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Transistor equivalent

Discussion in 'General Electronics Chat' started by Nikolai Petrenko, Oct 24, 2015.

  1. Nikolai Petrenko

    Nikolai Petrenko Member

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    I am searching something can replace these transistors:
    BF199, BF494, 2N4001, 2N4003, MPSA18
    And there are all I have:
    2N3904, S9012, S9014, S9015, S9018, 8050, 8550, C945, D965. But 8050 and 8550 seem by fit to make push-pull amplifer.
    All I need are for radio receiver or RF, light... detetors.
    I also want to know can 1N4148 replace 1N34 in crystal radio. I can't find 1N34, no stores I asked have 1N34 or other germanium diode. ( I can make 300ft antenna and good ground for strong signal ) ( 1N4148 heated or red LED in series with 1,5v cell will be good idea??? )
     
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  2. Mikebits

    Mikebits Well-Known Member

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    No the 1N4148 will not replace a 1N34 or 1n34A. What makes the 1N34 different is the low forward voltage drop, on the order of .3vdc, the 4148 is around .6vdc. So for your crystal radio you would need a germanium. I don't know why you are having trouble finding the 1N34 diode. Using Google, this was the first link on my search results.
    https://www.google.com/search?q=1n34&ie=utf-8&oe=utf-8#q=1n34&tbm=shop
    For your transistors, you can just compare datasheet specs, have you tried?
     
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  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The BF199, BF494, MPSA18, 2N4001 and 2N4003 are obsolete and have not been made for many years.
    1N34A germanium diodes are old but are still made and are available everywhere in The West. A 1N4148 is completely different but try one in your crystal radio.

    Here in The West there are so many radio stations that a crystal radio is too simple and picks up a few stations at the same time. A crystal radio has one tuned LC circuit. A REAL radio has many LC tuned circuits for good selectivity.
     
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  4. dave

    Dave New Member

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  5. Nikolai Petrenko

    Nikolai Petrenko Member

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    2n3904 might can replace most of them, but I not sure. Heated 1n4148 had similar characteristics like 1n34. I think can made an stable heated 1n4148 by wind few turns of nichrome around the diode than make a clay coat for it. The heat coil feed by battery.
    I also hear red led in series with battery can provide louder sound than 1n34 in some sites like techlib.com....
     
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  6. Willen

    Willen Well-Known Member

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    Hi Nikolai,
    You are living in the same place like I am living.

    For VERY high frequency RF you can find 9018 (around 1.1GHz) which is same as BF199). For general RF purpose (around 250MHz) use 2N3904 fast transistor, for audio use S9014 etc and for little more powerful audio etc, 8050 and 8550 are needed.

    You cannot use 4148 for crystal radio. You can find 1N34 in VERY VERY old AM radio. Or better solution is to search a black 1N5819 diode. Break a simple chinese cellphone power supply (charger), you can get it at output of the supply. I got it works better than 1N34 (louder audio). Or you ca use a condenser mic too. Here is my few experiment for rectifier of crystal radio-
    www.geopodium.com/files/Rajkumar/index.html

    Sure you got your all problems solved! :)
     
    Last edited: Oct 25, 2015
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  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    A heated 1N4148 diode will come down in forward voltage, but it takes maybe 100 degrees C to come down by maybe 0.25 volts bringing it down to maybe 0.35 volts.
    You can also try biasing it with a little DC to help get the dynamic forward voltage down. It's hard to adjust though, maybe with a DC feed back circuit.
    A red LED biased with 1.5v might come down to 0.2 volts but i've never tried that.
    They also make fast low voltage Schottky diodes for small signal high frequency use.
     
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  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A datasheet for the 1N34A germanium diode said its forward voltage is typically 1V at 5mA which is much too high. The current in a crystal radio is much less then the forward voltage will also be much less so I found the datasheet (47 years old!) of an OA90 germanium diode that was used in a TV because TV video is (was) AM modulated.

    Germanium diodes have extremely low capacitance. The capacitance of a 1N5817 Schottky diode is more than 100 times higher.
     

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  9. Mikebits

    Mikebits Well-Known Member

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    Clay and Nichrome, geezo, I don't understand why you can't get the right part as I showed you they are available on Ebay. Perhaps you are being held prisoner in a Gulag somewhere in Siberia in which case, you may be interested in building this:
    [​IMG]

    Good luck with your escape, freedom awaits you :) Okay, I am just kidding. The whole idea for using the 1N34 diode was that you could build a powerless radio due to the .3 vR drop of the diode (I don't understand why the datasheets say 1v?). I built my first crystal radio when I was 10 and it worked. The trick was winding the antenna. I would just try and get the right diode, and if you still can't get it, then maybe you should escape, I hear the weather in Canada is lovely this time of the year:)

    [​IMG]
     
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  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A diode has internal resistance so its forward voltage gradually increases as its current increases. The germanium diode I showed has a typical voltage of 1V at 10mA but is 0.35V at 1mA and is only 0.2V at 100uA.
    The 1N4148 silicon diode typically is 0.73V at 10mA, 0.62V at 1mA, 0.5V at 100uA, 0.38V at 10uA and only 0.27V at 1uA.

