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tn-transformer Transformers

common types of small transformers, (b) is a low-voltage transformer found in power supplies

primary vs. secondary

  • primary winding: input voltage \(V_p\)
    • turns: \(N_p\)
  • secondary winding: output voltage \(V_s\)
    • turns: \(N_s\)
  • \(V_S = \frac{N_S}{N_P}V_P\)
  • \(I_P = \frac{N_S}{N_P}I_S\)

hints for the primary

for a step-down set-up, the primary has:

  • "H" marking (vs. "X" for low voltage)
    • think of H = High voltage
  • higher V
  • lower I
  • higher \(\Omega\)
  • more turns
  • (thinner wire)

step-up vs. step down

  • step up: \(N_s > N_p\), \(V_s > V_p\)
    • \(I_s<I_p\) (with load)
  • step down: \(N_s < N_p\), \(V_s < V_p\)
    • \(I_s>I_p\) (with load)

reflected load / impedance ratio

  • reflected load \(R_p\) of \(R_L\)
  • \(R_P = (\frac{N_P}{N_S})^2 R_L\)

Thus, the resistance of a fixed load \(R_L\) can be transformed to any desired value at the primary side. This is called impedance matching.

phase dots

  • dot = \(+\)
    • primary: current enters at dot
    • secondary: current exits at dot

Most circuits don't care about phase shifs between primary and seconary circuit. This is the case for a AC-to-DC power supply.

minimum spec

  • \(V_p\) and \(V_s\)
    • voltages the transformer is designed for
  • frequency (typically 50/50Hz for power transformers)
    • operating with the wrong frequency may damage the transformer
  • apparent power rating [VA]
    • applicable to primary and secondary because power is roughly identical
      • ideal case: \(P_p=P_s\)
      • typical efficiency: 95-97%
    • \(P_p=V_p*I_p\)
    • \(P_s=V_s*I_s\)
  • (turns or turn ration are typically not given.)

open secondary / no load

shorted secondary

  • ❗ excessive primary current, can burn out primary winding
  • \(R_L= 0\) => reflected load \(R_P=0\)

1:1 isolating transformer for safety


https://electronics.stackexchange.com/questions/17496/how-is-using-a-transformer-for-isolation-safer-than-directly-connecting-to-the-p


https://www.circuitlab.com/circuit/hv2h6gndxw84/transformer-safety/

(dc) isolating transformer

  • \(N_p = N_s\)
  • DC isolation: DC does not reach secondary
  • AC coupling: AC passes through (perhaps filtered for frequencies)
    \(N_p=N_s => V_{ac}=V_{ind}\)

vintage transformers (for driving tubes)

typical secondaries

  • filament winding (5 to 6.3V)
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    high voltage (up to 800 \(V_{pk}\)) https://www.radioremembered.org/xfmr.htm

parts

transformer-6v-1w | transformer-6v-1w-center | transformer-6v-5w

resources

https://leetsacademy.blogspot.com/2017/04/how-to-identify-primary-and-secondary.html

https://www.etechnog.com/2018/09/why-transformer-cannot-work-on-dc.html

http://www.hunter.cuny.edu/physics/courses/physics222/repository/files/pdf/ElectronicsLab12.pdf/view

https://www.circuitlab.com/questions/6x42c39f/primaries-and-secondaries-in-transformers/

https://www.circuitlab.com/forums/general-electronics-discussion/topic/3uzy2ajp/transformer-parameters/

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