Resistor Info

Discussion in 'Archived Threads 2001-2004' started by Chris Tsutsui, Dec 6, 2002.

  1. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    I'm replacing some resistors on a transistor amplifier. I was wondering how I'd figure out what watt rating is on the resistors that are in there. The stripe color coding only reveals tolerance and ohms.

    The resistors are 1/8" long and beige/light brown colored. Is this 1/4 watt?

    Is it safe to go with a higher rated resistor such as 1/2W if it still fits?

    Another question is that some of the MOS transistors need replacing. They all say IRFZ48N on them followed by other numbers/letters. I was wondering if I ordered the part number IRFZ48N, whether it would be a direct swap. Or can the transistors be slightly different than eachother and have the same (IRFZ48N) tag on each of them?

    Thanks,

    I know that replacing these parts doesn't gaurantee the amp to work. But it sure would be nice to get it working for under $10.

    The amp is a car audio Alpine MRV-1507 I bought of the HTF hardware FS forum off Scott Falkler. It started to smoke after 5 minutes use and since I can't return it I was hoping to get it fixed. (It retails for like $600)

    I fear that if I take it to an Alpine repair center the cost will be high due to the damage. There's a quarter size hole burnt off the side of the amp and 6 transistors, and 12 resistors need replacing. I figured out the circuit wiring for the missing chunk and was going to re-wire it.

    Any thoughts?
     
  2. AaronBatiuk

    AaronBatiuk Second Unit

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    My thoughts:
    If you simply replace the components that were damaged during the 'burn out', then try to power it up again, it is likely to simply burn out the same way as before. You need to find the source of the problem and repair that. Now, it is entirely possible that, for example, one of the MOSFETs was damaged, and stuck 'on', causing the burn out, including burning itself up in the process. So in that case, replacing it (as you are planning to do) can fix the problem.

    As for the resistors, for the most part, size matters. Pick resistors of equal or slightly larger size. 1/2 watt resistors are substantially larger than what you describe. They sound like 1/8 watt to me. A common 1/4 W resisitor is about 1/4" long and they are typically beige.

    When replacing the MOSFETs, it is quite important to match them up, based primarily on gain (transconductance actually). This is especially critical for the ones that are ganged on each side of the amp. Often (certainly not always), there will be two (or more) of the same transistors connected in parallel, giving higher current capability. It is important that these are matched. If one has a higher gain, it will conduct more of the current, and will heat up more, increasing it's resistance, causing more heat, etc., until it basically self-destructs way before it should. Matching them helps ensure the current is loaded equally between them so one of them won't do this to itself.

    If you can buy matched MOSFETs, do it. Otherwise the only way is to buy way more than you need and measure them all, then buddy them up with others that measure the same. You can find out how to measure/match them by looking up some DIY amplifier designs/kits/plans, etc. on the web or in the library. They often explain how to build a simple circuit to measure and match the MOSFETs.

    Good luck!
     
  3. Dave Milne

    Dave Milne Supporting Actor

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    Aaron,
    Excellent advice about fixing the source of the problem first. Resistors rarely fail on their own. And physical size is indeed a good indicator of power rating.

    Matched MOSFETs are always a good idea for maximum performance. However, many amplifiers with paralleled MOSFETs don't use matched devices because of cost. The positive temperature coefficient of MOSFET's Rds is actually a good thing in this case. The FET that is drawing more current heats up, increases resistance, and draws less current. Remember the voltage across the paralleled devices is constant and so the current will follow the path (MOSFET) of least resistance. They're sort of self-matching in this regard. Bipolar transistors do exactly the opposite and are prone to thermal runaway as you suggest. Paralleled bipolars always employ emitter resistors to provide local negative feedback and counteract the runaway tendency.
     
  4. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Thanks,
    Yes the MOSFETs are wired in parallel, I can see so on the board. I think I will use 1/4W resistors if I can't find the 1/8W ones for the fix.
    I'll have to give things a try once I get the parts I need. The only thing now is finding a place that will sell the Mosfets without charging $12 for shipping.
    Any tips for finding the source? Would I look at the area where the fire started?
    Pictures of Amplifier
     
  5. Dave Milne

    Dave Milne Supporting Actor

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    Ouch Chris!! This baby cooked.

    IRFZ48Ns are 64A, 175C N-channel MOSFETs. This thing must have drawn some big current!

    Since you are running bridged mode, make absolutely sure no speaker leads ever touch vehicle ground. If this happened, it may have been the source of your problem.

    Do you have a schematic for this thing? If not, I'd try to get one. Since I can see the glass weave of the board, any traces in that area are destroyed. You want to make sure you can restore ALL connections.

    You might want to replace any electrolytic capacitors in the area --they don't like to be cooked. It's cheap insurance.

    If there's a convenient place to do it, you might want to temporarily put fuses in the supply rails to the outputs. That way, if you haven't fixed the inital problem when you power it up with new transistors, you won't destroy them, too. These amps probably run only a few hundred milliamps of bias current with no input signal, so a 2-3 amp fuse would protect the outputs while you get this baby fixed. After you're sure it's working with small signal, you can pull the fuses out and re-connect the rails directly. An easier but sometimes-not-as-effective alternative is to just fuse the +12V input. The problem with this is that the amp may draw a few amps at startup to charge the supply filter caps.

    Note that the tabs on TO220 MOSFETs are at drain (rail) potential. For big amps, this can be upwards of 100Vdc. Watch where you put your fingers when you power it up. And check to make sure that the transistor tabs didn't come into electrical contact with the heatsink. A lot of car amps rely on the dielectric strength of the heatsink anodize to provide electrical isolation. If it gets pitted or nicked, you're toast. Once you have things working, make sure you reinstall the transistor mounting hardware with thermal grease and appropriate insulators if they used them.

    Good luck!
     

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