible useful details..)
The resistors function is just to limit current to about 8mA during
the 20mS 170VDC pulse.
I may be able to use a device other than a resistor but can it compete
with 2512 footprint?? 6.4mmx3.2mm

D from BC
 
J

John Larkin

Jan 1, 1970
0
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Probably 0603. Certainly a 1206 wouldn't notice such a pulse. The
thermal tau is much longer than 20 ms.

It takes a couple of seconds for 1600 volts to blow a several-megohm
0603 resistor, suspended in free air.

We recently tested various precision (0.05% thinfilm) resistors for
power capability, measuring resistance and thermal imaging. Turns out
that the pads are dominant heat dissipation mechanisms. Soldered to
big slabs of copper foil (which I assume you can't do) an 0603 can
dissipate about as much power as a 1206, half a watt at reasonable
hot-spot temperatures.

But why don't you just try it? Find the destruct limit and then back
off by 5:1 or whatever. We do that a lot; sometimes we decide that we
can overload parts a lot (common for passives), sometimes we decide we
can't even run them at the mfr's ratings (typically semiconductors.)


John
 
D

D from BC

Jan 1, 1970
0
170V and 22k is 1.3W, which is far over the power rating for
small SMD resistors. You could use Panasonic's thick-film ERJ
2010 (0.5W, risky) or 2512 (1W) types - they allow 2.5x over-
power. Or better (and smaller if mounted standing up), use a
through-hole with appropriate construction (not a film type).

The Ohmite TFS part you found will be expensive and painfully
hard to get (Mouser). It has substantial excess capability
(3kV and 6J, vs your 170V and 26mJ). I wouldn't bother.

Thanks..I'll use the ERJ 2512 1W.
If space allows, I might resort to paralleling chip resistors.

But I'm curious if a current limiter transistor circuit can beat the
2512 (6.4mmx3.2mm) footprint ?
At this scale, maybe a transistor is better at handing a power pulse
beyond it's continuous power rating compared to resistors..?.
I don't know much about hot spots with pulse overloading devices.
I'll guess resistor wins..
I'll check this out later. For now a resistor is fine.
D from BC
 
D

D from BC

Jan 1, 1970
0
Probably 0603. Certainly a 1206 wouldn't notice such a pulse. The
thermal tau is much longer than 20 ms.

It takes a couple of seconds for 1600 volts to blow a several-megohm
0603 resistor, suspended in free air.

We recently tested various precision (0.05% thinfilm) resistors for
power capability, measuring resistance and thermal imaging. Turns out
that the pads are dominant heat dissipation mechanisms. Soldered to
big slabs of copper foil (which I assume you can't do) an 0603 can
dissipate about as much power as a 1206, half a watt at reasonable
hot-spot temperatures.

But why don't you just try it? Find the destruct limit and then back
off by 5:1 or whatever. We do that a lot; sometimes we decide that we
can overload parts a lot (common for passives), sometimes we decide we
can't even run them at the mfr's ratings (typically semiconductors.)


John

This is certainly food for thought..
Testing time vs trusting data sheets.
Heat sinking copper area vs resistor footprint.
Survivability vs reliability.
Interesting...

I think this is going to be my plan...
If I can't squeeze in a 2512 in the pcb design, I'll drop the chip
size down to 1206.
That way when pcb size goes down...risk goes up.. :)

Based on the above...sounds like a metal core pcb could provide some
good pulse overpower tolerability with tiny chip resistors.

Thanks for the thermals.. :)
D from BC
 
J

John Larkin

Jan 1, 1970
0
This is certainly food for thought..
Testing time vs trusting data sheets.

Datasheets almost never give typical destruct limits, and usually
don't give thermal taus or SOAR equivalents (except for some fraction
of power transistors, and not even all of them.)

And blowing up parts is fun.
Heat sinking copper area vs resistor footprint.
Survivability vs reliability.
Interesting...

I think this is going to be my plan...
If I can't squeeze in a 2512 in the pcb design, I'll drop the chip
size down to 1206.
That way when pcb size goes down...risk goes up.. :)

Really, this modest pulse power is zero risk on a regular thickfilm
1206.
Based on the above...sounds like a metal core pcb could provide some
good pulse overpower tolerability with tiny chip resistors.

