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snubber resistor power rating?

A

Arie de Muynck

Jan 1, 1970
0
"Ken Smith" ...
So, the spike like current in the resistor is a current that starts equal
to mains/R and then decreases very rapidly.

Does the triac get turned on only at zero crossings in this application or
is it phase controlled? If it is turned on at zero crossings, there is a
reduced requirement on the resistor. In the phase controlled case, the
resistor can end up with 4 spikes of almost a big per cycle.

Zerocrossing turnon does only load the resistor at turnoff - and then the
current wil be very low. No problem for the snubber R.

The circuit can be phase controlled which is a problem for the the resistor.
My problems with sparking resistors were in a phase-controlled motordrive,
mostly running at 50% phase and with a rather big C, so a heavy currentspike
at each turnon.

BTW: you must compute the proper values for a given circuit. Using a
low-impedance snubber with small solonoid controlled valves may cause the
valves to stay activated by the snubber current!

Regards,
Arie de Muynck
 
R

Robert Monsen

Jan 1, 1970
0
CBarn24050 said:
A typical snubber (0.1uf + 100r) on a triac at 60hz produces a very small
amount of power in the resistor. A half watt carbon resistor is more than
adequate.

I just simulated a triac circuit with a 100mH inductor. The current
through the 100 ohm resistor (using a 100nF capacitor as the other 1/2
of the snubber) was 1.4A, giving a burst of power that peaks at about
200W. Now, the average power through that resistor is only about 84mW,
so the average dissipation is small.

The question is whether a resistor will be able to absorb that power
spike without damage. There is anecdotal evidence given elsewhere in the
thread that it won't be able to in some cases.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.
 
K

Ken Smith

Jan 1, 1970
0
Arie de Muynck said:
The circuit can be phase controlled which is a problem for the the resistor.
My problems with sparking resistors were in a phase-controlled motordrive,
mostly running at 50% phase and with a rather big C, so a heavy currentspike
at each turnon.

BTW: you must compute the proper values for a given circuit. Using a
low-impedance snubber with small solonoid controlled valves may cause the
valves to stay activated by the snubber current!

I haven't thought this through but:

It seems to me that the problem could be helped by adding a bit of
complexity to the snubber circuit. This added stuff needs to do this:


When the triac is switching on, the snubber circuit is basically open
circuited.


When the triac is switching off, the snubber initially is just the
capacitor. As time passes the snubber starts to look resistive.


I have something like this in mind:

!
--- C1
---
!
!
-------+-------
! !
V ---
--- C2 ^
! !! !
+----!!---- S1 !
! !! ! / !
! +/ o-+
! X1 ! !
+--????---- !
! !
--- V
^ ---
! !
-------+--------
!


C1 is the capacitor that is normally part of the snubber.

The bridge and C2 makes a way to soak up the energy without having to turn
it into heat right at that instant. X1 is something that gets rid of the
energy. S1 closes when the voltage hits zero.

Yes, I know this actually won't work! It does seem that something with a
low parts count should be possible.
 
T

Terry Given

Jan 1, 1970
0
Ken said:
Arie de Muynck said:
AAArrghhh...

OK, here's the right drawing:

"Arie de Muynck" ...
Ken Smith" ....

Is this what we are dealing with here:
...
If so, how does the [triac] get turned off with a current flowing in the

load?

No. It is:

L (mainly inductive load)
---------UUUU---------------------------
^ | |
! | /
! | \ R
! | /
Mains | \
! ----- |
! A V Triac |
! ----- ___
! Trigger ckt----/ | ___ C
V | |
----------------------------------------

The triac turns off at the zerocrossing of the current through it. Since
the

mains voltage will be about maximum then, the snubber limits the slewrate,
preventing the turnon by excessive dV/dt.
The resistor provides damping of the turnoff efect. It also limits the
current when the triac fires at turnon:
I(pk) = Vmains(pk) / R
and this discussion is about how a 2W 47 Ohm resistor likes that hefty
spike...

Regards,
Arie de Muynck


Ok got it.

So, the spike like current in the resistor is a current that starts equal
to mains/R and then decreases very rapidly.

Does the triac get turned on only at zero crossings in this application or
is it phase controlled? If it is turned on at zero crossings, there is a
reduced requirement on the resistor. In the phase controlled case, the
resistor can end up with 4 spikes of almost a big per cycle.

47 Ohms is a lot of resistance to solve this way but at lower voltages, I
have made resistors to protect crowbar SCRs out just a length of hook up
wire folded back on its self. The accuracy of the value isn't good but
copper wire can take a huge spike with no trouble because the resistance
is spread over a large volume and it is very thermally conductive.


I have had a lot of trouble finding any resistor that has a good pulse
handling ability in surface mount. The ones I did find were very
expensive and not very available. They were from one of the Tyco
companies.

IRC (the company, I do Recall Correctly) make some great smt resistors.
And have peak pulse power curves.

Cheers
Terry
 
K

Ken Smith

Jan 1, 1970
0
Terry Given said:
IRC (the company, I do Recall Correctly) make some great smt resistors.
And have peak pulse power curves.


IRC person: Hello, how may I help you?

Ken: I need some pulse rated resistors.

IRC person: Good how many box cars full, and where the siding.

Ken: I only need about 100

IRC: 100 Box cars full is a small order but I think we can do it.

Ken: No, I need 100 resistors


IRC: Ha-ha-ha
(Craching sound as he falls out of chair)

Phone: Click Buzzzzzzzzzzzzzzzz


Also IIRC, their spec was weirdly written. It was some time ago but I
remember talking to the technical help from some resistor company and
pointing out that according to the specs, the part could handle less power
for 16mS than it could handle forever.
 
