Are these 2 transistors close enough?

Discussion in 'General Electronics Chat' started by slashmaster2, Apr 15, 2019 at 12:17 PM.

  1. slashmaster2

    Thread Starter New Member

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    Trying to replace this blown transistor from a dc motor speed controller. Looks like you can see the bulge of where it puffed up when it blew. Is my new replacement close enough?
     
  2. slashmaster2

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    Just realized because of the bulge in the one on the left you might not be able to read it, but they both say D635. It's what is underneath that makes me not too sure if they are the same.
     
  3. Dodgydave

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    Reloadron and slashmaster2 like this.
  4. Ylli

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    As long as it was purchased from a reliable source (i.e. not ebay).
     
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  5. mvas

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    The code underneath is a Date Code.
     
  6. Tonyr1084

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    Interesting thing about date codes: Well, not about the date codes themselves, but more about people understanding them. Last year I did some work for a Naval contractor. Worked as an incoming inspector. Did a lot of mechanical, electromechanical and electronic inspection. A set of boards were rejected because the inspector who inspected them didn't understand the markings. Her thinking was that ALL the transistors of a particular type should read EXACTLY the same. I don't recall the base number of the transistor but its base part number was the same. The only thing different from component to component was the date code. It was rejected and sent back to the supplier to be reworked. Unbeknownst to the company I was working for the supplier merely sat on them for a month, then sent them back. This time they came across my desk. The original inspector had family issues back home and had to leave the job for a while. When I researched the reject, along with photographs I could see the supplier had performed NO rework. I could clearly see and understand why the inspector rejected them, however, knowing about date codes, I accepted the boards with no further questioning.

    It came about later that the supplier complained about the mark against them and how they made no changes to the board and that the board was accepted as is the second time around, I was called to answer for my reasoning. After all, not only did the original inspector reject the boards - the engineers also agreed. So it was looking like I was on the hook for accepting something that was bad. Until I explained the date code. Something the original inspector was unaware of. And apparently the engineers as well.

    There ARE subtle differences in part numbers. While you may find a D635 to be the same as another D635, other designations such as H, HS, LS, and so forth, have meaning to the part. It can designate a percentage, a speed, a reliability, a working temperature range, and many more things. That's why it's important to read and understand the data sheet. You can use a high reliability part on a cheap child's toy but you can't use a low reliability part on a commercial jet liner or military equipment where someone's life will absolutely depend on the product to do its job. However, in your case, it appears to be an NPN Darlington transistor, its ability to perform its original job is not likely going to be compromised. However, I wouldn't take that to be the final word either. This is just something all people working with electronics should know. Those who don't know just don't know yet. Life is about learning new things, and I'm always learning something new too.

    Bottom line, it looks like the replacement will be just fine. But none of us can say for certainty exactly whether that is absolutely true or not. Remember, it's always OK to replace a component with a more reliable replacement, but depending on the use, going cheaper may mean failure sooner than later. Since the original transistor blew, there may be more problems with the circuit you're repairing than you are aware of. Simply pushing a new part into a problem circuit may easily result in a new blown component. Full diagnosis is always advised BEFORE you start throwing parts at the problem.

    Good luck with your repair.
     
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  7. slashmaster2

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    Thanks guys! I did buy it from ebay but at least now know bottom code is date code. Trying to use this to run a small 36 volt motor in a projector. Seems this transistor needs 15-20 volts at transistor base to run motor at original speed, but my motor controller is only giving me .9 volts. Is something wrong with my motor controller?
     
  8. MaxHeadRoom

    Expert

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    Do you have any info on the controller or nature of?
    Max.
     
  9. slashmaster2

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  10. MaxHeadRoom

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    It is best to keep to one relevant post, it is a forum no-no to have two running on the same subject.
    See if you can reverse-engineer the base input drive circuit.
    Max.
     
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  11. Tonyr1084

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    The transistor base is current based. Not voltage.

    Is the controller putting out 0.9 volts with the transistor in place or with the circuit open?
     
  12. slashmaster2

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    It's with the circuit open. To be more exact, I'm getting .906 volts with circuit open and .897 when it's hooked up to the transistor.
     