    The original poster cannot buy ordinary electronic parts because he is a kid living far from a city in Vietnam.

    58 years ago when I was a kid in a Canadian city I made a portable AM radio using a Japanese superhet AM radio module and an audio amplifier module. I made a wood case for it.

    EDIT: I am in Canada (not the OP) and the weather is lovely today, no frost and no snow (yet). The trees are dropping brightly colored leaves all over the place.
     
    Last edited: Oct 25, 2015
  11. Nikolai Petrenko

    Nikolai Petrenko Member

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    Yes. I will try all stuff to find what is the best:
    Razor blade diode
    1n4148
    Red led
    596 JFET
    And unknow code schottky diode from portable lamp inverter
     
  12. Willen

    Willen Well-Known Member

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    Performance depends on your antenna and earpiece too. I used my entire metal roof and it was large, so I got VERY efficient antenna so I easily heard AM radio with many parts. However I was unable to get audio from silicone diode like 4148. I just used an cellphone earpiece (plays little louder than others) and got loud audio like a volume 1 of my cellphone. AM station is 20KM far.
     
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there Willen,

    Hope you are doing ok over there.

    The way he wants to try the 1N4148 diode is by heating it up first. An Si diode has a negative temperature coefficient so the hotter it gets the lower it's forward voltage becomes. He will have to heat it up quite a bit though.

    audioguru:
    Thanks for the weather report :)
     
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  14. Willen

    Willen Well-Known Member

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    Wow, pretty interesting thing I saw today! I never thought that the a diode's junction voltage drop (forward voltage) is depended to temperature in such GREAT way! First I connected a 1N4148 diode in a crystal radio and heated a lead of the diode with soldering iron almost 10 seconds. I got the the AM radio working after almost 5 seconds with small volume and got it increasing second by second. At last it was working same as 1N34 or any schottky diode. (However it's not a solution/alternative for crystal radio. To get the radio working the diode need to heat up in such bad way, the diode would not be able to survive more minutes.)

    Another test is to measure its voltage drop by applying heat. I connected the diode in DMM's test probe and started to heat the lead of the diode. (I have no temperature meter.) I heated its one lead almost 45 seconds (I was thinking the diode is going to blast, but it's fine still). Voltage drop was 612mV at first and got around 250mV just after around 10 seconds of heating. Then decreasing voltage drop was not fast like till the 10 seconds. Around last seconds I got just 160mV hardly and I stopped heating (I do not want to waste parts actually). So nice part is to make temperature sensor! :)
     
    Last edited: Oct 26, 2015
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The change in forward voltage of a silicon (not silicone which is a synthetic rubber) diode when its temperature changes is what causes a poorly biased transistor not to work properly (it is a temperature sensor).
    The base-emitter forward voltage is 0.7V isn't it? Sometimes. Each transistor is different, the voltage increases when the current increases and the voltage decreases when the temperature increases. Then a transistor with no series emitter resistor and its base biased at 0.7V will conduct too much and be saturated when it heats and might get cutoff when it cools. The transistor amplifies the changes in the forward voltage of its base-emitter diode.
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Willen,

    Nice experiment there :)

    The diode can probably take about 175 to 200 degrees C so maybe 150 deg C wouldnt be too bad, heated up slowly. The reverse leakage will also increase, from maybe 10na to 10ua (1000 fold). The forward voltage will also be less for lower forward currents. A general purpose tester probably puts out more current than when used in a radio.
    Purposely changing the temperature higher and lower could act as a sort of squelch adjustment.
     
  17. Nikolai Petrenko

    Nikolai Petrenko Member

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    I have 300ft antenna but can longer (to 500ft ) . I don't have crystal earphone so I made one that attach piezoelectric diaphragm got from alarm chrono watch to a headphone. It has impedance about 2000Ω. but I don't like sound quality so I made a preamp to connect the radio to my diy amplifier I asked in previous thread.
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A crystal earphone has a very high impedance then it does not "load down" the low sound levels from a crystal radio. A piezo transducer squeaks awful sounds. You forgot to say the input impedance of your amplifier.
     
  19. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    An old RED LED is 1.2V
    An IR LED is 0.8V threshold
    Silicon LED 0.5 to 0.6
    Schottky diode 0.1V is best bet, Typical examples are the 1N5711 or BAT41.
     
  20. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    I live within 20km just North of Toronto GTA and agree... except we are getting remnants of Hurricane Patricia near Lake Ontario with 30 to 70mm of rain tonight.
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    AM radio sounds awful. Its high frequencies are cut above about 2.5kHz in an normal radio so the radio stations add pre-emphasis (treble frequencies boost) so that it sounds like it has a frequency response up to about 5kHz in a normal radio. We can hear to 20kHz if our hearing is not old or damaged. The "A" in AM is the word AMPLITUDE. Electrical sparks (car spark plugs or electrical switching) and lightning are heard.
    Make an FM radio instead.
     

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