50 ohm to 1 ohm Impedance matching

CosmicOrderMachines

Joined Oct 31, 2019
77
I'm trying to generate sufficient current into a primary of a tesla coil to get high voltage on the secondary. Using it as a transformer rather than to make sparks. Using a signal generator and 50 watt amplifier. The point of that is just to experiment. And I'm learning tons..

Since the amplifiers output impedance is 50 ohms. I'm wondering what gives the most power into the primary? I heard of impedance matching.. I know I can adjust the Inductance of the primary..

CosmicOrderMachines

Joined Oct 31, 2019
77
Can I just match the inductive reactance to 50 ohm?
$X_L=2\pi fL$

Say f = 200khz
I want L to = 39.7uH

Is this kind of thing ok?

ronsimpson

Joined Oct 7, 2019
1,459
Using a signal generator and 50 watt amplifier. .......... Since the amplifiers output impedance is 50 ohms
A 50 watt amp with a 50 ohm resistor on its output? I think a 50 watt amp might have 5 ohms or less. The signal generator does have 50 ohms.

CosmicOrderMachines

Joined Oct 31, 2019
77
A 50 watt amp with a 50 ohm resistor on its output? I think a 50 watt amp might have 5 ohms or less. The signal generator does have 50 ohms.
It says on the amplifier specs input and output impedance 50 ohm. It's a 100kHz-3MHz amp. Something off ebay ahaha

Last edited:

Yaakov

Joined Jan 27, 2019
3,483
50W in those RF bands suggests you are about to cause a great deal of havoc and violate the law.

The amount of RFI you are likely to generate is prodigious, I suggest you not do this.

CosmicOrderMachines

Joined Oct 31, 2019
77
50W in those RF bands suggests you are about to cause a great deal of havoc and violate the law.

The amount of RFI you are likely to generate is prodigious, I suggest you not do this.
Rip. Well I dont want that. Is it true aircoils wont work on lower frequencies? Ive also read that but never really understood why.

BobaMosfet

Joined Jul 1, 2009
1,848
I'm trying to generate sufficient current into a primary of a tesla coil to get high voltage on the secondary. Using it as a transformer rather than to make sparks. Using a signal generator and 50 watt amplifier. The point of that is just to experiment. And I'm learning tons..

Since the amplifiers output impedance is 50 ohms. I'm wondering what gives the most power into the primary? I heard of impedance matching.. I know I can adjust the Inductance of the primary..
Impedance matching: 50-Ohm output needs to go into 50-ohm input. The question I'm sensing is that I don't think you actually understand the concept of 'impedance'. What it really is. Math is fine, but meaningless if you don't understand what you're actually doing. In a nutshell, impedance means flow of current- or how strong the signal is. Volts is information (wave-form).

KeepItSimpleStupid

Joined Mar 4, 2014
5,072
You should look at a "matching network" and instrumentation to measure "forward and reflected power") or SWR,

The manual ones typically variable glass capacitors, a shunt and series cao (Jennings) and fixed inductors. They are adjusted to get the minimum reflected power.

There are automatic matching networks. They typically use fixed capacitors and a motorized roller inductor.

The max power transferred to the load occurs when there is no reactive component.

This article https://www.electronics-notes.com/articles/test-methods/oscilloscope/scope-probe-compensation.php on scope probe compensation could get you started. it's a simplistic view and it doesn't include inductance.

The idea is the scope has an input Z of 1M || 22 pf and the probe wires has some capacitance. No inductance is involved.
The scope probe divides by 10, so it has a variable capacitor and a 9M resistor. The object is to make the probel look resistive. You do that by measuring a known square wave signal and adjusting the compensation capacitor until the edges are square.

The matching network has to dothe same thing basically, but with power.

Here https://www.wirelessdesignonline.com/doc/understanding-forwardreflected-power-0001 is just a blurb.

CosmicOrderMachines

Joined Oct 31, 2019
77
Thanks for the help guys. I do kind of balance (joke) on understanding impedance matching. I get that change in impedance is going to affect the source negatively. Conjugate pairs in ac because otherwise the out of phase voltage/current interactions again would combine negatively.

Interestingly I was going down the rabbit hole of transformers to impedance match. I get how and why mentally that would work. But practically it has limits. Like unrealistic number of turns, core saturation. Idk, good amount to read.

I'm trying now to put it into frame of mind this low inductance primary air coil won't work for low frequencies. Why do low frequencies need higher inductance?