In 20 milliseconds, most of the hot-spot heat will remain trapped in
the part, so the extra foil won't help much. The thermal mass of a
larger resistor will absorb a shot of joules better than a small one.

John
 
W

Winfield Hill

Jan 1, 1970
0
This can be true for constant power-handling, which has much
higher energy-handling capability than the transient energy-
handling case.

The issue for short power transients, of course, is the thermal
mass of the power-dissipating element, which can be a thin film
without much thermal mass. Additional thermal mass located at
a distance isolated by a thermal-resistance pathway, necessarily
comes into play later. But 20ms is a fairly long time frame as
these things go.
This is certainly food for thought..
Testing time vs trusting data sheets.
Heat sinking copper area vs resistor footprint.
Survivability vs reliability.
Interesting...

Testing in your exact setup is not a bad idea, because the
datasheets are often excessively conservative. But you do
want to use a huge safety margin like 5x, as John suggested,
because of production variability, and because there may be
some long-term reliability issues that may not show up in
short-term testing. I've seen too many failed SMD resistors
in commercial equipment to be sanguine about this issue.
I think this is going to be my plan...
If I can't squeeze in a 2512 in the pcb design, I'll drop the
chip size down to 1206.

I'd try 2010 resistors before 1206, or use two 1206 parts.
That way when pcb size goes down...risk goes up.. :)

Based on the above...sounds like a metal core pcb could provide
some good pulse overpower tolerability with tiny chip resistors.

That would be expensive! And likely not so effective for
short very time scales.
 
J

John Larkin

Jan 1, 1970
0
This can be true for constant power-handling, which has much
higher energy-handling capability than the transient energy-
handling case.

The issue for short power transients, of course, is the thermal
mass of the power-dissipating element, which can be a thin film
without much thermal mass. Additional thermal mass located at
a distance isolated by a thermal-resistance pathway, necessarily
comes into play later. But 20ms is a fairly long time frame as
these things go.

Any idea what the thermal tau of typical surfmount resistors is like?

I did measure the thermal tau of a thinfilm platinum-on-ceramic RTD,
which is probably a reasonable model for a surface-mount resistor.
It's easy to fire a shot of energy into an rtd and then measure the
resistance versus time, directly measuring the film temperature. I
have that data at work and I'll try to find it monday/tuesday and
post.

Testing in your exact setup is not a bad idea, because the
datasheets are often excessively conservative. But you do
want to use a huge safety margin like 5x, as John suggested,
because of production variability, and because there may be
some long-term reliability issues that may not show up in
short-term testing. I've seen too many failed SMD resistors
in commercial equipment to be sanguine about this issue.

I don't think we've ever seen a surface-mount resistor fail, unless
some unusual thing overloaded it, or, in a few cases, it cracked from
some stress. They seem pretty tough.
I'd try 2010 resistors before 1206, or use two 1206 parts.


That would be expensive! And likely not so effective for
short very time scales.

I'd tend to agree. Heatsinking won't help much in 20 ms.

John
 
G

Gibbo

Jan 1, 1970
0
D said:
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Research effort:
I looked at a few datasheets without surge data..
Then I found this:
http://www.ohmite.com/catalog/pdf/tfs.pdf
This is smd ?? It's got leads..
This will work, but what else is there.?

Any suggestions...
D from BC

I was just doing a *very* similar thing (190 volts, 100uS) and came
across a datasheet in my collection. I can't remember where on the
Philips site I found it so I've stuck up here.......

http://www.smartgauge.co.uk/bits/misc/resistor_thick_film.pdf

Some handy graphs.
 
K

Ken Smith

Jan 1, 1970
0
It's needs to somewhat resist properly..
R=22k+/-5k and the resistor can be nonlinear. (Oops left out these
possible useful details..)
The resistors function is just to limit current to about 8mA during
the 20mS 170VDC pulse.
I may be able to use a device other than a resistor but can it compete
with 2512 footprint?? 6.4mmx3.2mm

How about:

http://www.supertex.com/products/selector_guides/102
and a resistor?
 
D

D from BC

Jan 1, 1970
0
Any idea what the thermal tau of typical surfmount resistors is like?

