My adapter is being stupid and I need help

Discussion in 'Wireless & RF Design' started by mik3ca, Apr 2, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    Now I have narrowed all my radio problems down to my power adapter.

    My power adapter is a "stringer" model BA-630.

    It outputs 6V, 300mA and it states its input is 117VAC 60Hz 5W.

    I cut the connector of my adapter off and replaced it with a battery snap connector so I can use it like a battery. The battery snap connector added maybe 1 inch to the total cord length. The total cord length that came with the adapter is about 1 meter.

    In my circuit, I have tried my own PI filter.

    I connected +ve to -ve through these capacitors that are connected in parallel: a 1mF, a 33nF, and a 4.7nF.

    then I connected +ve to a 2-pin large inductor that I made out of a 5-pin audio transformer. The other pin connects to the rest of the circuit that normally connects to +ve. The other pin alco connects to a 1mF capacitor which also connects to ground. a 33nF capacitor is tied in parallel with the 1mF capacitor.

    I just described my PI network. the 1mF capacitors are electrolytic and the rest of them are ceramic disk.

    It seems thatthe power cord functions as an antenna. If I have it in one position, I have constant hum. If I move the power cord to a different location, I don't get it the same. Sometimes, I get smooth white noise. It all depends on where I move the cord.

    How come my power cord is doing this, and how can I stop it from functioning as an antenna?

    Please help ASAP. Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The 1000uF caps are so large that they will have too much parasitic inductance & resistance to remove all but low frequency noise.

    The small caps are so small they'll only be effective at removing noise outside of the audible range.

    Try adding a 10uF and 0.1uF/100nF cap on either side of your makeshift choke.

    What's the voltage drop across your makeshift choke?

    Did you remove or short out the unused windings from the former audio transformer?
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You are trying to make a VHF RF filter but you used an AUDIO transformer?
    It has too much internal capacitance so it is not an inductor above about 1MHz.
    It doesn't have a high frequency ferrite core.

    A 1000uF electrolytic capacitor has too much inductance so it is not a capacitor above about 1MHz.

    Your "radio" picks up hum because it has only one tuned circuit and it has an AM detector.
    Mains hum radiates as AM. An FM radio has many tuned circuits, a limiting IF amplifier where the limiting removes AM and it has an FM detector.
     
  4. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    I also tried a standard radio, and it turns out that my laptop power adapter IS the culprit. When I switched outlets for the laptop power adapter, the reception greatly improved.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Merely a coincidence.
    Ditch the audio transformer.
    You could make a better choke by taking a 10-penny nail and winding 20 to 50 turns of #24 wire on it.
     
  6. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    why would that be better?
    I remember someone suggesting that I should use the largest inductance that I can afford. Isn't the total transformer inductance higher than the inductance of your suggested coil?
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You are NOT using a large inductor - it only looks like a large inductor.

    All inductors have stray capacitance. The inductor Sgt.Wookie suggest has vastly less stray capacitance than the audio transformer winding you are currently using.

    The audio transformer winding has such a high stray capacitance it passes the RF you want it to block. It is only inductive at audio frequencies. AudioGuru has already explained this concept herein.

    Similarly, all capacitors have stray inductance.

    Audio frequency components are called audio frequency components for a reason. Ditto for RF components. Use RF components to make an RF filter. It will work much better.
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Open a college savings fund, and provide better educational opportunities.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My clock radio uses a very cheap FM radio circuit. It tunes the signal about only 5 times.
    It has a limiting IF amplifier and it has an FM detector.
    It uses its power cord for its antenna and it does not pickup interference.
    It picks up Z103.5 very well but I am a little closer than mik3ca.
     
  10. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    I did an experiment last night. I made a transmitter to test with my receiver.

    Here's the circuit:
    [​IMG]

    I modified it so that R2 is 1K, R1 is 200K, and C1 and the microphone are connected in series instead of in parallel. my C1 is 10nF because 1nF might not give enough base. I might try 1nF later. I added a 150pF capacitor across R1.

    I tested my transmitter with my receiver, and this time, the receiver is running off a 9V battery without a PI filter. The transmitter is connected to my adapter. (6V transmitter to a 9V receiver).

    So far, I was able to transmit about 400 feet.

    If I were to use an inductor with a certain value instead of a transformer for the PI filter, what should it be?

    I want to order it.
     
Loading...