Are big wirewound resistors acceptable for snubbers

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.

Yep. Too bad.

So, what kind should I look for, carbon film, or what?

I took some formulas from a Fuji IGBT guide. I will appreciate some clarification.

As you know, I already have a working bridge:

but without snubber.

Feel free to disagree with what I will say: Essentially, in a shorting H bridge (one that is never fully open), the snubber is not used for much other than to suppress internal ringing on power rails going to half bridges that open and close. Which requires relatively small caps on each IGBT (which I will provide).

So, the job of the big snubber is to handle wrong situations like switching the bridge off while welding (ideally should not happen), wires going loose etc.

What I want is to never have voltage on the DC supply rails that exceeds 150 volts. That is so because my welder specifies max voltage as being 150 volts, and I do not want to ruin it.

That means that the resistor should allow all 200A of welding current (let's say 400A for safety sake) to go into the capacitor at 150 V, that means

or R = 150/400 = 0.375 ohm.

Got it. You are 100% right.

Well, I was not clear. The resistor that I tried was not that wirewound resistor. I tried a small, golden colored, heatsink mountable ribbed resistor. I am not sure what's inside it, but it does not seem to be wirewound.

Thanks Terry. Since I have the bridge basically working, having a super safe, idiot proof snubber is now my main focus.

Forgot to say. My Mom, whose job 20 years ago was to process stolen US technology for Soviet industrial and military applications, just gave me a big lecture about how dangerous it is for amateurs to create such big power projects.

So, that reinforces the safety message and I want the snubber circuit to be the pillar of safety of this whole thing. To save my ass in case if everything goes wrong and all my mistakes somehow create a domino effect.

I also installed a thermal normally closed switch that would turn the circuit off if the heatsink temperature exceeds 70 degrees C, for the same reasons.

Yeah. That's the stuff. Carbon comps can take a lot of abuse. Problem is they don't make them any more. A Russian physicist I worked with called these *bulk resistors*. Here's the modern equivalent:

Your dataset has expired. Please search again. $DEITY, I hate sites so heavily (and poorly) scripted that you can't deep-link to them.

Yup. We're on the same wavelength.

I forgot you're working from cast-offs. . . :::::I read in some document (cornell-dubilier snubbing guide) :::::that I should use "carbon composition resistors". ::::: Ignoramus14135 ::::: You may find some of the old stuff in your stash.

Yes, quite similar.

I think that you are right.

So, again: what kind of resistors should I use for snubbers...

I read in some document (cornell-dubilier snubbing guide) that I should use "carbon composition resistors".

I need about 0.2-0.3 ohms. (I have a huge inductor in front of the circuit)

I could parallel 20 of these resistors:

It's kind of pricey at $10 for 20 of them, but it would do what I want. Am I correct?

bingo. if you dig up the solenoid equation, or the modified version (google nagaoka) you can figure out roughly what inductance it is. XL =

one with less inductance. several in parallel would work better. are you sure about the value? it sounds awful low.

without R, all cap energy gets dumped into your IGBTs when they switch (I think, cant recall your sch). this may be a problem. The R is what provides the damping - converting the oscillatory energy to heat. without it, all you do is move the resonant frequency around. but you may be able to alter Fring such that the ESR of your main inductor provides the damping....

the "L" of the resistor no doubt.

if it gets up your gatedrive, you will care.

Cheers Terry

Try digikey item such as

TDH35PR250J-ND

TDH35PR150J-ND

sounds good. I will check my trash pile tonight also, I could have some such resistors, I recall seeing something in a TO package on a little heatsink, coming from a UPS.

I dont know the details of your project, but I would generally be inclined to prefer easily available parts. A standard power wire wound R can have a smaller powered higher R non inductive paralleled with it, in many cases this works well. Whether its good for your project I dont know, as I dont even know what youre making.

NT

Some things are castoffs, the stuff that I can get the fastest s what I already have...

I thought that the above ones (digikey) were carbon comp ones. Am I selecting wrong ones? If so, which ones would you suggest.

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:

The closest modern equivalent (as was my initial allusion) is *thick film*. http://66.102.7.104/search?q=cache:b0zDO-v-7n8J:

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. I ordered a 35 W carbon composition resistor in a TO-220 package. I have some TO-220 heatsinks lying around.

I am glad that you agree.

Yes. Exactly. I want it to pass the current along and then dampen it.

You are 100% right, it is definitely a wirewound POS.

I will play with my working bridge more over thanksgiving. I have a bigger DC power supply an will try to make an inductor out of low voltage winding of a transformer.

I have pulse-tested these resistors before. The peak pulse power rating is not much more than the average rating - its a thin film.

carbon composition resistors *are* still made. check out IRC, HVR to name a couple. We bought them from a japanese company, but I forget who.

Cheers Terry

cool!

I was doing some browing a while back, and found a whole bunch of white papers on nonlinear control and Liapunov's method - in the 50s/60s the US got very interested in russian control systems, and their analytical techniques.

smart move.

Cheers Terry

carbon composition.

yep.

OK

OK

max. Fair enough, lots of power so low impedance.

it will be. smash it to bits and have a look. you can (theoretically) get non-inductively wound wirewound resistors, where the wire zig-zags rather than spirals. I've never seen one though....

No worries mate :)

Cheers Terry

Thanks Terry. I bought this TO-220 resistor:

TDH35PR150J

Digikey part number TDH35PR150J-ND.

If it does not work, I will experiment a little more, I have my eyes set on something else as well. I appreciate your help. I am very eager to test that stuff under a little higher load and worse conditions.