Designing a receiver protection transformer.

Discussion in 'Homebrew and Kit Projects' started by SP4IT, Oct 5, 2021.

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  1. AI5DH

    AI5DH Ham Member

    Just how small would such a transformer be? Could you even see it clearly without a magnifying glass?
  2. SP4IT

    SP4IT Ham Member QRZ Page

    Magnetic flux depends on the number of turns and amps. If you need more flux (to saturate the core) at a given power level you need more turns.

    It also possible to add DC current to magnetise the core so it saturates faster.
  3. SM0GLD

    SM0GLD Ham Member QRZ Page

    Yes they are small.
    Array Solutions are using the MCL T16-1 transformer.

    MCL T16_1.jpg
  4. SP4IT

    SP4IT Ham Member QRZ Page

    If anyone found this thread looking for rf transformers and needs parts like this I suggest to look for Coilcraft make rf transformers like PWB-16-BLB. Electronic parts distributors (mouser, Farnell) have them for about $5~$8. While the parts mentioned previously seem to be pretty hard to find.

    However, using such parts for over power protection would require possibly destructive testing. Datasheets specify 0.25W as their power handling capability, but there is no mention what happens when more power is applied. Does the core saturate? Does it overheat first? Perhaps windings melt first? No idea.

    If one is looking for a receive side impedance matching transformer these are pretty nice.

    For overpower protection my main requirement is ability to withstand high power levels for a short amount of time(milliseconds) without damage. For example a transformer that saturates at 10W, but can survive short pulses of 1.5kW. Or one that let's through 1W, but repeatedly survives few ms of 200W.

    Perhaps the only way to meet the above requirement is to oversize the transformer and pre-saturate the core by sending DC through a winding. This is not ideal, because it stops working if dc power disappears, but I'll probably go down that route unless someone has a better idea :)
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    If you have to use a bigger core just to be able to handle high power peaks without destruction then you need more turns to meet your saturation criteria.
    With more turns your transformer will suffer in bandwidth.
    Ferrite cores saturate at about 300 to 400mT so you have to find out the number of Ampere-turns to reach 400mT on the core of your choice.

    B = ( AL * N * I )/Ae
    N*I = (Bmax * Ae)/AL
    Last edited: Oct 13, 2021
  6. AI5DH

    AI5DH Ham Member

    You are preaching to the choir. I asked a question I knew the answer to.
  7. W1BR

    W1BR Ham Member QRZ Page

    You said you wanted backup protection and was presented an option. Must protection devices at RX power levels are sacrificial in nature.
  8. AI5DH

    AI5DH Ham Member

    One tried and true practice commercial radios use is an RF Power Transistor as the front-end RF amp. You are correct, RF front-end is like a Surge Arrestor, made to be a sacrificial lamb front-line soldier.
  9. SP4IT

    SP4IT Ham Member QRZ Page

    Great. Please confirm what the terms stand for.

    N - number of turns
    I - current
    AL - length of coil?
    Ae - relative/absolute core permeability?
  10. SM0GLD

    SM0GLD Ham Member QRZ Page

    N = turns
    I = current
    AL (should be Al I think) = You will find it in the core datasheet. This is a value expressed in nanohenry to be used to calculate inductance / turn.
    Ae = Effective core Area. This is also to be found in the datasheet.

    Example (big core)
    Al = 400
    Ae = 43mm sq.
    I = 3A
    N = 10

    B = (400 * 3 * 10)/43 = 279mT.

    Lets calculate amp-turns at Bmax = 350mT

    N*I = (Bmax * Ae)/Al = (350 * 43)/400 = 37.6 amp-turns.

    using 10 turns this core will saturate at 37/10 = 3.7 A peak.

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