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Dummy Load II

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by jocanon, Jan 12, 2013.

  1. jocanon

    jocanon Member

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    Hold on a minute...if my memory (above) is right...does our conclusion change now that we are decreasing the ohms and increasing the watts from 5 to 10 of the current sense resistors...meaning do we now need to add a fuse(s)?

    I must be going crazy...I am talking to (quoting) myself...
     
  2. schmitt trigger

    schmitt trigger Active Member

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    "correct me if I am wrong, I think we decided the current sense resistors actually acted as fuses since they would blow first. "

    Fuses are THE safety devices of last resort, when everything else fails, to prevent smoke and fires. And a properly rated fuse will blow up faster than any resistor.
    Additionally, fuses are inexpensive and easily replaceable.

    I would not operate such a high power tester without a fuse. Just my two rupees on the subject.
     
  3. jocanon

    jocanon Member

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    I'm sold...so where do we put the fuse, right in line with the load device under test (DUT)? Or I wonder if it would be a better idea to have a smaller one for each FET.
     
  4. dave

    Dave New Member

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  5. schmitt trigger

    schmitt trigger Active Member

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    "Or I wonder if it would be a better idea to have a smaller one for each FET. "

    I would use an individual fuse per FET. Once that you decide how much current each FET will dissipate, I can guide you thru the fuse selection process.
     
  6. jocanon

    jocanon Member

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    I agree, seems this would be best... but () or ronv, help me remember, there is something nagging me in the back of my mind that we went down this road before and there was some reason, or something that was a hold up...sorry I can't be more specific, it is just a nagging thought in the back of my mind and I don't remember why. Was there some reason we didn't put fuses in the original schematic?
     
  7. ()blivion

    ()blivion Active Member

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    The good thing about forums, is they keep records. And better yet... they have a search function :) Start at *THIS* post, and read until post #304.
     
  8. jocanon

    jocanon Member

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    Yeah, thanks for digging that up...it's my turn to be lazy now ;)... 304 pretty much summed it up.
     
  9. ronv

    ronv Well-Known Member Most Helpful Member

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    Interesting discussion.
    I'm no expert on fuses but I don't think I have ever seen one blow without another bad part. It might have saved the resistor from burning up - and that may make it worth it, but the FET would still be a goner. The problem is they take time to blow and to make them meet their rating usually means the time is to long to protect a transistor.
    For example a 6 amp fast blow will hold its rated current for 4 hours, but may last 10 ms at 240 amps.
    I'm a little suprised the power supply over current didn't shut it down, but it could be the resistor was open before it could trip.
    We can add more FETs in parallel. No problem.
     
  10. ()blivion

    ()blivion Active Member

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    I've seen it 50/50 myself. But usually when just the fuse would be blown, it was either to conservatively rated, or the system would blow some time later after it was replaced. We could go with a crowbar + fuse? Seems like an active over current protection would allow us to fix the problems with fuses. And fuses would make up for the weaknesses in active over current.

    I think the main worry we should address is preventing fire, not really save transistors. That is just a secondary concern, though we should do it too if we can.
     
  11. jocanon

    jocanon Member

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    I agree with that statement.
     
  12. ronv

    ronv Well-Known Member Most Helpful Member

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  13. jocanon

    jocanon Member

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    So these would be an additional component, or replace an existing component?
     
  14. Mr RB

    Mr RB Well-Known Member

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    I would not be happy with that. Coming from an industrial electroncs background where things were always built to last, we were taught 50W is about the reliable max for TO-220 package and 80-90W for TO-3 pack (which are absolutely massive compared to TO-220).

    Modern TO-220 have pushed the W per package up seems like every year they are claiming higher wattage, but I still think >200W per TO-220 is not a good idea. I wonder how much is "manufacturer's enthusiasm" or if they just don't care that something blows up after a few months of use? Like light bulbs... ;)

    What temperature is the water exiting the pipe? The last FET in the chain of 8 on the pipe is being "cooled" into that hot water, so is getting "cooled" by hot water that already has >1500W of heat in the water! To me that alone indicates a poor design on 8 paralleled FETs.

