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Thread: Beta Matching!

  1. #1

    Beta Matching!

    Hi All,

    I am running a 4 transistor ss sg. Two of the devices get warm and the other two are cold. Is this a case of unmatched devices? I'm thinking so as I have not done any matching on any of my circuits yet but will be on my next build. Anyway, I am assuming that the beta of the two devices that are getting warm would be close and same for the two that are cold. Also then, the ones with the higher beta will be doing most of the work?

    This gets me thinking though....the beta matching has to do with getting all the devices switching at the same time correct? So how could two of them be doing most of the work when all four of them are switching. I suppose this could be that they are fighting one another. Meaning some are switching on slightly before the rest creating a crazy situation in the coil where not all the windings are in-phase.

    I have been having luck using mje13009 at $1 each. Roughly how many would I need to purchase to get 4 matched mje13009's??

    I am just having trouble understanding how the transistors could be switching at slightly different times even though they are all being driven by the same trigger signal. But I suppose that transistors with higher beta will turn on quicker than those with a lower beta.

    Just looking for confirmation here. Thanks.

    Thanks,

    Joster

  2. #2
    Senior Member Tom C's Avatar
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    it is not about when they switch but how far they open. if you look at the data sheet on a transistor there is a curve as to how much current on the base opens the junction, it varies a little bit to a lot. does nt matter on a single transistor but with multiple ones it does. on class A amps that is one of the main things that makes them class A. imagine 4 WATER VALVES all of them have 2 lbs of pressure applied to open the valve, but 2 valves open a quarter of a turn and 2 valves open a full turn, the full turn valves have more water flowing thru them even though they all have the same pressure applied to the valve handle.

    not a perfect picture.... you are trying to get all transistors to open their bases the same amount for the same milliamps coming off the trigger.

    Tom C


    experimental Kits, chargers and solar trackers

  3. #3
    Quote Originally Posted by Tom C View Post
    it is not about when they switch but how far they open. if you look at the data sheet on a transistor there is a curve as to how much current on the base opens the junction, it varies a little bit to a lot. does nt matter on a single transistor but with multiple ones it does. on class A amps that is one of the main things that makes them class A. imagine 4 WATER VALVES all of them have 2 lbs of pressure applied to open the valve, but 2 valves open a quarter of a turn and 2 valves open a full turn, the full turn valves have more water flowing thru them even though they all have the same pressure applied to the valve handle.

    not a perfect picture.... you are trying to get all transistors to open their bases the same amount for the same milliamps coming off the trigger.

    Tom C
    thanks now i get it. I just pulled out the 4 470 ohm base resistors and tested them all and they are all over the place too. I figure what I'll do is get a bunch of 1% tolerance resisters and match them from there.

    Any idea how many transistors I will need to get 4 matched devices...I understand it's sort of luck of the draw but from your experience how many do i need to buy?

  4. #4
    Senior Member Tom C's Avatar
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    we buy them in lots of 500 and match them... never bought less. if you want them already matched you can get them from teslagenx. yes they are more expensive but they are power matched and you don't have to buy 500.....

    Tom C
    Last edited by Tom C; 10-05-2014 at 04:51 PM.


    experimental Kits, chargers and solar trackers

  5. #5
    Quote Originally Posted by Tom C View Post
    we buy them in lots of 500 and match them... never bought less. if you want them already matched you can get them from teslagenx. yes they are more expensive but they are power matched and you don't have to buy 500.....

    Tom C
    Ok thanks...thats good to know. In your experience how many of the 500 do you have to test on average to get 8 matches?

    I did some poking around on the "all about circuits" forum and got this reply which I thought was interesting and wondered what you thought about it...

    "You don't want to go with matched transistors as that would be difficult to do and expensive. You should be able to design the circuit with some emitter negative feedback so the value of the beta has only a minor effect on circuit operation. That's the normal way to design a circuit. If you post more details about what you need (frequency response, current values, coil inductance, voltage values, etc.) someone on this forum can likely help you with the design."

    here is the link to the thread

    http://forum.allaboutcircuits.com/th...tching.102176/

  6. #6
    Quote Originally Posted by Joster View Post
    Ok thanks...thats good to know. In your experience how many of the 500 do you have to test on average to get 8 matches?
    Joster, Buying more transistors a time reduces the number of transistors we can't match. Start with a tube of 25 and you may get lucky and find 1 or more matched sets of 4, or you could end up with no matched sets. There is a reason we buy 500 or more at a time...the price of the transistors should give you a good idea of the loss we find but it is also a time consuming process.

    Quote Originally Posted by Joster View Post
    "You don't want to go with matched transistors as that would be difficult to do and expensive. You should be able to design the circuit with some emitter negative feedback so the value of the beta has only a minor effect on circuit operation. That's the normal way to design a circuit. If you post more details about what you need (frequency response, current values, coil inductance, voltage values, etc.) someone on this forum can likely help you with the design."
    You are free to design the circuit however you wish. Yes, it will run without matched transistors, but the SG isn't just a coil banger. It's designed as an audio amplifier, without matching the transistors, you reduce the efficiency of your SG. Our suggestion is build it like John Bedini says; after understanding how it works, then start making modifications.

    Erik

  7. #7
    Quote Originally Posted by ErikN View Post
    Joster, Buying more transistors a time reduces the number of transistors we can't match. Start with a tube of 25 and you may get lucky and find 1 or more matched sets of 4, or you could end up with no matched sets. There is a reason we buy 500 or more at a time...the price of the transistors should give you a good idea of the loss we find but it is also a time consuming process.



    You are free to design the circuit however you wish. Yes, it will run without matched transistors, but the SG isn't just a coil banger. It's designed as an audio amplifier, without matching the transistors, you reduce the efficiency of your SG. Our suggestion is build it like John Bedini says; after understanding how it works, then start making modifications.

    Erik
    ok thanks...that was what I was looking for...on a good day 4 matches out of 25

    Are your matches exact or is there a small tolerance window?

  8. #8
    Quote Originally Posted by Joster View Post
    Are your matches exact or is there a small tolerance window?
    Ours are typically matched to 1 percent or less. It's very unlikely you will achieve that low tolerance with a batch of 25 transistors, but again, you could get get lucky.

    Erik
    Last edited by ErikN; 10-12-2014 at 12:20 PM.

  9. #9
    ok! Ill try my luck with 25 thanks alot for the info...I'm sure having matched base resistors and output diodes would be awesome too. That should be easier at least with the resistors. Do you guys match the diodes too? If so what is your luck with that on a good day?

  10. #10
    found 100 tip41c's for $16 bucks

    I know its just 6A Collector Current and 100V collector/emitter but I think it will do for now anyways. I do not have alot of dough right now to play with.

    Just curious what people think of using the tip41c

    there are some 2n3055's on ebay for a good deal but they are only 60 Volts collector emitter. They do have a 12A collector max current which is good.....

    What should I go with?

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