  13. Tonyr1084

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    It's sounding to me like the source (where the 0.906V comes from) may have been hurt when the original Tx'r blew. You need sufficient current to turn that transistor on. If your source is not capable of doing that then you're putting new shoes on a lame horse. I'm not the expert on this, but I would expect to see a higher voltage from the source.

    As I said before, there may be more problems with the circuit than you're aware of.
     
  14. slashmaster2

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    At least now I know there is probably no transistor with enough gain to do that. Seems like there is a resistor that brings power to the source then there are 3 little transistors and a resistor that bleed off voltage from the source. Maybe I should see what happens to the source voltage when I remove one of them?
     
  15. Reloadron

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    Jan 15, 2015
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    Working from post #3 this is your transistor.which seems logical for your mentioned application. Just about anything you could want to know, including saturation data is in the data sheet. With a proper heat sink this darlington can drive 7 amps with a base current of 0.7 amp.

    Ron
     
  16. Tonyr1084

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    @slashmaster2 , And that's the key - base current, not voltage, is what turns a transistor on. Also notice the ratio; 10/1. It's a typical ratio and basic rule of thumb. But as Ron said, everything you need to know is in the data sheet.
     
  17. slashmaster2

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    Thanks Ron! Thanks Tony! I'm not too good at understanding those charts yet. So in other words I should be using the DC Amps part of my multimeter to measure that transistors base?
     
  18. Reloadron

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    If you want to know the base current then yes, but keep in mind that when we measure current the meter is placed in series with the load. You do not want to setup a meter to measure current and place it between ground (for example) and in this case the transistor base.

    Since you are just looking to replace a transistor which has failed I guess I don't understand why all this measuring? The more you try tinkering around the greater the chances something else can go wrong or very wrong.

    Ron
     
  19. slashmaster2

    Thread Starter New Member

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    Yeah I sure do agree!. I should have mentioned earlier that this motor controller worked perfectly fine before then it shorted out on it's own chassis and popped a transistor when I plugged it in. All because I didn't bother to put in all the screws for a test. Then I blew out 2 more transistors while messing with it. But the good news is I've studied all the paths of the circuit and think I know how it works now. There is that main transistor we talked about, a little transistor that bleeds off voltage from its base and 2 other transistors that bleed off voltage from the same base in 2 stages. So it's all about taking away voltage and not adding to it. I should ignore all capacitors in the circuit because they just prevent electrical noise don't they?
     
  20. Tonyr1084

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    On an NPN transistor you need a current going INTO the base lead. If your power source is putting out 12 volts (purely an example, don't take that number literal unless your supply source IS 12 volts) and the base forward voltage drops (typically) 0.7 volts then calculate the necessary resistor needed to achieve 0.7 amps.

    12 V - 0.7 V = 11.3 V
    11.3 V ÷ 0.7 A = 16.4 Ω

    You should have a 16 ohm resistor before the base of the transistor. Since in one of your measurements you reported seeing 0.9 volts (lets just call it 1 volt) your supply is not doing its job. If it's a typical transformer with rectifier circuitry then there are a few more calculations. First, a 12 VAC transformer should be able to put out 16.97 volts DC after rectification and filtration. Depending on the configuration you can be dropping 0.7 to 1.4 volts through the rectifier circuit. Lets assume you're using a full wave bridge rectifier. That'll drop 1.4 volts typically.

    16.97 V - 1.4 V - 0.7 drops the voltage to 14.87 (14.9) volts.
    14.9 ÷ 0.7 = 21.2 Ω

    Under that assumption you should be seeing about 15 volts at the base of the transistor through a 21 ohm resistor. Since you're only seeing 1 volt - I'm suspicious your rectifier circuit is faulty. Again, I've assumed a starting AC voltage of 12 volts. I'm dubious to believe replacing the transistor alone will solve all problems. The cap's before and after any regulators are there to smooth the DC voltage. If you don't have a regulator then the cap after the rectifier(s) will smooth the supply voltage and reduce noise.

    As yet we haven't seen a schematic of the circuitry. That'll be critical in our ability to correctly guide you in the repair of this project. If you can, draw on paper the circuit as best you can. Then take a picture of it and upload it to this site using the "Upload a File" right next to the "Post Reply" and select that picture. That way we can determine if just simply changing the transistor will be sufficient to achieve your goal.
     
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