Yaakov

Joined Jan 27, 2019
3,483
An LC circuit resonates as the input AC signal rises and falls. It has to charge the capacitor and create an increasing and collapsing magnetic field in the inductor. The pair work together to reinforce each other. Each has to be able to charge and discharge over the period of the waveform. So, a slower waveform needs a longer time constant. That means a lower frequency needs a larger value for the L and C.

Check out this calculator and the notes: https://goodcalculators.com/resonant-frequency-calculator/

CosmicOrderMachines

Joined Oct 31, 2019
77
An LC circuit resonates as the input AC signal rises and falls. It has to charge the capacitor and create an increasing and collapsing magnetic field in the inductor. The pair work together to reinforce each other. Each has to be able to charge and discharge over the period of the waveform. So, a slower waveform needs a longer time constant. That means a lower frequency needs a larger value for the L and C.

Check out this calculator and the notes: https://goodcalculators.com/resonant-frequency-calculator/
I must be getting this wrong somewhere fundamentally.. Since strictly inductor reactance goes down with lower frequency or lower inductance, that is a good thing right? Means more current? But if this view includes the parasitic capacitance then it will act as a choke for the very same low frequency?

Papabravo

Joined Feb 24, 2006
16,776
I must be getting this wrong somewhere fundamentally.. Since strictly inductor reactance goes down with lower frequency or lower inductance, that is a good thing right? Means more current? But if this view includes the parasitic capacitance then it will act as a choke for the very same low frequency?
The only way to truly appreciate the behavior of reactive components is to sweep them on a VNA and watch as they change from inductive to capacitive and back again, forever circling the origin in the complex plane. Blew my mind the first time I saw it, but it is fairly well explained in Feynmann Vol II.

CosmicOrderMachines

Joined Oct 31, 2019
77
The only way to truly appreciate the behavior of reactive components is to sweep them on a VNA and watch as they change from inductive to capacitive and back again, forever circling the origin in the complex plane. Blew my mind the first time I saw it, but it is fairly well explained in Feynmann Vol II.
Gotcha. I will try to do this. I appreciate that hidden secret here, thanks

Danko

Joined Nov 22, 2017
1,133
I'm trying to generate sufficient current into a primary of a tesla coil to get high voltage on the secondary. Using it as a transformer rather than to make sparks. Using a signal generator and 50 watt amplifier.
Tesla coil generates high voltage by two reasons:
1. It works on resonant frequency of tank (L secondary, C secondary).
2. It has very small coupling coefficient between primary and secondary windings.

Because of continuous changes of environment parameters, resonant frequency of tank changes too, so it should be driven by auto generator, not by signal generator + amplifier.

KeepItSimpleStupid

Joined Mar 4, 2014
5,072
Look up "Eli the Ice Man". Motors in a building have lots of inductance. An automatic power factor corrector has to add capacitance to fix it.

You have this:

And the fact that Xl and Xc are frequency dependent.

the whole point is you want xl-xc to equal to zero

Papabravo

Joined Feb 24, 2006
16,776
It was recently pointed out by a good friend, that tank circuits should be driven with current sources to ensure successful operation.

Janis59

Joined Aug 21, 2017
1,348
Tesla coils have a narrowly specific generator and narrow specific geometric attitudes in the construction let the both resonant frequencies fall in the same figure, only then may expect a zillion volts output. May illustrate with my mate experience to make a TC wrongly and create the 1 cm spark. And then made the coil in right way and got 40 cm long spark with the same turn count, same transistor and same power supply.

lmsele

Joined May 11, 2021
2
I'm trying to generate sufficient current into a primary of a tesla coil to get high voltage on the secondary. Using it as a transformer rather than to make sparks. Using a signal generator and 50 watt amplifier. The point of that is just to experiment. And I'm learning tons..

Since the amplifiers output impedance is 50 ohms. I'm wondering what gives the most power into the primary? I heard of impedance matching.. I know I can adjust the Inductance of the primary..
You are welcome.
View attachment 238082

lmsele

Joined May 11, 2021
2
I'm trying to generate sufficient current into a primary of a tesla coil to get high voltage on the secondary. Using it as a transformer rather than to make sparks. Using a signal generator and 50 watt amplifier. The point of that is just to experiment. And I'm learning tons..

Since the amplifiers output impedance is 50 ohms. I'm wondering what gives the most power into the primary? I heard of impedance matching.. I know I can adjust the Inductance of the primary..
You are welcome