This is one way of doing it. There is a model called transformer core
that draws two vertical lines.
Update: With neutral line and transformer core
Code
\documentclass[border=10pt]{standalone}
\usepackage[american,siunitx]{circuitikz}
\usetikzlibrary{arrows,shapes,calc,positioning}
\begin{document}
\begin{circuitikz}[american]
\draw (0,0) node [transformer core](T){} % reminded by @PaulGessler, thanks.
(T.A1) node[above] {A1}
(T.A2) node[below] {A2}
(T.B1) node[above] {B1}
(T.B2) node[below] {B2}
(T.base) node{K};
\draw (T.A1) --++(-2,0);
\draw (T.A2) --++(-2,0);
\draw (T.B1) --++(2,0) to[D, v=${V_\gamma=0.7}$, i>_=](5,0);
\draw (T.B2) --++(2,0) to[D, v=${V_\gamma=0.7}$ ,i>_=](5,-2.1);
\draw(T.A1) to[open,v<={$240V_{rms}$}](T.A2);
\draw(T.B2) to[open,v>=$$](T.B1);
% 2 new lines for neutral line on the secondary side.
\draw[thick] ($(T.B1)!0.515!(T.B2)-(0.7,0)$)--node[pos=0.5,above,inner sep=0pt](n){$12V_{rms}$}++ (3,0);
\draw (n) -- ++ (0,-0.3)node[ground]{};
\end{circuitikz}
\end{document}
Code
\documentclass[border=10pt]{standalone}
\usepackage[american,siunitx]{circuitikz}
\usetikzlibrary{arrows,shapes,calc,positioning}
\begin{document}
\begin{circuitikz}[american]
\draw (0,0) node [transformer](T){}
(T.A1) node[above] {A1}
(T.A2) node[below] {A2}
(T.B1) node[above] {B1}
(T.B2) node[below] {B2}
(T.base) node{K};
\draw (T.A1) --++(-2,0);
\draw (T.A2) --++(-2,0);
\draw (T.B1) --++(2,0) to[D, v=${V_\gamma=0.7}$, i>_=](5,0);
\draw (T.B2) --++(2,0) to[D, v=${V_\gamma=0.7}$ ,i>_=](5,-2.1);
\draw(T.A1) to[open,v<={$240V_{rms}$}](T.A2);
\draw(T.B2) to[open,v>=$12V_{rms}$](T.B1);
\draw ($(T.base)+(1mm,-2mm)$) -- ++(0,-1.8);
\draw ($(T.base)+(-1mm,-2mm)$) -- ++(0,-1.8);
\end{circuitikz}
\end{document}
Best Answer
If you don't have to use
circuitikz
, I can recommend the following (which is a modified version of a transformer that Thomas Söll drew and posted here):