Are big wirewound resistors acceptable for snubbers

[JeffM]

I have pulse-tested these resistors before. The peak pulse power rating

I thought that the above ones (digikey) were carbon comp ones.
Ignoramus1487

I have seen spec sheets for power tab resistors before
and all that I have encountered were thick film.
I ASSuME'd that these were as well.
Best practice:
Always check the manufacturer's spec sheet before ordering a part.

NOTE:
*Carbon Composition* refers to a cylindrical device with axial leads.
It has a slug of carbon, encased in phenolic.

The closest modern equivalent (as was my initial allusion)
is *thick film*.
http://66.102.7.104/search?q=cache:...y-20-Watt-Power-Resistors-in-a-TO-220-Package
http://66.102.7.104/search?q=cache:...unt+intitle:resistors+TO-*-package+thick-film
http://www.google.com/search?q=intitle:resistors+TO-*-package+thick-film

If Terry can post a current source for carbon comp parts,
that would be ideal.
The ones I found (several years back) were from the remaining stock
from a dealer in obsolete components.
I had to Google my brains out to find them.

 

[Ignoramus1487]

I have seen spec sheets for power tab resistors before
and all that I have encountered were thick film.
I ASSuME'd that these were as well.
Best practice:
Always check the manufacturer's spec sheet before ordering a part.

NOTE:
*Carbon Composition* refers to a cylindrical device with axial leads.
It has a slug of carbon, encased in phenolic.

The closest modern equivalent (as was my initial allusion)
is *thick film*.
http://66.102.7.104/search?q=cache:...y-20-Watt-Power-Resistors-in-a-TO-220-Package
http://66.102.7.104/search?q=cache:...unt+intitle:resistors+TO-*-package+thick-film
http://www.google.com/search?q=intitle:resistors+TO-*-package+thick-film

If Terry can post a current source for carbon comp parts,
that would be ideal.
The ones I found (several years back) were from the remaining stock
from a dealer in obsolete components.
I had to Google my brains out to find them.

Thanks Jeff. I will look around a little bit.

i
--

 

[Chris Jones]

Two questions:

Question 1.

I suspect that the answer is no, but want to doublecheck.

My tig inverter's snubber specifications call for a 0.16 Ohm snubber
resistor.

I have a large, about 4" long, wirewound (better called metal tape
wound), 0.2 ohm resistor. It is rated for 100 watts and is about 1"
thick and has maybe 20 turns of metal ribbon perhaps 3/16" wide.

Can I use this monster as a snubber resistor, or would it not be
suitable due to its inductance?

If not, what resistor would you suggest.

Question 2.

Can I simply skip the resistor in the RC snubber, in my application.
Especially since my bridge is shorting and there are no times when it
is open.

I experimented with a nice big cornell dubilier capacitor and found
that it reduces ripple to next to nothing, without a resistor. Adding
a 0.05 ohm resistor made things much worse for some reason.

I understand that resistor is there to suppress oscillations, but,
ultimately, why should I care too much about those oscillations.

i

When both the top and bottom IGBTs are on, basically you are shorting the
input to the bridge. If there is a big capacitor at the input to the
bridge, the currents in the IGBTs will be really high and will probably be
destructive if you use a high supply voltage. If there is a resistor in
the snubber then the majority of the capacitor voltage is dropped across
the resistor during the time that the top and bottom IGBTs are both on, and
Ohm's law will tell you what sort of current to expect, but at least it
will be lower than without the resistor.

I think you should go to using dead time if you are using a snubber with a
capacitor, and you must do so if there is no resistor in series with the
capacitor.

By the way, if you put a copper pipe inside your wirewound resistor (not
touching the resistor wire or terminals, but just sitting inside the
ceramic former) then this will reduce the inductance a bit. It will
probably still have far too much inductance though.

Chris

 

[Ignoramus22022]

When both the top and bottom IGBTs are on, basically you are shorting the
input to the bridge.
Yes.

If there is a big capacitor at the input to the
bridge, the currents in the IGBTs will be really high and will probably be
destructive if you use a high supply voltage.

While this is true, my Semikron 23 drivers sense shorting (which is
high voltage between collector and emitter) and shut down promptly and
softly.

If there is a resistor in
the snubber then the majority of the capacitor voltage is dropped across
the resistor during the time that the top and bottom IGBTs are both on, and
Ohm's law will tell you what sort of current to expect, but at least it
will be lower than without the resistor.

I think you should go to using dead time if you are using a snubber with a
capacitor, and you must do so if there is no resistor in series with the
capacitor.

What you say makes perfect sense indeed. I should try it using a
proper resistor in series with the cap.

By the way, if you put a copper pipe inside your wirewound resistor (not
touching the resistor wire or terminals, but just sitting inside the
ceramic former) then this will reduce the inductance a bit. It will
probably still have far too much inductance though.

That's a great thought. I have a large (4") wirewound resistor and I
could try placing a copper pipe in there. Or, maybe, I will just wait
until I get my carbon resistors.

By the way, I won some 0.33 ohm 1 watt resistors on ebay, a 6 pound
reel of them. I think that it's a good deal and I could sell them in
smaller batches and get my money back. I would use them in something
like 10x5 matrix (a series of 5 rows of 10 parallened resistors in
each) to get sensible resistance and wattage rating.

I will play some more, but I am limited in what I can do without
proper resistors. I will see if using inductance in series on the
supply line would change anything.

i