    Anyway I probably have a different design philosophy to most, so I will shut up. :)
     
  15. jocanon

    jocanon Member

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    You know, when we were first designing the dummy load I was half kidding but thought maybe it could double as a tankless water heater ;)...Fortunately for the dummy load's cooling properties however I was disappointed in that hope...even with the dummy load at max, the water exiting the pipe is almost nearly as cold as the water entering the pipe. While I have not actually measured the temperature, by touch alone I can say the difference in temperature is unperceivable. There is just too much water moving through the pipe too fast for it to really heat it up at all. Now if I turn the water off, it's a different story, that copper pipe will heat up in a hurry! I have the temperature probe at the end of the pipe where the water exits glued on with thermal transferring glue as close as I can get it to the last FET and once I turn the water on the temperature drops almost instantly to about 20C when on full load (full being about 1100 watts). Also, when I had to replace one of the FETs I did not get a good solder joint and noticed that the FET started smoking...I was trying to take a short cut and use my soldering iron...anyway, I ended up taking it out and using the torch to really solder another one on properly and then it was fine. Key is to get a good solder joint/connection to the copper pipe for transference of heat and then they seem to keep cool enough, at least they have for about 6 months of pretty heavy use. I have yet to have one go out on me (except the one I shorted). At any rate, as long as the dummy load works for my purposes I am happy with it. Now if I were making these for sale it would be a different story...since I am only making the dummy load for my own use I don't have to worry about it passing UL or anything like that ;).
     
  16. schmitt trigger

    schmitt trigger Active Member

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    "I think the main worry we should address is preventing fire, not really save transistors. That is just a secondary concern, though we should do it too if we can. "

    Yes safety is the main point.
    Now, with a properly rated fuse, the transistor could be saved too. Mosfets have a huge Safe Operating Area, and can sustain substantial current for short periods if properly heatsunk.

    The fusible wirewound resistors is an option. I was only suggesting fuses because they are easily replaceable and cheaper than a power resistor.
    Eventually, the OP will be the one to decide which path to follow.
     
  17. jocanon

    jocanon Member

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    I don't know, seems to me it's not going to hurt to put them in it only might help so why not? I am not concerned about the couple extra dollars it will cost, I mean we are not talking about $100's of here.
     
  18. ronv

    ronv Well-Known Member Most Helpful Member

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    Hi Mr. RB,
    Your absolutely right, heat is the bad guy. I worked for a disk drive company for a while and we built about 75,000 a day. It was easy to see the difference in failure rate between burn in and not burn in, but of course there were a lot more components than we have here.
    Here is a nice write up.

    http://www.google.com/url?sa=t&rct=...88xoHbmjqmVbmMfuhNjwRMA&bvm=bv.41018144,d.cGE

    The fusible resistor would replace the .13 ohm. They are made with a kind of ceramic filler that doesn't flame like the silicone one we were using.

    Trigger, Lets not give up one the fuse yet. Lets see if we can find a solution to make it work.
    We may have trouble getting the data we need to do it but we can try. ()blivion has lots of power supplies so he might be able to test our ideas. I think the basic problem with the fuse proteting the transistor is the low resistance of the circuit. So if the FET happens to turn all the way on like it did when Jeremy shorted the gate to +12 the current is only limited by the .13 ohm resistor and the output cap in his big power suppy.
     
  19. ()blivion

    ()blivion Active Member

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    Going with this thought...
    Rds-on for the FETs in question is 0.018 Ω, that would take 129 or so amps to reach 300 watts power dissipation in accordance with I[SUP]2[/SUP]R. The total circuit resistance with an FET fully on is 0.13 + 0.018 Ohms, or ~148 milliohms. Thus the max current is 24 Volts divided by 0.148, or ~162 Amps. Which is enough over the limit to be a problem. :( We could add in the stray resistance of the wiring, but that isn't a good idea as it is probably somewhat inductive. If we use smaller value current sense resistors, the problem will be even worse.

    Again, a full crowbar would probably allow us to side step the problem because it would let us sense over current when it happens, and then try to share it across all the FETs temporarily. This system wide over current could be made to pop a single large main fuse, or even better, a resetable breaker. On the flip side of the coin, a crowbar circuit is more parts added and more designing that we would have to do. It also has the unsavory effect of short circuiting the system for a small time. So I fully understand any reluctance from the group to go this route.

    I don't hate the idea of using the resistors as the fuse either mind you. It's nice to make parts dual propose, and if they were intended to do it then even better. The resistor is where all the power will go if a FET shorts anyway, that should virtually guarantee that it will blow. The only problem with this situation is if just one FET goes belly up, it may be a while before jocanon notices the discrepancy, as the system as a whole should still limp along. This could lead to improperly tested PSUs, or the rest of the dummy load bearing a larger part of the work than we have designed. Worst case scenario the old possibility of a cascading failure comes back.
     
  20. schmitt trigger

    schmitt trigger Active Member

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    Ok, fuse issue is settled.

    How about the loop stability...have you decided upon a scope yet? Check E-bay also, there are some used scopes at bargain prices.
     
  21. jocanon

    jocanon Member

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    I was thinking I would build it and then IF it is still unstable with the mods that ronv made I will then look into getting a scope. I think the only risk in that strategy is I may have to do some re-work...but if it is stable I may be able to say the cost of scope.
     

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