Are inductors in resonant circuits dangerous? They produce very high voltages

A simple series LC resonant circuit has theoretically infinite voltage in the middle between L and C, if we assume 0 ESR and exact resonant frequency:

$$\V_{at LC junc} = V_{in} \frac{X_{C1}}{X_{C1}-X_{L1}} \to \frac {X_{C1}}{0} \to \infty\$$

Of course, in real life, we will not hit the resonant frequency exactly, and there's also some ESR. Nonetheless, we get very high voltages, over 60 V in this case:

And a simple Clapp oscillator showed voltages even higher at the LC junction.

Can these high voltages be dangerous? They are above the safe voltage limit. I can think of reasons why they might be safe, but when I asked about similar circuits, the answer was a unanimous they are not safe.

• I'm afraid that the JFET model you have used in your linked question about the Clapp oscillator is rather misguiding you. It's almost certain the the positive gate voltage will not exceed +1 volt in any circumstances. The problem here appears to be that you are trusting the model without using theoretical JFET information to tell you that the model is poor. Also the transient analysis image doesn't have any information about the waveforms. Commented Jul 1 at 17:46
• @Andyaka It's true that gate voltage is limited. But the voltage at the junction between L1 and C2 can get very high, perhaps 80V. Commented Jul 1 at 17:52
• Yes, I misread the circuit and apologize. Clearly the gate voltage can rise if the source voltage also rises. However, given that the source voltage cannot be greater than the supply voltage on the drain, the peak positive voltage is limited to no more than the supply voltage. Commented Jul 1 at 17:54
• Actually, in real life, if your design counts on never hitting resonance exactly, then just by luck, you will :) Commented Jul 1 at 17:58
• Also note, that when you touch it, the human body will be adding capacitance to that circuit model, so it can quickly drop out of resonance. Thus, only the small charge in the cap at the time of touch will discharge into and "zing" you. Then it won't do much after that, most likely drop to 12v source. Commented Jul 1 at 18:24

When you touch it, the human body will be adding capacitance to that circuit model, so it will quickly drop out of resonance. Thus, only the small charge in the cap at the time of touch will discharge into and "zing" you. Then it won't do much after that, most likely drop to 12v source.
Danger to the human body is related to how much energy is going through it. In this case it's the charge in the capacitor. $$\U=\frac{1}{2}CV^2\$$
$$\\frac{1}{2}*1nF*60V^2=1.8uJ\$$
So will this cause a burn?
Approximately 16.4 J/cm2 of heat transfer are necessary to cause second-degree burns. So that 1.8uJ would need to be in tiny area of 109nm2 (nano meters squared).
No danger here!

• Thanks. Do you mean there's no energy stored in the inductor at the time, because the voltage across it is high? Commented Jul 1 at 18:43
• @SRobertJames The energy is equal in each part, and it bounces back and forth between the two. That's kinda the definition of resonance. Commented Jul 1 at 18:50
• Useful reference (or at least interesting). Rule of thumb - aim for well under 1000 degrees C X seconds exposure Commented Jul 1 at 20:13

They can be, just like any circuit that generates high voltages can be dangerous, regardless of if it uses resonance, charge pumps, electromagnetic conversion, etc. Resonance requires specific conditions, as you mentioned, and if you are driving a LC circuit with a sine wave then frequency response is probably a big part of your design, so the chances of "accidentally" creating a dangerous voltage this way are fairly low.

It is true that at resonance, voltage across an inductor in a resonant circuit can be large...especially so if resonant components have high Q (quality factor). If one is building a power oscillator, care should be exercised.
Even with a JFET Clapp-type oscillator, high-Q resonators should be treated with some respect. A low-power JFET oscillator built by Harold Johnson used a carefully-built high-Q helical resonator. He says, "I have managed to get little RF burns off the top end of a helical resonator oscillator with the active device running at 15 volts!"
His circuit is Clapp-like in that a 9-turn helical coil was tapped at one-half turn from its cold end (a helical resonator has no discrete resonating capacitors). However, even with great care, it is unlikely that a conventional inductor could approach quality factors of a well-designed helical resonator.

High-power LC oscillators can shift frequency as they heat. Where stable frequency is desired, power is often kept lower. Leeson's criteria for low phase noise oscillators wants to maximize power, along with high-Q resonator components.
Heat can cause a trade-off between phase noise and stability, at least for LC oscillators.

With low DC supply voltage, keeping transistor junctions from forward-biasing during parts of the oscillator's swing can be difficult. At low power, design has more latitude. Transistor junctions that swing into forward bias reduce resonator Q - not what you want.

For crystal oscillators, resonator Q is very high, but crystals can only accept limited power before they shift frequency or are damaged.