Impedance Matching

[Burnt]

Hi All,

A few questions.

Just wondering if someone can help me out with Impedance matching.

In the circuit attached does the completed Primary circuit including Mosfets & Primary Transformer coil have to match the Impedance of the completed Secondary circuit , including the work coil?

If this includes the work coil do i just remove one end of the work coil & measure the impedance across the Secondary circuit from there.
I guess on the primary side i read across the + & - inputs on the Mosfets, is this correct?

Also is Impedance just a fancy word for Resistance, if not can i check the Impedance without an Impedance Meter or just read Resistance?

Can i just measure Inductance for this circuit instead of Impedance, if the Impedance is the same on the Primary & Secondary will the Inductance be the same?

Thanks

 

[aibelectronics]

As regards the second question Impedance is the opposition to current flow from a circuit comprising resistances and reactive components like capacitors and inductors. It's associated only with AC systems. Impedance would only be a 'fancy' word for resistance in a DC circuit.

And as regards the last question, the inductances may or may not be the same. As your circuit shows, you have capacitors as well and they too contribute to the overall impedance.

 

[crutschow]

Matching the impedance is not required for your circuit. If it's operating at the resonant point the theoretical impedance of the L and C in series is equal to the resistance of the coil and wires which is generally very small.

There are two reasons to match impedances:

1) To get maximum power transfer, such as from an amplifier to a load. This was common for tube audio amps, not common for transistor audio amps since transistor amps generally have very low output impedance.

2) Match the impedance of a transmission line to avoid signal reflections and minimize SWR.

 

[Willbe]

There is something called the complex conjugate, to do with impedance matching.

It's been awhile since I've done this, but I think the problem is the output impedance of the FETs is resistive. I don't see how you can ever match an RC driving an LC circuit, unless you assume the L has some resistive component.

What's your operating freq. and other values?

 

[Burnt]

Thanks for the replies,

It's best i direct you to the site that the Schematic come from for more information as i am just learning about all of this & i am somewhat confused?.

uzzors2k - Series Resonant Induction Heater

I am attemting to make a small reliable induction heater & any advise would be greatly appreciated, i need to heat small pieces of steel maximum size 50mm x 50mm solid sections.

Thank You

 

[Burnt]

Hi Again,

Anybody care to comment about the Impedance matching of the particular circuit.

I need to make this but i would like to get a better understanding of it before just jumping in head first & just making it without a full understanding of it.

I have built the driver for it already & have purchased some components but i am missing the knowledge to understand it without help.

I realise that some will not comment on things like this for obvious reasons & the dangers involved, i am a newby to electronics only.

I need to build this to heat treat components that i make for Automotive purposes.

Thanks

 

[crutschow]

For the induction heater he is matching the high current of the resonant induction coil and capacitor to the lower output current ability of the transistors using a transformer. I would recommend following his design as closely as possibly and observing his precautions to avoid zapping the circuit, since it's delivering a large amount of energy to the load.

The transformer is not really matching impedances as such, just transforming currents and voltages, the same as the transformer to your house.

 

[MrAl]

Hi Again,

Anybody care to comment about the Impedance matching of the particular circuit.

I need to make this but i would like to get a better understanding of it before just jumping in head first & just making it without a full understanding of it.

I have built the driver for it already & have purchased some components but i am missing the knowledge to understand it without help.

I realise that some will not comment on things like this for obvious reasons & the dangers involved, i am a newby to electronics only.

I need to build this to heat treat components that i make for Automotive purposes.

Thanks

Hello,


The impedance matching aspect of the transformer is basically to be
able to drive the output coil resistance with the maximum current that
is possible. Without the transformer the coil is subject to be driven
by the series impedance of the transistors and the 1uf capacitors,
and even the power supply,
which can be high relative to the resistance of the coil. By using
a step down transformer between driver and load we effectively
lower the output impedance of the driver by 400 (20 to 1 transformer)
and so are better able to drive the coil. The best match is obtained
when the transformer presents the same impedance as the
raw output of the transistors, to the transistors, with the true load
connected, and this will be 400 times higher than the actual load
impedance because the impedance reflected to the primary is
as the turns ratio in that direction squared (20 squared = 400).

The other view is that we want to load the power supply as much
as possible, but not overload it, and this can only happen when the
load on the very output of the transistors is a certain value, and this
only happens when the transformer ratio is such that the reflected
impedance is that certain value.

There are practical considerations that make this a little harder to
do then simply sit down and do a few calculations. This is partly
due to the proposed frequency of operation (125kHz) and also because
there are component imperfections abounding. The frequency
of operation here means that skin effect is going to have a
significant impact on the actual component parameters.
The component imperfections include this skin effect as well as
inherent deficiencies like ESR and leakage inductance.
What this means is getting the right transformer ratio is going to
take some experimentation, unless suboptimal performance can be
tolerated. This also means it would be best to 'tune' the frequency
in using a current meter to measure dc current from the power
supply, or possibly just looking for a dip in the power supply voltage.
As the frequency is tuned, the power supply current increases
as resonance is obtained.
To tune the transformer, it would be necessary to try different
ratios to see which one caused the maximum power in the coil.
This may have already been done and 20 seemed like a good
ratio which causes enough heating so that it doesnt have
to be changed. If not, or if your setup is unique, changing
the turns ratio could result in more heating.

I would try it with 20, and also tune the frequency and
see if it flies like that first.


Simple Example...

We have a 10v power supply that can put out 10 amps max.
To get this 10 amps max, we need a load of 1 ohm because
anything larger wont draw the full power and anything smaller
will draw too much current.
Our load, unfortunately, is 100 times smaller (0.01 ohms),
so if we connect this load to the output we will attempt
to draw 1000 amps, which obviously will blow the fuse.
If we use a series resistor of 0.99 ohms, we get the
right current (10 amps) but now we loose much of the
power in the 0.99 ohm resistor, so instead we use a
matching transformer. Since we want to step 'up' the
load from 0.01 to 1, this is 100 times, so we need a
turns ratio of 10 to 1 (because 10 squared is 100).
This transformer will allow the 10 amps to be drawn from
the power supply and the load will get the max power
available without damaging the power supply.

 

[Burnt]

Thanks to all who have taken the time to explain this to me.

Once again MrAl you have written an explanation that is easy to understand & very informative & helpful to me.


Thank You very much!