J

John Popelish

Jan 1, 1970
0
Terry said:
Ken said:
Arie de Muynck said:
AAArrghhh...

OK, here's the right drawing:

"Arie de Muynck" ...

Ken Smith" ....

Is this what we are dealing with here:
...
If so, how does the [triac] get turned off with a current flowing in the

load?

No. It is:

L (mainly inductive load)
---------UUUU---------------------------
^ | |
! | /
! | \ R
! | /
Mains | \
! ----- |
! A V Triac |
! ----- ___
! Trigger ckt----/ | ___ C
V | |
----------------------------------------


The triac turns off at the zerocrossing of the current through it. Since

the

mains voltage will be about maximum then, the snubber limits the slewrate,
preventing the turnon by excessive dV/dt.
The resistor provides damping of the turnoff efect. It also limits the
current when the triac fires at turnon:
I(pk) = Vmains(pk) / R
and this discussion is about how a 2W 47 Ohm resistor likes that hefty
spike...

Regards,
Arie de Muynck


Ok got it.

So, the spike like current in the resistor is a current that starts equal
to mains/R and then decreases very rapidly.

Does the triac get turned on only at zero crossings in this application or
is it phase controlled? If it is turned on at zero crossings, there is a
reduced requirement on the resistor. In the phase controlled case, the
resistor can end up with 4 spikes of almost a big per cycle.

47 Ohms is a lot of resistance to solve this way but at lower voltages, I
have made resistors to protect crowbar SCRs out just a length of hook up
wire folded back on its self. The accuracy of the value isn't good but
copper wire can take a huge spike with no trouble because the resistance
is spread over a large volume and it is very thermally conductive.


I have had a lot of trouble finding any resistor that has a good pulse
handling ability in surface mount. The ones I did find were very
expensive and not very available. They were from one of the Tyco
companies.

IRC (the company, I do Recall Correctly) make some great smt resistors.
And have peak pulse power curves.

Caddock does too:
http://www.caddock.com/Online_catalog/smt/smt.html
 
H

HARRY DELLAMANO

Jan 1, 1970
0
John Popelish said:
But that 1.5 factor is allowed for 5 seconds. I was assuming that
from an I^2*t fusing effect that the dissipation capability would go
up quite a bit if the time was in milliseconds.

A-huh, assuming.
Harry
 
J

John Popelish

Jan 1, 1970
0
Harry said:
Caddock is a perfect example of a poor surge (Pulse) power resistor. They
are thin film and X1.5 rated peak power. A good surge rated resistor is
X5000 or maybe 5 Joules. Call Richard Caddock and see if they speak in
Joules.

But that 1.5 factor is allowed for 5 seconds. I was assuming that
from an I^2*t fusing effect that the dissipation capability would go
up quite a bit if the time was in milliseconds.
 
K

Ken Smith

Jan 1, 1970
0
But that 1.5 factor is allowed for 5 seconds. I was assuming that
from an I^2*t fusing effect that the dissipation capability would go
up quite a bit if the time was in milliseconds.

No, X1.5 applies to 0.1 attoseconds to 1.5 seconds. 1.500001 applies below
0.1 attoseconds.

(or something like that)

Thermally the situation looks a lt like this:


Power in
= I in R1 R2 R3 R4 R5
-> ---/\/\/\------/\/\/----/\/\/----/\/\----/\/\--- GND
! ! ! ! !
--- --- --- --- ---
--- C1 ---C2 --- C3 --- C4 --- C5
! ! ! ! !
GND GND GND GND GND

C1 is the thermal mass of the actual resistive material.

R1 is the thermal conductivity of the resistive material

C2 is the thermal mass of the substrate the resistive material is on.

R2, R3 with C3, C4 are a lumped constant representation of the substrates
bulk.


C5 and R5 are for the outer package to the air.

Unless you have some feel for R1 and C1 you really can't extrapolate from
1.5 seconds down. I know this because I did it and ended up with 1206
open circuits after a few months of service. The power spike was only
300uS long.
 
T

Terry Given

Jan 1, 1970
0
Ken said:
No, X1.5 applies to 0.1 attoseconds to 1.5 seconds. 1.500001 applies below
0.1 attoseconds.

(or something like that)

Thermally the situation looks a lt like this:


Power in
= I in R1 R2 R3 R4 R5
-> ---/\/\/\------/\/\/----/\/\/----/\/\----/\/\--- GND
! ! ! ! !
--- --- --- --- ---
--- C1 ---C2 --- C3 --- C4 --- C5
! ! ! ! !
GND GND GND GND GND

C1 is the thermal mass of the actual resistive material.

R1 is the thermal conductivity of the resistive material

C2 is the thermal mass of the substrate the resistive material is on.

R2, R3 with C3, C4 are a lumped constant representation of the substrates
bulk.


C5 and R5 are for the outer package to the air.

Unless you have some feel for R1 and C1 you really can't extrapolate from
1.5 seconds down. I know this because I did it and ended up with 1206
open circuits after a few months of service. The power spike was only
300uS long.

Philips (or whatever the hell they are called now) have nice peak pulse
power curves in their discretes book. an 0603 RC21 5% resistor has an
average power rating of 63mW, and can take a repetitive 1ns 2W pulse
every 1us.

It would be nice if they just gave us the damn RC thermal model, instead
of maybe giving us peak pulse power curves. I got a nice surprise the
other day when looking at an Infineon FET - a complete thermal model, as
part of the datasheet. yay.

Cheers
Terry
 
J

John Popelish

Jan 1, 1970
0
Terry Given said:
John Popelish wrote:

Wow, thats GREAT! pisses all over the CHP series!

I'll remember that part, thanks John.

Now, find me someone who stocks them.
 
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