I did measure the thermal tau of a thinfilm platinum-on-ceramic RTD,
which is probably a reasonable model for a surface-mount resistor.
It's easy to fire a shot of energy into an rtd and then measure the
resistance versus time, directly measuring the film temperature. I
have that data at work and I'll try to find it monday/tuesday and
post.



I don't think we've ever seen a surface-mount resistor fail, unless
some unusual thing overloaded it, or, in a few cases, it cracked from
some stress. They seem pretty tough.


I'd tend to agree. Heatsinking won't help much in 20 ms.

John


I just mentioned metal core pcbs because I make my own..
I was just trying to find another excuse to use it.. :)

So... it's not very effective to heat sink (by Cu trace or mcpcb) a
2512,2010 or 1206 chip resistor in this case to handle a 26mJ energy
burst..
I'll go for that. :) Thanks..

I guess my weak spot in heat dynamics is showing :)
All I know is this:
Heat has a propagation time.
Materials have thermal resistance.
Most materials increase in resistance with heat.
Materials decompose if spot is hot enough.
Heating an object is like charging a capacitor???
At first I didn't quite understand what was meant by tau...It clicked
in about 1 hour later... :)
Gotta do the google heat review someday...
D from BC
 
G

Gareth

Jan 1, 1970
0
D said:
Problem: Smallest chip resistor that can handle a one shot pulse of
170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

Research effort:
I looked at a few datasheets without surge data..
Then I found this:
http://www.ohmite.com/catalog/pdf/tfs.pdf
This is smd ?? It's got leads..
This will work, but what else is there.?

Any suggestions...
D from BC

Welwyn have some data for short pulses here:

http://www.welwyn-tt.com/products/pulse-data.asp

http://www.welwyn-tt.com/pdf/application_notes/pwc_appnote.pdf

--
 
D

D from BC

Jan 1, 1970
0

I noticed a SOT 23 in the table..
Found app note on:
http://www.supertex.com/pdf/app_notes/AN-D25.pdf
mmmm depletion mosfet I limiting...There's an animal I haven't played
with yet..

I'm going to dodge the Supertex reading& math for now...
As posted here, I should be ok with a 1206 chip in my app..
A SOT23 + small chip resistor at first guess has a larger footprint
than the 1206.

By the way, the resistor I'm looking for is part of a smps startup
cct.. But I like my homebrew cct. more than the Supertex sol'n.

Thanks...I might use DMOS someday...
D from BC
 
J

John Larkin

Jan 1, 1970
0
I just mentioned metal core pcbs because I make my own..
I was just trying to find another excuse to use it.. :)

So... it's not very effective to heat sink (by Cu trace or mcpcb) a
2512,2010 or 1206 chip resistor in this case to handle a 26mJ energy
burst..
I'll go for that. :) Thanks..

I guess my weak spot in heat dynamics is showing :)

Join the club. Even if you know the theory, the practical issues -
finding reliable data, modeling 3D distributed diffusive systems,
estimating reliability vs temp - are still fuzzy. [envision global
warming rant here]. At least most thermal conduction systems are
linear, sort of.

All I know is this:
Heat has a propagation time.
Materials have thermal resistance.
Most materials increase in resistance with heat.
Materials decompose if spot is hot enough.
Heating an object is like charging a capacitor???

Yes. There's a direct similarity between thermal and electrical
circuits. You can model thermal things with Spice, making the simple
equivalences

1 volt == 1 degree C

1 amp electrical == 1 watt thermal

1 farad == 1 gram aluminum

1 ohm == 1 degc/watt

1 second == 1 second

which is good to about 5%. There's no thermal equivalent of an
inductor. The bitch is that, while circuits are usually lumped,
thermal systems are often distributed, so diffusion equations apply.
Yuk.

At first I didn't quite understand what was meant by tau...It clicked
in about 1 hour later... :)

Thermal time constant, how fast a thing transitions to its
steady-state temperature. Can range from nanoseconds to kiloyears.
They estimate time of death by how much a body has cooled down.

If a device has a thermal tau of, say 1 second, it will jump about
1/10th of the way to its steady-state temp if you pulse it for 0.1
second.

This is excellent, and free:

http://web.mit.edu/lienhard/www/downloadform1.html

John
 
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