> For the complete documentation index, see [llms.txt](https://mujoseph.gitbook.io/mujoseph-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mujoseph.gitbook.io/mujoseph-docs/emma-mk1-drsstc/hardware-overview/electrical-design/secondary-tank-circuit.md).

# Secondary Tank Circuit

The *Emma Mk1* secondary tank circuit consists of a secondary coil and toroidal topload.

<div data-with-frame="true"><figure><img src="https://2297250131-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzNHSl7dsTOS0XjlMXnIe%2Fuploads%2F5kGQDpYJSmfKkSmeIwZa%2Fimage.png?alt=media&amp;token=add5b9d0-8a07-4401-9004-e09c0510cfd5" alt="" width="373"><figcaption><p><em>Emma Mk1</em> secondary tank circuit assembly:<br>spun-aluminum toroidal topload, secondary coil, secondary coil<br>holder (blue) and secondary coil ground lug (green)</p></figcaption></figure></div>

## Measuring Secondary Tank Circuit Resonant Frequency

Using this [method](https://www.easternvoltageresearch.com/tesla-coil-workshop/measuring-the-resonant-frequency-of-your-secondary-coil/), the resonant frequency of the [secondary coil](/mujoseph-docs/emma-mk1-drsstc/hardware-overview/electrical-design/secondary-tank-circuit/secondary-coil.md) and the [toroidal topload](/mujoseph-docs/emma-mk1-drsstc/hardware-overview/electrical-design/secondary-tank-circuit/topload.md) series combination was measured as : $$f\_{res(sec)}=264,\text{kHz}$$.

<div data-with-frame="true"><figure><img src="https://2297250131-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzNHSl7dsTOS0XjlMXnIe%2Fuploads%2Fdgxpjf6uY5NANElHNboY%2Fimage.png?alt=media&amp;token=69e10a82-9b4c-4f45-ba8f-a68b30680dab" alt="" width="563"><figcaption><p>LTSpice sim showing behavior of voltage divider when swept across the <br>resonant frequency of the secondary tank circuit. At resonance the<br>capacitive and inductive reactance cancel, leaving only the ESR. ESR &#x3C;&#x3C; R1, leading to a minima in <br>the voltage divider output (null in frequency response).</p></figcaption></figure></div>

{% embed url="<https://youtube.com/shorts/-wcT47Rd_sU>" %}
Measuring the resonant frequency of the *Emma Mk1* secondary tank circuit
{% endembed %}

## Calculating Topload Capacitance

With the resonant frequency ($$f\_{res(sec)}$$)and the secondary coil inductance ($$L\_{sec}$$) known, the topload capacitance can be calculated using the standard equation for the resonant frequency of an LC circuit:

$$
C\_{tl}=\bigg(\frac{1}{2\pi f\_{res(sec)}\sqrt{L\_{sec}}}\bigg)^2= 18.45 \text,{pF}
$$